FAILED: patch "[PATCH] unshare: don't drop active namespace references that were" 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 0b6f31a3c0b16066f812e94b52a704174a6f23cd
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026101253-shape-overstep-3470@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

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

From 0b6f31a3c0b16066f812e94b52a704174a6f23cd Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Wed, 30 Sep 2026 15:32:08 +0200
Subject: [PATCH] unshare: don't drop active namespace references that were
never taken

Active references on the namespaces of an nsproxy are taken when the nsproxy is installed into a task in switch_task_namespaces() and copy_namespaces() and dropped again by put_nsproxy() through deactivate_nsproxy().

But ksys_unshare() calls put_nsproxy() on an nsproxy that was never installed when set_cred_ucounts() fails. The new namespaces of that nsproxy go from zero to minus one and the namespaces shared with the caller lose a reference that belongs to the nsproxy the caller keeps using:

  WARNING: kernel/nscommon.c:171 at __ns_ref_active_put+0x1cd/0x230
   nsproxy_ns_active_put
   deactivate_nsproxy
   ksys_unshare

Afterwards the namespaces the caller lives in aren't listed by listns() anymore. set_cred_ucounts() only fails when alloc_ucounts() can't allocate and it only allocates when the real uid of the caller differs from its effective uid.

Commit cefd55bd2159 ("nsproxy: fix free_nsproxy() and simplify create_new_namespaces()") separated the two cases on purpose. nsproxy_free() frees an nsproxy that was prepared but never installed and that's what a failed setns() uses in put_nsset(). Export it and use it for a failed unshare() as well.

index 5a67648721c7..dc2447f5f092 100644
@@ -100,6 +100,7 @@ void exit_cred_namespaces(struct task_struct *tsk);
 void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 int exec_task_namespaces(void);
 void deactivate_nsproxy(struct nsproxy *ns);
+void nsproxy_free(struct nsproxy *ns);
 int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
 	struct cred *, struct fs_struct *);
 int __init nsproxy_cache_init(void);

index da48168c504f..d442cd68a37a 100644
@@ -3338,8 +3338,9 @@ int ksys_unshare(unsigned long unshare_flags)
 	perf_event_namespaces(current);
 
 bad_unshare_cleanup_nsproxy:
+	/* never installed, so no active references to drop */
 	if (new_nsproxy)
-		put_nsproxy(new_nsproxy);
+		nsproxy_free(new_nsproxy);
 bad_unshare_cleanup_cred:
 	if (new_cred)
 		put_cred(new_cred);

index d9d3d5973bf5..3fb1595a4a59 100644
@@ -61,7 +61,7 @@ static inline struct nsproxy *create_nsproxy(void)
 	return nsproxy;
 }
 
-static inline void nsproxy_free(struct nsproxy *ns)
+void nsproxy_free(struct nsproxy *ns)
 {
 	put_mnt_ns(ns->mnt_ns);
 	put_uts_ns(ns->uts_ns);