Re: [syzbot] [net?] KASAN: slab-use-after-free Write in __xfrm_state_delete

[Hillf Danton]

> Date: Sun, 24 Aug 2025 12:21:23 -0700	[thread overview]
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    b1c92cdf5af3 Merge branch 'net-wangxun-complete-ethtool-co..
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1411b062580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=67b99ceb67d33475
> dashboard link: https://syzkaller.appspot.com/bug?extid=a25ee9d20d31e483ba7b
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14221862580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=159fba34580000

#syz test

--- x/net/xfrm/xfrm_state.c
+++ y/net/xfrm/xfrm_state.c
@@ -798,6 +798,7 @@ void xfrm_dev_state_free(struct xfrm_sta
 void __xfrm_state_destroy(struct xfrm_state *x)
 {
 	WARN_ON(x->km.state != XFRM_STATE_DEAD);
+	WARN_ON(!hlist_unhashed(&x->bydst));
 
 	spin_lock_bh(&xfrm_state_gc_lock);
 	hlist_add_head(&x->gclist, &xfrm_state_gc_list);
@@ -934,14 +935,15 @@ restart:
 			if (!xfrm_state_kern(x) &&
 			    xfrm_id_proto_match(x->id.proto, proto)) {
 				xfrm_state_hold(x);
+				if (x->km.state == XFRM_STATE_DEAD)
+					x->km.state++;
 				spin_unlock_bh(&net->xfrm.xfrm_state_lock);
 
 				err = xfrm_state_delete(x);
 				xfrm_audit_state_delete(x, err ? 0 : 1,
 							task_valid);
 				xfrm_state_put(x);
-				if (!err)
-					cnt++;
+				cnt++;
 
 				spin_lock_bh(&net->xfrm.xfrm_state_lock);
 				goto restart;
--