[PATCH 3.16 160/305] mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED mounts

From: Ben Hutchings
Date: Sun Feb 03 2019 - 09:27:59 EST

3.16.63-rc1 review patch. If anyone has any objections, please let me know.


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.

# Hide a path to all users using a tmpfs
root@castiana:~# mount -t tmpfs tmpfs /sys/devices/

# 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.

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: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
fs/namespace.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1574,8 +1574,14 @@ struct mount *copy_tree(struct mount *mn
struct mount *t = NULL;
if (!(flag & CL_COPY_UNBINDABLE) &&
- 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)) {