During a hash set resize, mtype_resize() copies each element into the
new table with memcpy(), so the new-table element shares the old-table
element's comment extension.  An xt_SET delete on the old table during
the resize destroys that shared comment via ip_set_ext_destroy() and
queues a replayed delete on h->ad.  After the table swap mtype_resize()
replays it with mtype_del() on the new table, whose copy still points at
the freed comment, so ip_set_ext_destroy() frees it a second time:

 ODEBUG: activate active (active state 1) object: ... object type: rcu_head
 WARNING: CPU: 3 PID: 5311 at lib/debugobjects.c:514 debug_print_object
 Call Trace:
  <IRQ>
  kvfree_call_rcu (kernel/rcu/tree.c:3825)
  ip_set_comment_free (net/netfilter/ipset/ip_set_core.c:397)
  hash_ip4_del (net/netfilter/ipset/ip_set_hash_gen.h:1098)
  hash_ip4_kadt (net/netfilter/ipset/ip_set_hash_ip.c:96)
  ip_set_del (net/netfilter/ipset/ip_set_core.c:813)
  set_target_v3 (net/netfilter/xt_set.c:412)
  ipt_do_table (net/ipv4/netfilter/ip_tables.c:346)
  __ip_local_out (net/ipv4/ip_output.c:119)
  icmp_push_reply (net/ipv4/icmp.c:397)
  __icmp_send (net/ipv4/icmp.c:804)
  __udp4_lib_rcv (net/ipv4/udp.c:2521)
  ip_local_deliver (net/ipv4/ip_input.c:254)
  ip_rcv (net/ipv4/ip_input.c:569)
  </IRQ>

The replay passes a NULL ext (the kernel-side delete that queued it
already destroyed the extensions), so skip ip_set_ext_destroy() when ext
is NULL.  This also avoids the NULL ext->target dereference that was only
kept safe by the new table's ref being zero.

Reachable from an unprivileged user namespace.

Fixes: f66ee0410b1c ("netfilter: ipset: Fix \"INFO: rcu detected stall in hash_xxx\" reports")
Reported-by: Xiang Mei <redacted>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <redacted>
---
 net/netfilter/ipset/ip_set_hash_gen.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/ipset/ip_set_hash_gen.h b/net/netfilter/ipset/ip_set_hash_gen.h
index 5e4453e9e..bc909ae2d 100644
--- a/net/netfilter/ipset/ip_set_hash_gen.h
+++ b/net/netfilter/ipset/ip_set_hash_gen.h

@@ -1080,9 +1080,11 @@ mtype_del(struct ip_set *set, void *value, const struct ip_set_ext *ext,
 			mtype_del_cidr(set, h,
 				       NCIDR_PUT(DCIDR_GET(d->cidr, j)), j);
 #endif
-		ip_set_ext_destroy(set, data);
+		/* On a resize replay the extensions were already destroyed. */
+		if (ext)
+			ip_set_ext_destroy(set, data);
 
-		if (atomic_read(&t->ref) && ext->target) {
+		if (ext && atomic_read(&t->ref) && ext->target) {
 			/* Resize is in process and kernel side del,
 			 * save values
 			 */

-- 
2.43.0