FAILED: patch "[PATCH] namespace: don't copy an unmounted mount tree" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 8bacbf6be959973b28b691713a872c12eae69eec
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026101251-collage-refried-0dba@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8bacbf6be959973b28b691713a872c12eae69eec Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Sat, 26 Sep 2026 15:44:27 +0200
Subject: [PATCH] namespace: don't copy an unmounted mount tree

may_copy_tree() accepts a mount whose root is an nsfs or pidfs file before it checks whether that mount is mounted. This exception exists for nsfs and pidfs mounts. They are part of no mount namespace so neither check_mnt() nor is_mounted() would let them through and bind-mounting a nsfsfd or a pidfd wouldn't work.

Problem is that exception also admits a bind mount of such a file when it was lazily unmounted and is only kept alive by a file descriptor:

  mount --bind /proc/self/ns/net /tmp/x
  exec 3< /tmp/x
  umount -l /tmp/x
  mount --rbind /proc/self/fd/3 /tmp/y

With MS_REC or AT_RECURSIVE copy_tree() doesn't stop at the unmounted mount. It copies every mount that stayed attached to it as well. So if you have a bunch of overmounts over the nsfsfd or the pidfd the whole unmounted stack is copied. That's a bug.

So refuse to walk an unmounted tree. Copying its root alone stays possible. So a bind mount of a namespace that is gone can still be recreated from a file descriptor on the old one. The file descriptor pins that mount and its root and __has_locked_children() only reads the direct children which don't leave the parent's list without namespace_sem. The internal mounts carry MNT_NS_INTERNAL instead of NULL and are still walked. Copying an unmounted tree recursively fails with EINVAL.

index 6d4ecde682df..c7e252eb249e 100644
@@ -3007,6 +3007,9 @@ static struct mount *__do_loopback(const struct path *old_path,
 	if (!may_copy_tree(old_path))
 		return ERR_PTR(-EINVAL);
 
+	if (recurse && !old->mnt_ns)
+		return ERR_PTR(-EINVAL);
+
 	if (!recurse && __has_locked_children(old, old_path->dentry))
 		return ERR_PTR(-EINVAL);
 

From: Christian Brauner <[email protected]>

[ Upstream commit 8bacbf6be959973b28b691713a872c12eae69eec ]

may_copy_tree() accepts a mount whose root is an nsfs or pidfs file before it checks whether that mount is mounted. This exception exists for nsfs and pidfs mounts. They are part of no mount namespace so neither check_mnt() nor is_mounted() would let them through and bind-mounting a nsfsfd or a pidfd wouldn't work.

Problem is that exception also admits a bind mount of such a file when it was lazily unmounted and is only kept alive by a file descriptor:

  mount --bind /proc/self/ns/net /tmp/x
  exec 3< /tmp/x
  umount -l /tmp/x
  mount --rbind /proc/self/fd/3 /tmp/y

With MS_REC or AT_RECURSIVE copy_tree() doesn't stop at the unmounted mount. It copies every mount that stayed attached to it as well. So if you have a bunch of overmounts over the nsfsfd or the pidfd the whole unmounted stack is copied. That's a bug.

So refuse to walk an unmounted tree. Copying its root alone stays possible. So a bind mount of a namespace that is gone can still be recreated from a file descriptor on the old one. The file descriptor pins that mount and its root and __has_locked_children() only reads the direct children which don't leave the parent's list without namespace_sem. The internal mounts carry MNT_NS_INTERNAL instead of NULL and are still walked. Copying an unmounted tree recursively fails with EINVAL.

---
 fs/namespace.c | 3 +++
 1 file changed, 3 insertions(+)

index 42ecf070b6f76..78e521ee3ceb3 100644
@@ -2428,6 +2428,9 @@ static struct mount *__do_loopback(struct path *old_path, int recurse)
 	if (!check_mnt(old) && old_path->dentry->d_op != &ns_dentry_operations)
 		return mnt;
 
+	if (recurse && !old->mnt_ns)
+		return ERR_PTR(-EINVAL);
+
 	if (!recurse && has_locked_children(old, old_path->dentry))
 		return mnt;
 

2.53.0