[trusty 1/4] mnt: Only change user settable mount flags in remount

[Andy Whitcroft]

From: "Eric W. Biederman" <ebiederm at xmission.com>

Kenton Varda <kenton at sandstorm.io> discovered that by remounting a
read-only bind mount read-only in a user namespace the
MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user
to the remount a read-only mount read-write.

Correct this by replacing the mask of mount flags to preserve
with a mask of mount flags that may be changed, and preserve
all others.   This ensures that any future bugs with this mask and
remount will fail in an easy to detect way where new mount flags
simply won't change.

Cc: stable at vger.kernel.org
Acked-by: Serge E. Hallyn <serge.hallyn at ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>

(cherry picked from commit a6138db815df5ee542d848318e5dae681590fccd)
CVE-2014-5206
BugLink: http://bugs.launchpad.net/bugs/1356318
Signed-off-by: Andy Whitcroft <apw at canonical.com>
---
 fs/namespace.c        | 2 +-
 include/linux/mount.h | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index b6f4942..c795e83 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1957,7 +1957,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 		err = do_remount_sb(sb, flags, data, 0);
 	if (!err) {
 		lock_mount_hash();
-		mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
+		mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
 		mnt->mnt.mnt_flags = mnt_flags;
 		touch_mnt_namespace(mnt->mnt_ns);
 		unlock_mount_hash();
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 31617fc..5ea2a80 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -42,7 +42,9 @@ struct mnt_namespace;
  * flag, consider how it interacts with shared mounts.
  */
 #define MNT_SHARED_MASK	(MNT_UNBINDABLE)
-#define MNT_PROPAGATION_MASK	(MNT_SHARED | MNT_UNBINDABLE)
+#define MNT_USER_SETTABLE_MASK  (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
+				 | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
+				 | MNT_READONLY)
 
 #define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
 			    MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED)
-- 
1.9.1