Prevent overlayfs from being mounted on top of idmapped mounts until we
have ported it to handle this case and added proper testing for it.

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <redacted>
---
/* v2 */
patch introduced

/* v3 */
- Amir Goldstein [off-list ref]:
  - Move check for idmapped lower layers into ovl_mount_dir_noesc().
- David Howells [off-list ref]:
  - Adapt check after removing mnt_idmapped() helper.

/* v4 */
unchanged

/* v5 */
base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
---
 fs/overlayfs/super.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 24404a167367..c64a6644203e 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c

@@ -856,6 +856,10 @@ static int ovl_mount_dir_noesc(const char *name, struct path *path)
 		pr_err("filesystem on '%s' not supported\n", name);
 		goto out_put;
 	}
+	if (mnt_user_ns(path->mnt) != &init_user_ns) {
+		pr_err("idmapped layers are currently not supported\n");
+		goto out_put;
+	}
 	if (!d_is_dir(path->dentry)) {
 		pr_err("'%s' not a directory\n", name);
 		goto out_put;

-- 
2.30.0