umount: Disallow unprivileged mount force

commit b2f5d4dc38e034eecb7987e513255265ff9aa1cf upstream.

Forced unmount affects not just the mount namespace but the underlying
superblock as well.  Restrict forced unmount to the global root user
for now.  Otherwise it becomes possible a user in a less privileged
mount namespace to force the shutdown of a superblock of a filesystem
in a more privileged mount namespace, allowing a DOS attack on root.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index 6f270b459320f01dc1f95e62f4954a633c7641f4..bbde147196556fbf3d04fc6c8b37de43a3b1430d 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1546,6 +1546,9 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
                goto dput_and_out;
        if (mnt->mnt.mnt_flags & MNT_LOCKED)
                goto dput_and_out;
+       retval = -EPERM;
+       if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
+               goto dput_and_out;
 
        retval = do_umount(mnt, flags);
 dput_and_out: