From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from Chamillionaire.breakpoint.cc (Chamillionaire.breakpoint.cc [91.216.245.30]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6BD9333987; Sun, 19 Apr 2026 08:00:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.216.245.30 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776585638; cv=none; b=LBqtXD2RaneOIjDy5cf0okaXdMmaqWzLGfb5Py7a/xDRODUu08WprerB9iOykI/5AQadqWLKUXKnI+YOT4OxXFp+OSiCxHWrzzC9SlAz9gDRda9WmGkKA/Dyl4zWVYq3pLu9dxigbCkDd+TIiDvuZzUgHnW8TYwNQRodPrcQ4ng= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776585638; c=relaxed/simple; bh=Z/bcn2OK8NOgU6UH+vEzzrrUVNXWK29iBoH9LmnHRUo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=rNH4La7aDnT88AuCdquBHkSxMR4T5mTi9cjtKGgbioReo/H+Nm+j3YsakRCb2vpFxeLtGGiVfXvXHTbmBtvMQGlmHXnEYQqPYoYdBvZVirupxusePLH7csufuBdv9wBwpXLjkpNMyl1kqeW++yIg5pF4kzAHD8J9fUl12FX+Q18= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de; spf=pass smtp.mailfrom=strlen.de; arc=none smtp.client-ip=91.216.245.30 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=strlen.de Received: by Chamillionaire.breakpoint.cc (Postfix, from userid 1003) id 2891B60420; Sun, 19 Apr 2026 10:00:34 +0200 (CEST) Date: Sun, 19 Apr 2026 10:00:33 +0200 From: Florian Westphal To: Weiming Shi Cc: Pablo Neira Ayuso , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Phil Sutter , Simon Horman , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, netdev@vger.kernel.org, Xiang Mei Subject: Re: [PATCH nf] netfilter: xt_TCPMSS: check skb_dst before path-MTU clamping Message-ID: References: <20260418163057.2611503-2-bestswngs@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Florian Westphal wrote: > Weiming Shi wrote: > > When TCPMSS with CLAMP_PMTU is used via nft_compat in a non-base > > chain, par->hook_mask is set to 0, bypassing the checkentry hook > > validation. The target can then run at PRE_ROUTING where skb_dst is > > NULL, causing a null-ptr-deref in tcpmss_mangle_packet(): > > > > KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f] > > RIP: 0010:tcpmss_mangle_packet (include/net/dst.h:219 net/netfilter/xt_TCPMSS.c:105) > > tcpmss_tg4 (net/netfilter/xt_TCPMSS.c:202) > > nft_target_eval_xt (net/netfilter/nft_compat.c:87) > > nft_do_chain (net/netfilter/nf_tables_core.c:287) > > nf_hook_slow (net/netfilter/core.c:623) > > > > Check skb_dst() for NULL before calling dst_mtu(). > > FWIW I will apply this patch even though its wrong. > > nft_compat.c is just too broken, I don't see how it can be > fixed in any reasonable amount of time. net/netfilter/xt_TCPMSS.c: (par->hook_mask & ~((1 << NF_INET_FORWARD) | net/netfilter/xt_addrtype.c: if (par->hook_mask & ((1 << NF_INET_PRE_ROUTING) | net/netfilter/xt_devgroup.c: par->hook_mask & ~((1 << NF_INET_PRE_ROUTING) | net/netfilter/xt_physdev.c: par->hook_mask & (1 << NF_INET_LOCAL_OUT)) { net/netfilter/xt_policy.c: if (par->hook_mask & ((1 << NF_INET_POST_ROUTING) | net/netfilter/xt_set.c: (par->hook_mask & ~(1 << NF_INET_FORWARD | Look at this I don't see an alternative to mixing nft specific bits into x_tables, i.e.: diff --git a/include/linux/netfilter/x_tables.h b/include/linux/netfilter/x_tables.h --- a/include/linux/netfilter/x_tables.h +++ b/include/linux/netfilter/x_tables.h @@ -187,6 +187,8 @@ struct xt_target { /* Should return 0 on success or an error code otherwise (-Exxxx). */ int (*checkentry)(const struct xt_tgchk_param *); + int (*nft_validate_chain)(const void *targinfo, unsigned int hook_mask); + /* Called when entry of this type deleted. */ void (*destroy)(const struct xt_tgdtor_param *); #ifdef CONFIG_NETFILTER_XTABLES_COMPAT .. and then call that from nft_compat.c for TCPSS. Same for matches.