• Eric W. Biederman's avatar
    mount: Prevent MNT_DETACH from disconnecting locked mounts · 03004f41
    Eric W. Biederman authored
    commit 9c8e0a1b683525464a2abe9fb4b54404a50ed2b4 upstream.
    
    Timothy Baldwin <timbaldwin@fastmail.co.uk> wrote:
    > As per mount_namespaces(7) unprivileged users should not be able to look under mount points:
    >
    >   Mounts that come as a single unit from more privileged mount are locked
    >   together and may not be separated in a less privileged mount namespace.
    >
    > However they can:
    >
    > 1. Create a mount namespace.
    > 2. In the mount namespace open a file descriptor to the parent of a mount point.
    > 3. Destroy the mount namespace.
    > 4. Use the file descriptor to look under the mount point.
    >
    > I have reproduced this with Linux 4.16.18 and Linux 4.18-rc8.
    >
    > The setup:
    >
    > $ sudo sysctl kernel.unprivileged_userns_clone=1
    > kernel.unprivileged_userns_clone = 1
    > $ mkdir -p A/B/Secret
    > $ sudo mount -t tmpfs hide A/B
    >
    >
    > "Secret" is indeed hidden as expected:
    >
    > $ ls -lR A
    > A:
    > total 0
    > drwxrwxrwt 2 root root 40 Feb 12 21:08 B
    >
    > A/B:
    > total 0
    >
    >
    > The attack revealing "Secret":
    >
    > $ unshare -Umr sh -c "exec unshare -m ls -lR /proc/self/fd/4/ 4<A"
    > /proc/self/fd/4/:
    > total 0
    > drwxr-xr-x 3 root root 60 Feb 12 21:08 B
    >
    > /proc/self/fd/4/B:
    > total 0
    > drwxr-xr-x 2 root root 40 Feb 12 21:08 Secret
    >
    > /proc/self/fd/4/B/Secret:
    > total 0
    
    I tracked this down to put_mnt_ns running passing UMOUNT_SYNC and
    disconnecting all of the mounts in a mount namespace.  Fix this by
    factoring drop_mounts out of drop_collected_mounts and passing
    0 instead of UMOUNT_SYNC.
    
    There are two possible behavior differences that result from this.
    - No longer setting UMOUNT_SYNC will no longer set MNT_SYNC_UMOUNT on
      the vfsmounts being unmounted.  This effects the lazy rcu walk by
      kicking the walk out of rcu mode and forcing it to be a non-lazy
      walk.
    - No longer disconnecting locked mounts will keep some mounts around
      longer as they stay because the are locked to other mounts.
    
    There are only two users of drop_collected mounts: audit_tree.c and
    put_mnt_ns.
    
    In audit_tree.c the mounts are private and there are no rcu lazy walks
    only calls to iterate_mounts. So the changes should have no effect
    except for a small timing effect as the connected mounts are disconnected.
    
    In put_mnt_ns there may be references from process outside the mount
    namespace to the mounts.  So the mounts remaining connected will
    be the bug fix that is needed.  That rcu walks are allowed to continue
    appears not to be a problem especially as the rcu walk change was about
    an implementation detail not about semantics.
    
    Cc: stable@vger.kernel.org
    Fixes: 5ff9d8a6 ("vfs: Lock in place mounts from more privileged users")
    Reported-by: 's avatarTimothy Baldwin <timbaldwin@fastmail.co.uk>
    Tested-by: 's avatarTimothy Baldwin <timbaldwin@fastmail.co.uk>
    Signed-off-by: 's avatar"Eric W. Biederman" <ebiederm@xmission.com>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    03004f41