vfs: fix infinite loop caused by clone_mnt race

If clone_mnt() happens while mnt_make_readonly() is running, the
cloned mount might have MNT_WRITE_HOLD flag set, which results in
mnt_want_write() spinning forever on this mount.

Needs CAP_SYS_ADMIN to trigger deliberately and unlikely to happen
accidentally.  But if it does happen it can hang the machine.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -595,7 +595,7 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 				goto out_free;
 		}
 
-		mnt->mnt_flags = old->mnt_flags;
+		mnt->mnt_flags = old->mnt_flags & ~MNT_WRITE_HOLD;
 		atomic_inc(&sb->s_active);
 		mnt->mnt_sb = sb;
 		mnt->mnt_root = dget(root);