JakeHillion/dissertation
1
0
Fork 0
mirror of https://git.overleaf.com/6227c8e96fcdc06e56454f24 synced 2024-11-24 11:10:23 +00:00
Code Issues Projects Releases 8 Wiki Activity

Update on Overleaf.

Browse Source
This commit is contained in:
jsh77 2022-05-06 17:55:11 +00:00 committed by node
parent 6c2ce57a6f
commit 1078e34d73
1 changed files with 94 additions and 75 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

169
dissertation.tex
View File

@ -104,7 +104,7 @@
\setuptodonotes{inline}
% listing settings
\lstset{basicstyle=\small\ttfamily}
\lstset{basicstyle=\footnotesize\ttfamily}
%%
%% end of the preamble, start of the body of the document source.
@ -140,9 +140,9 @@
%% The abstract is a short summary of the work to be presented in the
%% article.
\begin{abstract}
Operating systems are providing more facilities for process isolation than ever before, realised in technologies such as Docker containers \citep{merkel_docker_2014} and systemd slices \citep{the_systemd_authors_systemdslice_2022}. These systems separate the design of the program from the systems that create privilege separation. Void Processes take these techniques to the extreme, removing access to everything but syscalls from a process by default. This work focuses on adding back slivers of privilege to achieve functional applications with minimal privilege.
Operating systems are providing more facilities for process isolation than ever before, realised in technologies such as Docker containers \citep{merkel_docker_2014} and systemd slices \citep{the_systemd_authors_systemdslice_2022}. Void Processes take these techniques to the extreme, removing access to everything but syscalls from a process by default. This work focuses on adding back slivers of privilege to achieve functional applications with minimal privilege.
I present a summary of the privilege separation features in modern Linux, the system design of void processes, and an evaluation on a series of example applications.
I present a summary of the privilege separation features in modern Linux, the system design of Void Processes, and an evaluation on a series of example applications.
\end{abstract}
%%
@ -192,13 +192,19 @@ I present a summary of the privilege separation features in modern Linux, the sy
\section{Introduction}
Void processes take advantage of modern Linux namespaces to run applications with minimal exposure to the system itself. Void processes use a mixture of Linux namespaces and file descriptor based capabilities to allow running purpose-built applications without expecting the support of the standard Linux system. During the process of building such a system, gaps in the kernel were exposed. Namespaces were intended to emulate an ordinary Linux system, rather than creating something new. This work will go on to detail the mechanisms for creating void processes themselves, re-adding features that these processes need to do useful work, and the learnings of what features are missing in the user-space kernel APIs to succeed in creating processes this way.
Void Processes take advantage of modern Linux namespaces to run applications with minimal exposure to the host system. Void Processes use a mixture of Linux namespaces and file descriptor based capabilities to allow running purpose-built applications without expecting the support of a full Linux system. During the process of building such a system, gaps in the kernel were exposed - namespaces were intended to emulate an ordinary Linux system, rather than build something new. This work will go on to detail the mechanisms for creating Void Processes themselves, re-adding features that these processes need to do useful work, and the learnings of what features are missing in the user-space kernel APIs to succeed in creating processes this way.
\iffalse
This work explores the question of what is an operating system by taking a novel approach to running applications with the system exposed in a very different way. Rather than limiting the access of a process or set of processes to the operating system, such as in containers, we instead limit the access to the operating system with more explicit methods per process. Interaction between processes is allowed by specifying such interaction statically at compile time, removing any separation between the application developer and the system controlling access to the application, unlike solutions such as SELinux \citep{loscocco_security-enhanced_2000}.
\fi
The question of what makes an operating system has been asked many times. This work looks for an answer by running applications in a very different way.
\todo{Comparison to unikernels.}
\begin{table*}
\caption{Table showing the date and kernel version each namespace was added. The date provides the first commit where they appeared date of creation, and the kernel version the kernel release they appear in the changelog of. Namespaces are ordered by kernel version then alphabetically. Some examples are provided of CVEs for each namespace.}
\caption{Table showing the date and kernel version each namespace was added. The date provides the date of the first commit where they appeared, and the kernel version the kernel release they appear in the changelog of. Namespaces are ordered by kernel version then alphabetically. Some examples are provided of CVEs for each namespace.}
\begin{minipage}{\textwidth}
\begin{center}
@ -259,55 +265,54 @@ This work explores the question of what is an operating system by taking a novel
\section{Namespaces}
\label{sec:voiding}
Isolating parts of a Linux system from the view of certain processes is achieved by using namespaces. Namespaces are commonly used to provide isolation in the context of containers, which are very close to virtual machines - they provide an isolated complete Linux environment. Instead, with Void Processes, we target complete isolation. Rather than using namespaces to provide a view of an alternate full Linux system, they are used to provide a view of a system that is as minimal as possible. In this section each namespace available in Linux is detailed, including how to create a void.
Isolating parts of a Linux system from the view of certain processes is achieved by using namespaces. Namespaces are commonly used to provide isolation in the context of containers, which provide the appearance of an isolated complete Linux environment to contained processes. Instead, with Void Processes, we target complete isolation. Rather than using namespaces to provide a view of an alternate full Linux system, they are used to provide a view of a system that is as minimal as possible, while still sitting atop the Linux kernel. In this section each namespace available in Linux is detailed, including how to take each namespace and convert it to completely empty for a Void Process. Section \ref{sec:filling} goes on to explain how necessary features for applications are added back in.
The full set of namespaces are represented in Table \ref{tab:namespaces}, in chronological order. The chronology of these is important in understanding the thought process behind some of the design decisions.
Preparing a void process takes advantage of the namespaces feature in Linux. However, many of the namespaces are not designed for this purpose, so this is a more difficult prospect than one might hope. Details of when each namespace was added and some of the relevant features are given in Table \ref{tab:namespaces}.
The full set of namespaces are represented in Table \ref{tab:namespaces}, in chronological order. The chronology of these is important in understanding the thought process behind some of the design decisions. The ease of creating an empty namespace varies massively, as although adding namespaces shared the goal of containerisation, they were completed by many different teams of people over a number of years. Some namespaces maintain strong connections to their parent, while others are created completely separate. We start with those that are most trivial to add, working up to the namespaces most intensely linked to their parents.
\subsection{ipc namespaces}
\label{sec:voiding-ipc}
Creating a void process with IPC namespaces is pleasantly easy in comparison. From the manual page \citep{free_software_foundation_ipc_namespaces7_2021}:
IPC namespaces isolate two mechanisms that Linux provides for IPC which aren't controlled by the filesystem. System V IPC and POSIX message queues are each accessed in a global namespace of keys. This has created issues in the past with attempting to run multiple instances of PostgreSQL on a single machine, as both instances tried to create a System V IPC entry with the same key [CN]. IPC namespaces solve this effectively for containers by creating a new scoped namespace. Processes are a member of one and only one IPC namespace, allowing the familiar global key APIs. IPC namespaces are optimal for creating Void Processes. From the manual page \citep{free_software_foundation_ipc_namespaces7_2021}:
\say{Objects created in an IPC namespace are visible to all other processes that are members of that namespace, but are not visible to processes in other IPC namespaces.}
This provides exactly the correct semantics for a void, particularly because it is not copy-on-write. IPC objects are visible within a namespace if and only if they are created within that namespace. Therefore, a new namespace is an entirely empty void.
This provides exactly the correct semantics for a Void Process. IPC objects are visible within a namespace if and only if they are created within that namespace. Therefore, a new namespace is entirely empty.
\subsection{uts namespaces}
\label{sec:voiding-uts}
UTS namespaces provide isolation of the hostname and domain name of a system between processes. Similarly to IPC namespaces, all processes in the same namespace see the same results for each of these. Unlike IPC namespaces, UTS namespaces are copy-on-write. That is, the value of each of these in the parent namespace is the same in the child.
UTS namespaces provide isolation of the hostname and domain name of a system between processes. Similarly to IPC namespaces, all processes in the same namespace see the same results for each of these values. This is useful when creating containers. If unable to hide the hostname, each container would look like the same machine. Unlike IPC namespaces, UTS namespaces are copy-on-write. Each of these values in the child is initialised as the same as the parent.
As the copied value does give information about the world outside of the void process, slightly more must be done than placing the process in a new namespace. Fortunately this is easy for UTS namespaces, as the host name and domain name can be set to constants, removing any link to the parent.
As the copied value does give information about the world outside of the Void Process, slightly more must be done than placing the process in a new namespace. Fortunately this is easy for UTS namespaces, as the host name and domain name can be set to constants, removing any link to the parent.
\subsection{time namespaces}
\label{sec:time-namespaces}
\label{sec:voiding-time}
Time namespaces are the final namespace added at the time of writing, added in kernel version 5.6 \citep{noauthor_linux_2020}. The motivation for adding time namespaces is given in the manual page \citep{free_software_foundation_time_namespaces7_2021}:
\say{The motivation for adding time namespaces was to allow the monotonic and boot-time clocks to maintain consistent values during container migration and checkpoint/restore.}
That is, time namespaces virtualise the appearance of system uptime to processes, rather than attempting to virtualise the wall clock time. This is important for processes that depend on it in one specific situation: migration. If an uptime dependent process is migrated from a machine that has been up for a week to a machine that was booted a minute ago, the guarantees provided by the clocks \texttt{CLOCK\_MONOTONIC} and \texttt{CLONE\_BOOTTIME} no longer hold.
That is, time namespaces virtualise the appearance of system uptime to processes, rather than attempting to virtualise the wall clock time. This is important for processes that depend on time in primarily one situation: migration. If an uptime dependent process is migrated from a machine that has been up for a week to a machine that was booted a minute ago, the guarantees provided by the clocks \texttt{CLOCK\_MONOTONIC} and \texttt{CLOCK\_BOOTTIME} no longer hold. This results in time namespaces having very limited usefulness in a system that does not support migration, such as the one presented here. Perhaps randomised offsets would hide some information about the system, but the usefulness is debatable and the quantity of bespoke syscalls would slow down the application. Time namespaces are thus avoided in this implementation.
This results in time namespaces having very limited usefulness in a system that does not support migration, such as the one presented here. Perhaps randomised offsets would hide some information about the system, but the usefulness is debatable and the quantity of bespoke syscalls would slow down the application. Time namespaces are thus avoided in this implementation.
\subsection{network namespaces}
\label{sec:voiding-net}
\subsection{net namespaces}
Network namespaces were added in kernel version 2.6.24 \citep{noauthor_linux_2008}, some time after the initial namespace boom. They present the optimal namespace for creating a void. Creating a new network namespace immediately creates an entirely empty namespace. That is, the new network namespace has no link whatsoever to the creating network namespace. To add a link, one can create a virtual Ethernet pair, with one adapter in each namespace [RN]. Alternatively, one can create a Wireguard adapter with sending and receiving sockets in one namespace and the VPN adapter in another \citep[§7.3]{donenfeld_wireguard_2017}. This allows for very high levels of separation while still maintaining access to the primary resource - the Internet or wider network.
Network namespaces were added in kernel version 2.6.24 \citep{noauthor_linux_2008}. Similarly to IPC, they present the optimal namespace for running a Void Process. Creating a new network namespace immediately creates a namespace containing only a loopback adapter. This means that the new network namespace has no link whatsoever to the creating network namespace, only supporting internal communication. To add a link, one can create a virtual Ethernet pair with one adapter in each namespace [RN]. Alternatively, one can create a Wireguard adapter with sending and receiving sockets in one namespace and the VPN adapter in another \citep[§7.3]{donenfeld_wireguard_2017}. These methods allow for very high levels of separation while still maintaining access to the primary resource - the Internet or wider network.
\subsection{pid namespaces}
\label{sec:voiding-pid}
pid namespaces add a mapping from the process IDs inside the namespace to process IDs in the parent namespace. This continues until processes reach the top-level pid namespace. This isolation behaviour is different to that of some other namespaces, as each process within the namespace represents a process in the parent namespace too.
PID namespaces add a mapping from the process IDs inside the namespace to process IDs in the parent namespace. This continues until processes reach the top-level PID namespace. This isolation behaviour is different to that of some other namespaces, as each process within the namespace represents a process in the parent namespace too, albeit with different identifiers.
Although pid namespaces work quite well for creating a void process from the perspective of the inside process, some care must be taken in the implementation, as the actions of pid namespaces are highly affected by others. Some examples of this slightly unusual behaviour are shown in Listing \ref{lst:unshare-pid}.
Although PID namespaces work quite well for creating a Void Process from the perspective of the inside process, some care must be taken in the implementation, as the actions of PID namespaces are highly affected by others. Some examples of this slightly unusual behaviour are shown in Listing \ref{lst:unshare-pid}.
The first behaviour shown is that an \texttt{unshare(CLONE\_PID)} call followed immediately by an \text{exec} does not have the desired behaviour. The reason for this is that the first process created in the new namespace is given PID 1 and acts as an init process. That is, whichever process the shell spawns first becomes the init process of the namespace, and when that process dies, the namespace can no longer create new processes. This behaviour is avoided by either calling \texttt{unshare/fork}, or utilising \texttt{clone(2)} instead. The \texttt{unshare(1)} binary provides a fork flag to solve this, while the implementation of the void orchestrator uses \texttt{clone(2)} which combines the two into a single syscall.
The first behaviour shown is that an \texttt{unshare(CLONE\_PID)} call followed immediately by an \text{exec} does not have the desired behaviour. The reason for this is that the first process created in the new namespace is given PID 1 and acts as an init process. That is, whichever process the shell spawns first becomes the init process of the namespace, and when that process dies, the namespace can no longer create new processes. This behaviour is avoided by either calling \texttt{unshare(2)} followed by \texttt{fork(2)}, or utilising \texttt{clone(2)} instead. The \texttt{unshare(1)} binary provides a fork flag to solve this, while the implementation of the Void Orchestrator uses \texttt{clone(2)} which combines the two into a single syscall.
Secondly, we see that even in a shell that appears to be working correctly, processes from outside of the new pid namespace are still visible. This behaviour occurs because the mount of \texttt{/proc} visible to the process in the new pid namespace is the same as the init process. This is solved by remounting \texttt{/proc}, available to \texttt{unshare(3)} with the \texttt{--mount-proc} flag. Care must be taken that this mount is completed in a new mount namespace, or else processes outside of the pid namespace will be affected. The void orchestrator again avoids this by voiding the mount namespace entirely, so any access to proc must be either bound to outside the namespace, or freshly mounted.
Secondly, we see that even in a shell that appears to be working correctly, processes from outside of the new PID namespace are still visible. This behaviour occurs because the mount of \texttt{/proc} visible to the process in the new PID namespace is the same as the init process. This is solved by remounting \texttt{/proc}, available to \texttt{unshare(3)} with the \texttt{---mount-proc} flag. Care must be taken that this mount is completed in a new mount namespace, or else processes outside of the PID namespace will be affected. The Void Orchestrator again avoids this by voiding the mount namespace entirely, so any access to proc must be either bound to outside the namespace, or freshly mounted, allowing either required behaviour.
\lstset{caption={Unshare behaviour with pid namespaces, with and without forking and remounting proc.}}
\lstset{caption={Unshare behaviour with PID namespaces, with and without forking and remounting proc.}}
\begin{lstlisting}[float,label={lst:unshare-pid}]
$ unshare -p
-bash: fork: Cannot allocate memory
@ -331,48 +336,35 @@ $ unshare --fork --mount-proc -p
\end{lstlisting}
\subsection{cgroup namespaces}
cgroup namespaces provide limited isolation of the cgroup hierarchy between processes. Rather than showing the full cgroups hierarchy, they instead show only the part of the hierarchy that the process was in on creation of the new cgroup namespace. Correctly creating a void process is hence as follows:
\begin{enumerate}
\item Create an empty cgroup leaf.
\item Move the new process to that leaf.
\item Unshare the cgroup namespace.
\end{enumerate}
This process excludes the cgroup namespace from the initial \texttt{clone(3)} call, as the cloned process must be moved before creating the new namespace. By following this sequence of calls, the process in the void can only see the leaf which contains itself and nothing else, limiting access to the host system. This is the approach taken in this piece of work.
Although good isolation of the host system from the void process is provided, the void process is in no way hidden from the host. There exists only one cgroups v2 hierarchy on a system (cgroups v1 are ignored for clarity), where resources are delegated through each. This means that all processes contained within the hierarchy must appear in the primary hierarchy, such that the distribution of the single set of system resources can be centrally controlled. This behaviour is similar to the aforementioned pid namespaces, where each process has a distinct pid in each of its parents, but does show up in each. Hiding from the host has little value as a root user there can inspect each namespace manually.
\subsection{mount namespaces}
\label{sec:voiding-mount}
Mount namespaces were by far the most challenging part of this project. When adding new features, they continuously raised problems in both API description, expected behaviour, and availability of tools in user-space. A comparison will be given in this section to two other namespaces, network and UTS, to show the significant differences in the design goals of mount namespaces. Many of the implementation problems here comes from a fundamental lack of consistency between mount namespaces and other namespaces in Linux.
\subsubsection{Copy-on-Write}
Comparing to network namespaces, a slightly more modern namespace [Table \ref{tab:namespaces}], we see a huge difference in what occurs when a new namespace is created. When creating a new network namespace, the ideal conditions for a void process are created - a network namespace containing only a loopback adapter. That is, the process has no ability to interact with the outside network, and no immediate relation to the parent network namespace. To interact with alternate namespaces, one must explicitly create a connection between the two, or move a physical adapter into the new (empty) namespace. Mount namespaces, rather than creating a new and empty namespace, made the choice to create a copy of the parent namespace, in a copy-on-write fashion. That is, after creating a new mount namespace, the mount hierarchy appears much the same as before.
Comparing to network namespaces, a slightly more modern namespace [Table \ref{tab:namespaces}], we see a huge difference in what occurs when a new namespace is created. When creating a new network namespace, the ideal conditions for a Void Process are created - a network namespace containing only a loopback adapter. That is, the process has no ability to interact with the outside network, and no immediate relation to the parent network namespace. To interact with alternate namespaces, one must explicitly create a connection between the two, or move a physical adapter into the new (empty) namespace. Mount namespaces, rather than creating a new and empty namespace, made the choice to create a copy of the parent namespace, in a copy-on-write fashion. That is, after creating a new mount namespace, the mount hierarchy appears much the same as before. This is shown in Listing \ref{lst:unshare-cat-passwd}, where the file \texttt{/etc/passwd} is shown before and after an unshare, revealing the same content.
\lstset{caption={Reading the same file before and after unsharing the mount namespace.}}
\begin{lstlisting}[float,label={lst:unshare-cat-passwd}]
int main() {
int fd;
int fd;
if ((fd = open("/etc/passwd", O_RDONLY)) < 0)
perror("open");
print_file(fd);
if (close(fd))
perror("close");
if ((fd = open("/etc/passwd", O_RDONLY)) < 0)
perror("open");
print_file(fd);
if (close(fd))
perror("close");
if (unshare(CLONE_NEWNS))
perror("unshare");
printf("----- unshared -----\n");
if (unshare(CLONE_NEWNS))
perror("unshare");
printf("----- unshared -----\n");
if ((fd = open("/etc/passwd", O_RDONLY)) < 0)
perror("open");
print_file(fd);
if (close(fd))
perror("close");
if ((fd = open("/etc/passwd", O_RDONLY)) < 0)
perror("open");
print_file(fd);
if (close(fd))
perror("close");
}
--
@ -387,13 +379,12 @@ daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
...
umount: Device or resource busy
\end{lstlisting}
\subsubsection{Shared Subtrees}
\label{sec:shared-subtrees}
While some other namespaces are copy-on-write, for example UTS namespaces, they do not present the same problem as mount namespaces. Although UTS namespaces are copy-on-write, it is trivial to create the conditions for a void process by setting the hostname of the machine to a constant. This removes any relation to the parent namespace and to the outside machine. Mount namespaces instead maintain a shared pointer with most filesystems, more akin to not creating a new namespace than a copy-on-write namespace.
While some other namespaces are copy-on-write, for example UTS namespaces, they do not present the same problem as mount namespaces. Although UTS namespaces are copy-on-write, it is trivial to create the conditions for a Void Process by setting the hostname of the machine to a constant. This removes any relation to the parent namespace and to the outside machine. Mount namespaces instead maintain a shared pointer with most filesystems, more akin to not creating a new namespace than a copy-on-write namespace.
Shared subtrees \citep{pai_shared_2005} were introduced to provide a consistent view of the unified hierarchy between namespaces. Consider the example in Figure \ref{fig:shared-subtrees}. \texttt{unshare(1)} creates a non-shared tree, which presents the behaviour shown. Although \texttt{/mnt/cdrom} from the parent namespace has been bind mounted in the new namespace, the content of \texttt{/mnt/cdrom} is not the same. This is because the filesystem newly mounted on \texttt{/mnt/cdrom} is unavailable in the separate mount namespace. To combat this, shared subtrees were introduced. That is, as long as \texttt{/mnt/cdrom} resides on a shared subtree, the newly mounted filesystem will be available to a bind of \texttt{/mnt/cdrom} in another namespace. \texttt{systemd} made the choice to mount \texttt{/} as a shared subtree \citep{free_software_foundation_mount_namespaces7_2021}:
@ -439,7 +430,7 @@ This means that when creating a new namespace, mounts and unmounts are propagate
\subsubsection{Lazy unmounting}
Mount namespaces present further interesting behaviour when unmounting the initial root filesystem. Although this may initially seem isolated to void processes, it is also a problem in a container type system. Consider again the container created in Figure \ref{fig:shared-subtrees} - the existing root must be unmounted after pivoting, to avoid keeping the container fully connected to the outside root.
Mount namespaces present further interesting behaviour when unmounting the initial root filesystem. Although this may initially seem isolated to Void Processes, it is also a problem in a container type system. Consider again the container created in Figure \ref{fig:shared-subtrees} - the existing root must be unmounted after pivoting, to avoid keeping the container fully connected to the outside root.
Referring again to network namespaces, sockets continue to exist in their initial namespace, allowing for regular file-descriptor passing semantics \citep{biederman_re_2007}. Extending upon this socket behaviour is Wireguard, which creates adapters that may be freely moved between namespaces while continuing to connect externally from their initial parent \citep[§7.3]{donenfeld_wireguard_2017}.
@ -448,13 +439,13 @@ Something which behaves differently is the memory mapping of a currently running
\lstset{caption={Behaviour when attempting to unmount / after an unshare.}}
\begin{lstlisting}[float,label={lst:unshare-umount}]
int main() {
if (unshare(CLONE_NEWNS))
perror("unshare");
if (mount("none", "/", NULL,
MS_REC|MS_PRIVATE, NULL))
perror("mount");
if (umount("/"))
perror("umount");
if (unshare(CLONE_NEWNS))
perror("unshare");
if (mount("none", "/", NULL,
MS_REC|MS_PRIVATE, NULL))
perror("mount");
if (umount("/"))
perror("umount");
}
--
umount: Device or resource busy
@ -508,54 +499,82 @@ When setting up a container environment, one calls \texttt{pivot\_root(2)} to re
If, instead, one wishes to continue running the existing binary, this is possible with lazy unmounting. However, the kernel only exposes a recursive lazy unmount. With shared subtrees, this results in destroying the parent tree. While this is avoidable by removing the shared propagation from the subtree before unmounting, the choice to have \texttt{MNT\_DETACH} aggressively cross shared subtrees can be highly confusing, and perhaps undesired behaviour in a world with shared subtrees by default.
Mount namespaces were the first [Table \ref{tab:namespaces}] namespaces introduced to Linux, in kernel version 2.5.2 \citep{torvalds_linux_2002}. In contrast to network namespaces, the API is particularly unfriendly to creating a Void process. The creation of mount namespaces is copy-on-write, and many filesystems are mounted shared. This means that they propagate changes back through namespace boundaries. As the mount namespace does not allow for creating an entirely empty root, extra care must be taken in separating processes. The method taken in this system is mounting a new \texttt{tmpfs} file system in a new namespace, which doesn't propagate to the parent, and using the \texttt{pivot\_root(8)} command to make this the new root. By pivoting to the \texttt{tmpfs}, the old root exists as the only reference in the otherwise empty \texttt{tmpfs}. Finally, after ensuring the old root is set to \texttt{MNT\_PRIVATE} to avoid propagation (more details in §\ref{sec:shared-subtrees}), the old root can be lazily detached. This allows the binary from the parent namespace, the shim in this case, to continue running correctly. Any new processes only have access to the materials in the empty \texttt{tmpfs}. This new \texttt{tmpfs} never appears in the parent namespace, separating the void process effectively from the parent namespace.
Mount namespaces were the first [Table \ref{tab:namespaces}] namespaces introduced to Linux, in kernel version 2.5.2 \citep{torvalds_linux_2002}. In contrast to network namespaces, the API is particularly unfriendly to creating a Void Process. The creation of mount namespaces is copy-on-write, and many filesystems are mounted shared. This means that they propagate changes back through namespace boundaries. As the mount namespace does not allow for creating an entirely empty root, extra care must be taken in separating processes. The method taken in this system is mounting a new \texttt{tmpfs} file system in a new namespace, which doesn't propagate to the parent, and using the \texttt{pivot\_root(8)} command to make this the new root. By pivoting to the \texttt{tmpfs}, the old root exists as the only reference in the otherwise empty \texttt{tmpfs}. Finally, after ensuring the old root is set to \texttt{MNT\_PRIVATE} to avoid propagation (more details in §\ref{sec:shared-subtrees}), the old root can be lazily detached. This allows the binary from the parent namespace, the shim in this case, to continue running correctly. Any new processes only have access to the materials in the empty \texttt{tmpfs}. This new \texttt{tmpfs} never appears in the parent namespace, separating the Void Process effectively from the parent namespace.
\subsection{user namespaces}
\label{sec:voiding-user}
User namespaces provide isolation of security between processes. They isolate uids, gids, the root directory, keys and capabilities. This provides massive utility for rootless containers [CN], and also this shim. Rather than the shim being a \texttt{setuid} or \texttt{CAP\_SYS\_ADMIN} binary, it can instead operate with ambient authority. This vastly simplifies the logic for opening file descriptors to pass the child processes, as the shim itself is already operating with correctly limited authority.
Similarly to many other namespaces, user namespaces suffer from needing to limit their isolation. For a user namespace to be useful, some relation needs to exist between processes in the user namespace and objects outside. That is, if a process in a user namespace shares a filesystem with a process in the parent namespace, there should be a way to share credentials. To achieve this with user namespaces a mapping between users in the namespace and users outside exists. The most common use-case is to map root in the user namespace to the creating user outside, meaning that a process with full privileges in the namespace will be constrained to the creating user's ambient authority.
To create an effective void process content must be written to the files \texttt{/proc/[pid]/uid\_map} and \texttt{/proc/[pid]/gid\_map}. In the case of the shim uid 0 and gid 0 are mapped to the creating user. This is done first such that the remaining stages in creating a void process can have root capabilities within the user namespace - this is not possible prior to writing to these files. Otherwise, \texttt{CLONE\_NEWUSER} combines effectively with other namespace flags, ensuring that the user namespace is created first. This enables the other namespaces to be created without additional permissions.
To create an effective Void Process content must be written to the files \texttt{/proc/[pid]/uid\_map} and \texttt{/proc/[pid]/gid\_map}. In the case of the shim uid 0 and gid 0 are mapped to the creating user. This is done first such that the remaining stages in creating a Void Process can have root capabilities within the user namespace - this is not possible prior to writing to these files. Otherwise, \texttt{CLONE\_NEWUSER} combines effectively with other namespace flags, ensuring that the user namespace is created first. This enables the other namespaces to be created without additional permissions.
\subsection{cgroup namespaces}
\label{sec:voiding-cgroup}
cgroup namespaces provide limited isolation of the cgroup hierarchy between processes. Rather than showing the full cgroups hierarchy, they instead show only the part of the hierarchy that the process was in on creation of the new cgroup namespace. Correctly creating a Void Process is hence as follows:
\begin{enumerate}
\item Create an empty cgroup leaf.
\item Move the new process to that leaf.
\item Unshare the cgroup namespace.
\end{enumerate}
This process excludes the cgroup namespace from the initial \texttt{clone(3)} call, as the cloned process must be moved before creating the new namespace. By following this sequence of calls, the process in the void can only see the leaf which contains itself and nothing else, limiting access to the host system. This is the approach taken in this piece of work. This presents the one point where running the shim with ambient authority rather than high capabilities is potentially limiting. In order to move the process into a leaf the shim must have sufficient authority to modify the cgroup hierarchy. On systemd these processes will be launched underneath a user slice and will have sufficient permissions, but this may vary between systems. This leaves cgroups the most weakly implemented namespace at the moment.
Although good isolation of the host system from the Void Process is provided, the Void Process is in no way hidden from the host. There exists only one cgroups v2 hierarchy on a system (cgroups v1 are ignored for clarity), where resources are delegated through each. This means that all processes contained within the hierarchy must appear in the primary hierarchy, such that the distribution of the single set of system resources can be centrally controlled. This behaviour is similar to the aforementioned pid namespaces, where each process has a distinct PID in each of its parents, but does show up in each. Hiding from the host has little value as a root user there can inspect each namespace manually.
An alternative implementation that would make implementing with the cgroups namespace easier would be one that condenses all of the processes in the sea groups name space into one parent process in the parent main space. This would have the effect of hiding underlying processes from the parent name space, while still allowing control over the sea groups tree as a whole. It would further provide better isolation of the child, as a newly spawned cgroups space would show an empty route that only contains the child process. This would also allow more effective interaction with user namespaces, as the child namespace would only have control over itself, allowing for full control without risking the rest of the tree. This is opposed to the current limited view of the cgroups tree, which appears to have limited usefulness.
\section{Filling the Void}
\label{sec:filling}
Once a set of namespaces to contain the void process have been created the goal is to reinsert enough to run the application, and nothing more. To allow for running applications as void processes with minimal kernel changes, this is done using a mixture of file-descriptor capabilities and adding elements to the namespaces. Capabilities allow for a clean experience where suitable, while adding elements to namespaces creates a more Linux-like experience for the application.
Once a set of namespaces to contain the Void Process have been created the goal is to reinsert enough to run the application, and nothing more. To allow for running applications as Void Processes with minimal kernel changes, this is done using a mixture of file-descriptor capabilities and adding elements to the namespaces. Capabilities allow for a clean experience where suitable, while adding elements to namespaces creates a more Linux-like experience for the application.
\subsection{Files and directories} There are two options to provide access to files and directories in the void. Firstly, for a single file, an already open file descriptor can be offered. Consider the TLS broker of a TLS server with a persistent certificate and keyfile. Only these files are required to correctly run the application - no view of a filesystem is necessary. Providing an already opened file descriptor gives the process a capability to those files while requiring no concept of a filesystem, allowing that to remain a complete void. This is possible because of the semantics of file descriptor passing across namespaces - the file descriptor remains a capability, regardless of moving into a namespace without access to the file in question.
\subsection{Files and directories}
\label{sec:filling-mount}
Alternatively, files and directories can be mounted in the void process's namespace. This supports three things which the capabilities do not: directories, dynamic linking, and applications which have not been adapted to use file descriptors. Firstly, the existing \texttt{openat(2)} calls are not suitable by default to treat directory file descriptors as capabilities, as they allow the search path to be absolute. This means that a process with a directory file descriptor in another namespace can access any files in that namespace [RN] by supplying an absolute path. Secondly, dynamic linking is best served by binding files, as these read only copies and the trusted binaries ensure that only the required libraries can be linked against. Finally, support for individual required files can be added by using file descriptors, but many applications will not trivially support it. Binding files allows for a form of backwards compatibility.
There are two options to provide access to files and directories in the void. Firstly, for a single file, an already open file descriptor can be offered. Consider the TLS broker of a TLS server with a persistent certificate and keyfile. Only these files are required to correctly run the application - no view of a filesystem is necessary. Providing an already opened file descriptor gives the process a capability to those files while requiring no concept of a filesystem, allowing that to remain a complete void. This is possible because of the semantics of file descriptor passing across namespaces - the file descriptor remains a capability, regardless of moving into a namespace without access to the file in question.
Alternatively, files and directories can be mounted in the Void Process's namespace. This supports three things which the capabilities do not: directories, dynamic linking, and applications which have not been adapted to use file descriptors. Firstly, the existing \texttt{openat(2)} calls are not suitable by default to treat directory file descriptors as capabilities, as they allow the search path to be absolute. This means that a process with a directory file descriptor in another namespace can access any files in that namespace [RN] by supplying an absolute path. Secondly, dynamic linking is best served by binding files, as these read only copies and the trusted binaries ensure that only the required libraries can be linked against. Finally, support for individual required files can be added by using file descriptors, but many applications will not trivially support it. Binding files allows for a form of backwards compatibility.
\subsection{Networking}
\label{sec:filling-net}
Reintroducing networking to a void process follows a similar capability-based paradigm to reintroducing files. Rather than providing the full Linux networking view to a void process, it is instead handed a file descriptor that already has the requisite networking permissions. A capability for an inbound networking socket can be requested statically in the application's specification, which fits well with the earlier specified threat model. This socket remains open and allows the application to continuously accept requests, generating the appropriate socket for each request within the application itself, which can be dealt with through the mechanisms provided - specifically file descriptor based sockets.
Reintroducing networking to a Void Process follows a similar capability-based paradigm to reintroducing files. Rather than providing the full Linux networking view to a Void Process, it is instead handed a file descriptor that already has the requisite networking permissions. A capability for an inbound networking socket can be requested statically in the application's specification, which fits well with the earlier specified threat model. This socket remains open and allows the application to continuously accept requests, generating the appropriate socket for each request within the application itself, which can be dealt with through the mechanisms provided - specifically file descriptor based sockets.
Outbound networking is more difficult to re-add to a void process than inbound networking. The approach that containerisation solutions such as Docker take is using NAT with bridged adapters by default [RN]. That is, the container is provided an internal IP address that allows access to all networks via the host. Virtual machine solutions take a similar approach, creating bridged Ethernet adapters on the outside network or on a private NAT by default. Each of these approaches give the container/machine the appearance of unbounded outbound access, relying on firewalls to limit this afterwards. This does not fit well with the ethos of creating a void process - minimum privilege by default. An ideal solution would provide precise network access to the void, rather than adding all access and restricting it in post. This is achieved with inbound sockets by providing the precise and already connected socket to an otherwise empty network namespace, which does not support creating inbound sockets of its own.
Outbound networking is more difficult to re-add to a Void Process than inbound networking. The approach that containerisation solutions such as Docker take is using NAT with bridged adapters by default [RN]. That is, the container is provided an internal IP address that allows access to all networks via the host. Virtual machine solutions take a similar approach, creating bridged Ethernet adapters on the outside network or on a private NAT by default. Each of these approaches give the container/machine the appearance of unbounded outbound access, relying on firewalls to limit this afterwards. This does not fit well with the ethos of creating a Void Process - minimum privilege by default. An ideal solution would provide precise network access to the void, rather than adding all access and restricting it in post. This is achieved with inbound sockets by providing the precise and already connected socket to an otherwise empty network namespace, which does not support creating inbound sockets of its own.
Consideration is given to providing outbound access in the same way as inbound - with statically created and passed sockets. For example, a socket to a database could be specified in the specification, or even one per worker process. The downside of this approach is that the socket lifecycle is still handled by the kernel. While this would work well with UDP sockets, TCP sockets can fail because the remote was closed or a break in the path caused a timeout to be hit.
Given that statically giving sockets is infeasible and adding a firewall does not fit well with creating a void, I sought an alternative API. \texttt{pledge(2)} is a system call from OpenBSD which restricts future system calls to an approved set \citep{the_openbsd_foundation_pledge2_2022}. This seems like a good fit, though operating outside of the operating system makes the implementation very different. Acceptable sockets can be specified in the application specification, then an interaction socket provided to request various pre-approved sockets from the shim layer. This allows limited access to the host network, approved or denied at request time instead of by a firewall. That is, access to a precisely configured socket can be injected to the void, with a capability to request such sockets and a capability given for the socket.
\subsection{User capabilities}
\label{sec:filling-user}
\todo{Write section on filling user namespaces.}
\subsection{Remaining namespaces}
\subsubsection{uts namespaces}
\label{sec:filling-uts}
\todo{Write about filling uts namespaces.}
\subsubsection{ipc namespaces}
\label{sec:filling-ipc}
\todo{Write about (lack of) filling ipc namespaces.}
\subsubsection{pid namespaces}
\label{sec:filling-pid}
\todo{Write about there being no need to fill pid namespaces.}
\subsubsection{cgroup namespaces}
\label{sec:filling-cgroup}
\todo{Write about how cgroup namespaces are filled by default, as it is a subtree.}
@ -569,9 +588,9 @@ Given that statically giving sockets is infeasible and adding a firewall does no
\label{fig:self-compartmentalisation-interactions}
\end{figure}
An example of running a void process application is given in Figure \ref{fig:self-compartmentalisation-interactions}. What was originally a monolithic application becomes a set of applications that communicate with a new shim. The shim does not replace the kernel, and instead supplements it with new higher-level abilities. Each entrypoint receives input from the shim, and can return data to the shim where appropriate. Most of this data is in the form of file descriptors, which are treated as capabilities in this system.
An example of running a Void Process application is given in Figure \ref{fig:self-compartmentalisation-interactions}. What was originally a monolithic application becomes a set of applications that communicate with a new shim. The shim does not replace the kernel, and instead supplements it with new higher-level abilities. Each entrypoint receives input from the shim, and can return data to the shim where appropriate. Most of this data is in the form of file descriptors, which are treated as capabilities in this system.
A void process application stores the requirements for running it as static data in the ELF of the binary. When launched, \texttt{binfmt\_misc} is used to launch the application with the multi-entrypoint shim. The shim decodes this data and sets up processes and inter-process communication (IPC) accordingly.
A Void Process application stores the requirements for running it as static data in the ELF of the binary. When launched, \texttt{binfmt\_misc} is used to launch the application with the multi-entrypoint shim. The shim decodes this data and sets up processes and inter-process communication (IPC) accordingly.
\section{Building Applications}
@ -636,7 +655,7 @@ Capsicum \citep{watson_capsicum_2010} extends UNIX file descriptors in FreeBSD t
\subsection{Dynamic Linking}
Dynamic linking works correctly under the shim, however, it currently requires a high level of manual input. Given that the threat model in Section \ref{section:threat-model} specifies trusted binaries, it is feasible to add a pre-spawning phase which appends read-only libraries to the specification for each spawned process automatically before creating appropriate voids. This would allow anything which can link correctly on the host system to link correctly in void processes.
Dynamic linking works correctly under the shim, however, it currently requires a high level of manual input. Given that the threat model in Section \ref{section:threat-model} specifies trusted binaries, it is feasible to add a pre-spawning phase which appends read-only libraries to the specification for each spawned process automatically before creating appropriate voids. This would allow anything which can link correctly on the host system to link correctly in Void Processes.
\section{Conclusion}