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 739a00760dc138c9988e68d8d8a07323f20aeb9a..2e0e452bc0b4d31991f8b6e56b71f3ea17d8c9ab 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1365,6 +1365,9 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
                goto dput_and_out;
        if (!check_mnt(mnt))
                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: