From: Nicolas Schier <nicolas@fjasle.eu>
Subject: ovl: permit overlayfs mounts in user namespaces (taints kernel)
Date: Mon, 19 Nov 2018 20:36:14 +0100

Permit overlayfs mounts within user namespaces to allow utilisation of e.g.
unprivileged LXC overlay snapshots.

Except by the Ubuntu community [1], overlayfs mounts in user namespaces are
expected to be a security risk [2] and thus are not enabled on upstream
Linux kernels.  For the non-Ubuntu users that have to stick to unprivileged
overlay-based LXCs, this meant to patch and compile the kernel manually.
Instead, adding the kernel tainting 'permit_mounts_in_userns' module
parameter allows a kind of a user-friendly way to enable the feature.

Testable with:

    sudo modprobe overlay permit_mounts_in_userns=1
    sudo sysctl -w kernel.unprivileged_userns_clone=1
    mkdir -p lower upper work mnt
    unshare --map-root-user --mount \
        mount -t overlay none mnt \
              -o lowerdir=lower,upperdir=upper,workdir=work

[1]: Ubuntu allows unprivileged mounting of overlay filesystem
https://lists.ubuntu.com/archives/kernel-team/2014-February/038091.html

[2]: User namespaces + overlayfs = root privileges
https://lwn.net/Articles/671641/

Signed-off-by: Nicolas Schier <nicolas@fjasle.eu>

Index: linux/fs/overlayfs/super.c
===================================================================
--- linux.orig/fs/overlayfs/super.c
+++ linux/fs/overlayfs/super.c
@@ -56,6 +56,11 @@ module_param_named(xino_auto, ovl_xino_a
 MODULE_PARM_DESC(ovl_xino_auto_def,
 		 "Auto enable xino feature");
 
+static bool ovl_permit_mounts_in_userns;
+module_param_named_unsafe(permit_mounts_in_userns, ovl_permit_mounts_in_userns,
+			  bool, 0444);
+MODULE_PARM_DESC(permit_mounts_in_userns, "Permit mounts in user namespaces");
+
 static void ovl_entry_stack_free(struct ovl_entry *oe)
 {
 	unsigned int i;
@@ -1715,6 +1720,11 @@ static int __init ovl_init(void)
 	if (ovl_inode_cachep == NULL)
 		return -ENOMEM;
 
+	if (unlikely(ovl_permit_mounts_in_userns)) {
+		pr_warn("overlayfs: Allowing overlay mounts in user namespaces bears security risks\n");
+		ovl_fs_type.fs_flags |= FS_USERNS_MOUNT;
+	}
+
 	err = register_filesystem(&ovl_fs_type);
 	if (err)
 		kmem_cache_destroy(ovl_inode_cachep);