FAILED: patch "[PATCH] namespace: check the target before walking it in" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-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-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 08f4288a37ef4b3d19b4a9599ec3048219cac828
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026101210-barge-alkalize-e6de@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 08f4288a37ef4b3d19b4a9599ec3048219cac828 Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Sat, 26 Sep 2026 15:44:28 +0200
Subject: [PATCH] namespace: check the target before walking it in
do_mount_setattr()

When mount_setattr() changes the propagation type to MS_SHARED do_mount_setattr() takes namespace_sem and calls invent_group_ids() which walks the whole target tree with next_mnt() and allocates a peer group id for every mount that doesn't have one.

Whether the caller may touch the target at all is only checked afterwards under mount_lock. If it may not, cleanup_group_ids() walks the tree a second time on the way out to free the ids again.

A detached tree that a file descriptor keeps alive, a mount in another mount namespace or a mount below the root of an anonymous mount namespace does a bunch of allocation work that is completely useless because it will fail and revert right after.

Do the right thing and check the target right after taking namespace_sem instead. A mount leaves its namespace only under namespace_sem so the mount stays stable. The check under mount_lock stays for the case where no propagation change is requested and namespace_sem isn't taken. Callers get the same EINVAL as before.

index c7e252eb249e..de28fe9058ab 100644
@@ -4976,6 +4976,11 @@ static int do_mount_setattr(const struct path *path, struct mount_kattr *kattr)
 		 * propagation.
 		 */
 		namespace_lock();
+		/* invent_group_ids() walks the tree, only walk a mounted one */
+		if (!anon_ns_root(mnt) && !check_mnt(mnt)) {
+			namespace_unlock();
+			return -EINVAL;
+		}
 		if (kattr->propagation == MS_SHARED) {
 			err = invent_group_ids(mnt, kattr->kflags & MOUNT_KATTR_RECURSE);
 			if (err) {

@@ -4988,6 +4993,7 @@ static int do_mount_setattr(const struct path *path, struct mount_kattr *kattr)
 	err = -EINVAL;
 	lock_mount_hash();
 
+	/* Checked under namespace_sem already if the propagation changes. */
 	if (!anon_ns_root(mnt) && !check_mnt(mnt))
 		goto out;
 

From: Christian Brauner <[email protected]>

[ Upstream commit 08f4288a37ef4b3d19b4a9599ec3048219cac828 ]

When mount_setattr() changes the propagation type to MS_SHARED do_mount_setattr() takes namespace_sem and calls invent_group_ids() which walks the whole target tree with next_mnt() and allocates a peer group id for every mount that doesn't have one.

Whether the caller may touch the target at all is only checked afterwards under mount_lock. If it may not, cleanup_group_ids() walks the tree a second time on the way out to free the ids again.

A detached tree that a file descriptor keeps alive, a mount in another mount namespace or a mount below the root of an anonymous mount namespace does a bunch of allocation work that is completely useless because it will fail and revert right after.

Do the right thing and check the target right after taking namespace_sem instead. A mount leaves its namespace only under namespace_sem so the mount stays stable. The check under mount_lock stays for the case where no propagation change is requested and namespace_sem isn't taken. Callers get the same EINVAL as before.

---
 fs/namespace.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index ff8d16a3eeb2d..e81aa0fdb397b 100644
@@ -4063,6 +4063,7 @@ static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
 {
 	struct mount *m = mnt, *last = NULL;
 
+	/* Checked under namespace_sem already if the propagation changes. */
 	if (!is_mounted(&m->mnt)) {
 		*err = -EINVAL;
 		goto out;

@@ -4172,6 +4173,13 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 		 * propagation.
 		 */
 		namespace_lock();
+		/* invent_group_ids() walks the tree, only walk a mounted one */
+		if (!is_mounted(&mnt->mnt) ||
+		    !(mnt_has_parent(mnt) ? check_mnt(mnt) :
+		      is_anon_ns(mnt->mnt_ns))) {
+			namespace_unlock();
+			return -EINVAL;
+		}
 		if (kattr->propagation == MS_SHARED) {
 			err = invent_group_ids(mnt, kattr->recurse);
 			if (err) {

2.53.0