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[34.85.134.180]) by smtp.gmail.com with UTF8SMTPSA id 956f58d0204a3-64eabe7e343sm2510457d50.11.2026.03.20.18.36.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Mar 2026 18:36:55 -0700 (PDT) Date: Fri, 20 Mar 2026 21:36:54 -0400 From: Willem de Bruijn To: Willem de Bruijn , Guoyu Su , edumazet@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com Cc: willemdebruijn.kernel@gmail.com, netdev@vger.kernel.org, horms@kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com, Guoyu Su Message-ID: In-Reply-To: References: <20260320141459.9691-1-yss2813483011xxl@gmail.com> Subject: Re: [PATCH net v5] net: use skb_header_pointer() only for DODGY TCPv4 GSO skbs Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Willem de Bruijn wrote: > Guoyu Su wrote: > > Syzbot reported a KMSAN uninit-value warning in gso_features_check() > > called from netif_skb_features() [1]. > > > > The current direct skb->len check is not sufficient for SKB_GSO_DODGY > > packets. In the AF_PACKET/PACKET_VNET_HDR path, packet_snd() can build > > a DODGY GSO skb whose total length is large enough, while the IPv4 > > header is not fully available as initialized linear data for a direct > > iph->frag_off access. > > The fix looks fine, but the AI review of an earlier revision brings up > a good point: __virtio_net_hdr_to_skb calls pskb_may_pull in all paths > to ensure the network header is fully in skb linear. What kind of packet > is this that managed to escape those checks? The packets I got out of the C repro just after virtio_net_hdr_to_skb look as below. [ 76.539562] vnet_hdr: flags=0x75 gso_type=0x1 hlen=0x6a gso_sz=0x416d cstart=0x58 [ 76.539755] skb len=56584 data_len=56476 headroom=4 headlen=108 tailroom=0 [ 76.539755] end-tail=208 mac=(4,76) mac_len=0 net=(80,12) trans=92 [ 76.539755] shinfo(txflags=0 nr_frags=3 gso(size=16749 type=3 segs=0)) [ 76.539755] csum(0x10005c start=92 offset=16 ip_summed=3 complete_sw=0 valid=0 level=0) [ 76.539755] hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=0 iif=0 [ 76.539755] priority=0x0 mark=0x0 alloc_cpu=0 vlan_all=0x0 [ 76.539755] encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0) [ 76.540713] dev name=ip6gretap0 feat=0x0000000e401d4869 [ 76.540843] sk family=17 type=3 proto=0 Clearly fishy. They do have VIRTIO_NET_HDR_F_NEEDS_CSUM set, so we know which branch they take. skb_reset_mac_header(skb); if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) { u32 start = __virtio16_to_cpu(little_endian, hdr->csum_start); u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset); u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16)); // start == 88 // needed == 88 + 18 == 106 if (!pskb_may_pull(skb, needed)) return -EINVAL; if (!skb_partial_csum_set(skb, start, off)) return -EINVAL; if (skb_transport_offset(skb) < nh_min_len) return -EINVAL; nh_min_len = skb_transport_offset(skb); // nh_min_len == 88 p_off = nh_min_len + thlen; // p_off == 108 if (!pskb_may_pull(skb, p_off)) return -EINVAL; // headlen == 108 At the end of this headlen == 108, so all of iphdr should be in linear. Since the syz repro requires repeat it is possible that I simply did not capture the right packet, but I don't see the C program vary the packet contents.