mnt: Only change user settable mount flags in remount

commit a6138db815df5ee542d848318e5dae681590fccd upstream.

Kenton Varda <kenton@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@vger.kernel.org
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/fs/namespace.c b/fs/namespace.c
index 84447dbcb650eecba1821d9ef58d1f638f743c9d..34fa7a52f37397cf14a4bc6edb5e9b88b0eca208 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1847,7 +1847,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
                err = do_remount_sb(sb, flags, data, 0);
        if (!err) {
                br_write_lock(&vfsmount_lock);
-               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;
                br_write_unlock(&vfsmount_lock);
        }

diff --git a/include/linux/mount.h b/include/linux/mount.h
index 38cd98f112a0e4ccb62e32d94a2d1065ce76b2dd..8707c9e9dbb9faff21c2df530ff018829ba0226f 100644 (file)
--- 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   0x4000