linux-kernel - [PATCH 3.16 160/305] mount: Don't allow copying MNT_UNBINDABLE|MNT

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Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Jonathan Calmels" <jcalmels@...dia.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 3.16 160/305] mount: Don't allow copying MNT_UNBINDABLE|MNT_LOCKED
 mounts

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

------------------

From: "Eric W. Biederman" <ebiederm@...ssion.com>

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@...tiana:~# mount -t tmpfs tmpfs /sys/devices/
  root@...tiana:~#

  # As an unprivileged user, unshare user namespace and mount namespace
  stgraber@...tiana:~$ unshare -U -m -r

  # Confirm the path is still not accessible
  root@...tiana:~# ls /sys/devices/

  # Make /sys recursively unbindable and private
  root@...tiana:~# mount --make-runbindable /sys
  root@...tiana:~# mount --make-private /sys

  # Recursively bind-mount the rest of /sys over to /mnnt
  root@...tiana:~# mount --rbind /sys/ /mnt

  # Access our hidden /sys/device as an unprivileged user
  root@...tiana:~# 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@...dia.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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) &&
 			    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)) {