FAILED: patch "[PATCH] xfrm: defensively unhash xfrm_state lists in" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-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-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 14acf9652e5690de3c7486c6db5fb8dafd0a32a3
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026051215-richness-lagoon-e3ee@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 14acf9652e5690de3c7486c6db5fb8dafd0a32a3 Mon Sep 17 00:00:00 2001
From: Michal Kosiorek <[email protected]>
Date: Wed, 29 Apr 2026 10:54:51 +0200
Subject: [PATCH] xfrm: defensively unhash xfrm_state lists in
__xfrm_state_delete

KASAN reproduces a slab-use-after-free in __xfrm_state_delete()'s hlist_del_rcu calls under syzkaller load on linux-6.12.y stable (reproduced on 6.12.47, also reachable via the same code path on torvalds/master and on the ipsec tree). Nine unique signatures cluster in the xfrm_state lifecycle, the load-bearing one being:

  BUG: KASAN: slab-use-after-free in __hlist_del include/linux/list.h:990 [inline]
  BUG: KASAN: slab-use-after-free in hlist_del_rcu include/linux/rculist.h:516 [inline]
  BUG: KASAN: slab-use-after-free in __xfrm_state_delete net/xfrm/xfrm_state.c
  Write of size 8 at addr ffff8881198bcb70 by task kworker/u8:9/435

  Workqueue: netns cleanup_net
  Call Trace:
   __hlist_del / hlist_del_rcu
   __xfrm_state_delete
   xfrm_state_delete
   xfrm_state_flush
   xfrm_state_fini
   ops_exit_list
   cleanup_net

The other observed signatures hit the same slab object from __xfrm_state_lookup, xfrm_alloc_spi, __xfrm_state_insert and an OOB write variant of __xfrm_state_delete, all on the byseq/byspi hash chains.

__xfrm_state_delete() guards its byseq and byspi unhashes with value-based predicates:

	if (x->km.seq)
		hlist_del_rcu(&x->byseq);
	if (x->id.spi)
		hlist_del_rcu(&x->byspi);

while everywhere else in the file (e.g. state_cache, state_cache_input) the safer hlist_unhashed() check is used. xfrm_alloc_spi() sets x->id.spi = newspi inside xfrm_state_lock and then immediately inserts into byspi, but a path that observes x->id.spi != 0 outside of xfrm_state_lock can still skip-or-hit the byspi unhash inconsistently with whether x is actually on the list. The same holds for x->km.seq versus byseq, and the bydst/bysrc unhashes have no predicate at all, so a second __xfrm_state_delete() on the same object writes through LIST_POISON pprev.

The defensive change here:

- Use hlist_del_init_rcu() instead of hlist_del_rcu() on bydst, bysrc, byseq and byspi so a second deletion is a no-op rather than a write through LIST_POISON pprev. The byseq/byspi nodes are already initialised in xfrm_state_alloc().
- Test hlist_unhashed() rather than the value predicate for byseq/byspi, so the unhash decision tracks list state rather than mutable scalar fields.

Empirical verification: applied this patch on top of v6.12.47, rebuilt, and re-ran the same syzkaller harness for 1h16m on a previously-crashy configuration that produced ~100 hits each of slab-use-after-free Read in xfrm_alloc_spi / Read in __xfrm_state_lookup / Write in __xfrm_state_delete. After the patch, 7.1M execs across 32 VMs at ~1550 exec/sec produced zero xfrm_state UAF/OOB hits. /proc/slabinfo confirms the xfrm_state slab is actively allocated and freed during the run (~143 KiB resident), so the fuzzer is still exercising those code paths -- they just no longer crash.

Reproduction:

- Linux 6.12.47 x86_64 + KASAN_GENERIC + KASAN_INLINE + KCOV
- syzkaller @ 746545b8b1e4c3a128db8652b340d3df90ce61db
- 32 QEMU/KVM VMs x 2 vCPU on AWS c5.metal bare metal
- 9 unique signatures collected in ~9h, all within xfrm_state lifecycle

index 1748d374abca..686014d39429 100644
@@ -818,17 +818,17 @@ int __xfrm_state_delete(struct xfrm_state *x)
 
 		spin_lock(&net->xfrm.xfrm_state_lock);
 		list_del(&x->km.all);
-		hlist_del_rcu(&x->bydst);
-		hlist_del_rcu(&x->bysrc);
-		if (x->km.seq)
-			hlist_del_rcu(&x->byseq);
+		hlist_del_init_rcu(&x->bydst);
+		hlist_del_init_rcu(&x->bysrc);
+		if (!hlist_unhashed(&x->byseq))
+			hlist_del_init_rcu(&x->byseq);
 		if (!hlist_unhashed(&x->state_cache))
 			hlist_del_rcu(&x->state_cache);
 		if (!hlist_unhashed(&x->state_cache_input))
 			hlist_del_rcu(&x->state_cache_input);
 
-		if (x->id.spi)
-			hlist_del_rcu(&x->byspi);
+		if (!hlist_unhashed(&x->byspi))
+			hlist_del_init_rcu(&x->byspi);
 		net->xfrm.state_num--;
 		xfrm_nat_keepalive_state_updated(x);
 		spin_unlock(&net->xfrm.xfrm_state_lock);