util-linux: fix unshare -r on linux >=3.19 (close #6130)

pkgs/os-specific/linux/util-linux/default.nix

@@ -8,7 +8,9 @@ stdenv.mkDerivation rec {
     sha256 = "e0457f715b73f4a349e1acb08cb410bf0edc9a74a3f75c357070f31f70e33cd6";
   };
 
-  patches = [ ./rtcwake-search-PATH-for-shutdown.patch ];
+  patches = [ ./rtcwake-search-PATH-for-shutdown.patch
+              ./unshare-fix-map-root-user.patch
+            ];
 
   #FIXME: make it also work on non-nixos?
   postPatch = ''

pkgs/os-specific/linux/util-linux/unshare-fix-map-root-user.patch

@@ -0,0 +1,66 @@
+In rare cases droping groups with setgroups(0, NULL) is an operation
+that can grant a user additional privileges.  User namespaces were
+allwoing that operation to unprivileged users and that had to be
+fixed.
+
+Update unshare --map-root-user to disable the setgroups operation
+before setting the gid_map.
+
+This is needed as after the security fix gid_map is restricted to
+privileged users unless setgroups has been disabled.
+
+Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
+---
+ include/pathnames.h |  1 +
+ sys-utils/unshare.c | 19 +++++++++++++++++++
+ 2 files changed, 20 insertions(+)
+
+diff --git a/include/pathnames.h b/include/pathnames.h
+index 1cc4e15e6e4f..1c53e4554268 100644
+--- a/include/pathnames.h
++++ b/include/pathnames.h
+@@ -92,6 +92,7 @@
+ 
+ #define _PATH_PROC_UIDMAP	"/proc/self/uid_map"
+ #define _PATH_PROC_GIDMAP	"/proc/self/gid_map"
++#define _PATH_PROC_SETGROUPS	"/proc/self/setgroups"
+ 
+ #define _PATH_PROC_ATTR_CURRENT	"/proc/self/attr/current"
+ #define _PATH_PROC_ATTR_EXEC	"/proc/self/attr/exec"
+diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
+index 95e4afbd055e..d409a7c936b6 100644
+--- a/sys-utils/unshare.c
++++ b/sys-utils/unshare.c
+@@ -39,6 +39,24 @@
+ #include "pathnames.h"
+ #include "all-io.h"
+ 
++static void disable_setgroups(void)
++{
++	const char *file = _PATH_PROC_SETGROUPS;
++	const char *deny = "deny";
++	int fd;
++
++	fd = open(file, O_WRONLY);
++	if (fd < 0) {
++		if (errno == ENOENT)
++			return;
++		 err(EXIT_FAILURE, _("cannot open %s"), file);
++	}
++
++	if (write_all(fd, deny, strlen(deny)))
++		err(EXIT_FAILURE, _("write failed %s"), file);
++	close(fd);
++}
++
+ static void map_id(const char *file, uint32_t from, uint32_t to)
+ {
+ 	char *buf;
+@@ -178,6 +196,7 @@ int main(int argc, char *argv[])
+ 	}
+ 
+ 	if (maproot) {
++		disable_setgroups();
+ 		map_id(_PATH_PROC_UIDMAP, 0, real_euid);
+ 		map_id(_PATH_PROC_GIDMAP, 0, real_egid);
+ 	}