Re: [PATCH nf] netfilter: ipset: skip extension destroy on hash resize replay

Hi,

On Fri, 3 Jul 2026, Weiming Shi wrote:

> 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 <[email protected]>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Weiming Shi <[email protected]>
> ---
> 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
> 			 */

Please rebase your patch against the nf-next tree 
(git://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf-next.git): the 
second chunk of your patch is not needed as it has been fixed.

Thank you and best regards,
Jozsef