This is a note to let you know that I've just added the patch titled mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED mounts to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: mount-don-t-allow-copying-mnt_unbindable-mnt_locked-mounts.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From df7342b240185d58d3d9665c0bbf0a0f5570ec29 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Date: Thu, 25 Oct 2018 09:04:18 -0500 Subject: mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED mounts From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> commit df7342b240185d58d3d9665c0bbf0a0f5570ec29 upstream. Jonathan Calmels from NVIDIA reported that he's able to bypass the mount visibility security check in place in the Linux kernel by using a combination of the unbindable property along with the private mount propagation option to allow a unprivileged user to see a path which was purposefully hidden by the root user. Reproducer: # Hide a path to all users using a tmpfs root@castiana:~# mount -t tmpfs tmpfs /sys/devices/ root@castiana:~# # As an unprivileged user, unshare user namespace and mount namespace stgraber@castiana:~$ unshare -U -m -r # Confirm the path is still not accessible root@castiana:~# ls /sys/devices/ # Make /sys recursively unbindable and private root@castiana:~# mount --make-runbindable /sys root@castiana:~# mount --make-private /sys # Recursively bind-mount the rest of /sys over to /mnnt root@castiana:~# mount --rbind /sys/ /mnt # Access our hidden /sys/device as an unprivileged user root@castiana:~# ls /mnt/devices/ breakpoint cpu cstate_core cstate_pkg i915 intel_pt isa kprobe LNXSYSTM:00 msr pci0000:00 platform pnp0 power software system tracepoint uncore_arb uncore_cbox_0 uncore_cbox_1 uprobe virtual Solve this by teaching copy_tree to fail if a mount turns out to be both unbindable and locked. Cc: stable@xxxxxxxxxxxxxxx Fixes: 5ff9d8a65ce8 ("vfs: Lock in place mounts from more privileged users") Reported-by: Jonathan Calmels <jcalmels@xxxxxxxxxx> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/namespace.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1734,8 +1734,14 @@ struct mount *copy_tree(struct mount *mn for (s = r; s; s = next_mnt(s, r)) { if (!(flag & CL_COPY_UNBINDABLE) && IS_MNT_UNBINDABLE(s)) { - s = skip_mnt_tree(s); - continue; + if (s->mnt.mnt_flags & MNT_LOCKED) { + /* Both unbindable and locked. */ + q = ERR_PTR(-EPERM); + goto out; + } else { + s = skip_mnt_tree(s); + continue; + } } if (!(flag & CL_COPY_MNT_NS_FILE) && is_mnt_ns_file(s->mnt.mnt_root)) { Patches currently in stable-queue which might be from ebiederm@xxxxxxxxxxxx are queue-4.19/mount-retest-mnt_locked-in-do_umount.patch queue-4.19/mount-don-t-allow-copying-mnt_unbindable-mnt_locked-mounts.patch queue-4.19/mount-prevent-mnt_detach-from-disconnecting-locked-mounts.patch queue-4.19/mnt-fix-__detach_mounts-infinite-loop.patch