vfs: Add a mount flag to lock read only bind mounts

When a read-only bind mount is copied from mount namespace in a higher
privileged user namespace to a mount namespace in a lesser privileged
user namespace, it should not be possible to remove the the read-only
restriction.

Add a MNT_LOCK_READONLY mount flag to indicate that a mount must
remain read-only.

CC: stable@vger.kernel.org
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/mount.h b/include/linux/mount.h
index d7029f4a191a..73005f9957ea 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -47,6 +47,8 @@ struct mnt_namespace;
 
 #define MNT_INTERNAL    0x4000
 
+#define MNT_LOCK_READONLY       0x400000
+
 struct vfsmount {
         struct dentry *mnt_root;        /* root of the mounted tree */
         struct super_block *mnt_sb;     /* pointer to superblock */