fs: Add a missing permission check to do_umount

Accessing do_remount_sb should require global CAP_SYS_ADMIN, but
only one of the two call sites was appropriately protected.

Fixes CVE-2014-7975.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index ef42d9bee212..7f67b463a5b4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1356,6 +1356,8 @@ static int do_umount(struct mount *mnt, int flags)
                  * Special case for "unmounting" root ...
                  * we just try to remount it readonly.
                  */
+                if (!capable(CAP_SYS_ADMIN))
+                        return -EPERM;
                 down_write(&sb->s_umount);
                 if (!(sb->s_flags & MS_RDONLY))
                         retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);