[PATCH review 03/18] umount: Disallow unprivileged mount force

Eric W. Biederman
2014-12-12T22:48:14Z

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.

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
---
fs/namespace.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index 3a1a87dc33df..43b16af8af30 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1544,6 +1544,9 @@ 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:
--
1.9.1