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Fri, 21 Aug 2026 10:10:54 +0800 (CST) Received: from whupemk200012.china.huawei.com (unknown [7.152.185.169]) by mail.maildlp.com (Postfix) with ESMTPS id CA72D203D1; Fri, 21 Aug 2026 10:21:34 +0800 (CST) Received: from [10.174.177.223] (10.174.177.223) by whupemk200012.china.huawei.com (7.152.185.169) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.45; Fri, 21 Aug 2026 10:21:32 +0800 Message-ID: Date: Fri, 21 Aug 2026 10:21:31 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net] net: iptunnel: fix stale transport header during tunnel decapsulation To: Eric Dumazet CC: , , , , , , , , , , , , , , , References: <20260813033855.3372172-1-dongchenchen2@huawei.com> <53e86f6f-43b9-4e06-b185-1344f69cd650@huawei.com> <9a80f88e-c8a6-44fb-a7b2-7cdbaa3bba24@huawei.com> From: "dongchenchen (A)" In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems200001.china.huawei.com (7.221.188.67) To whupemk200012.china.huawei.com (7.152.185.169) 在 2026/8/21 0:32, Eric Dumazet 写道: > On Thu, Aug 20, 2026 at 12:04 PM dongchenchen (A) > wrote: >> >> >> >> 在 2026/8/20 17:16, Eric Dumazet 写道: >>> On Thu, Aug 20, 2026 at 10:58 AM dongchenchen (A) >>> wrote: >>>> >>>> >>>>> On Thu, Aug 13, 2026 at 5:30 AM Dong Chenchen wrote: >>>>>> >>>>>> Syzbot reported a crash in qdisc_pkt_len_segs_init() caused by a stale >>>>>> transport_header offset after tunnel decapsulation. >>>>>> >>>>>> BUG: unable to handle page fault for address: ffffed102091a42e >>>>>> Oops: Oops: 0000 [#1] SMP KASAN NOPTI >>>>>> CPU: 0 UID: 0 PID: 340 Comm: qdisc_uaf_repro Not tainted 7.2.0-rc4-00061-g248951ddc14d #256 PREEMPT(full) >>>>>> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 >>>>>> RIP: 0010:__asan_load2 >>>>>> >>>>>> qdisc_pkt_len_segs_init (net/core/dev.c:4145) >>>>>> __dev_queue_xmit (net/core/dev.c:4787) >>>>>> br_dev_queue_push_xmit (net/bridge/br_forward.c:53) >>>>>> br_handle_frame_finish (net/bridge/br_input.c:229) >>>>>> br_handle_frame (net/bridge/br_input.c:315) >>>>>> __netif_receive_skb_core.constprop.0 (net/core/dev.c:6099) >>>>>> __netif_receive_skb_list_core (net/core/dev.c:6287) >>>>>> netif_receive_skb_list_internal (net/core/dev.c:6445) >>>>>> napi_complete_done (net/core/dev.c:6813) >>>>>> gro_cell_poll (net/core/gro_cells.c:74) >>>>>> __napi_poll (net/core/dev.c:7735) >>>>>> net_rx_action (net/core/dev.c:7798 net/core/dev.c:7955) >>>>>> handle_softirqs (kernel/softirq.c:622) >>>>>> do_softirq (kernel/softirq.c:523 kernel/softirq.c:510 ) >>>>>> __local_bh_enable_ip (kernel/softirq.c:450) >>>>>> tun_get_user (drivers/net/tun.c:1986 (discriminator 1)) >>>>>> tun_chr_write_iter (drivers/net/tun.c:2032) >>>>>> >>>>>> The crash requires four conditions to line up: >>>>>> >>>>>> 1. The incoming packet is encapsulated and carries GSO metadata. The outer >>>>>> transport header offset is stored in skb->transport_header while the >>>>>> packet is still in the outer tunnel context. >>>>>> 2. The tunnel receiver strips the outer headers. skb->data is advanced to >>>>>> the inner frame, but skb->transport_header is left pointing to the >>>>>> now-removed outer L4 header, so it becomes a negative offset relative to >>>>>> the new data. >>>>>> 3. The inner frame is not delivered to the local IP stack. Instead, it >>>>>> is forwarded at L2 by a bridge or HSR, so ip_rcv_core() never runs and >>>>>> the transport header is not reset to the inner L4 offset. >>>>>> 4. The forwarding path calls __dev_queue_xmit(), which enters >>>>>> qdisc_pkt_len_segs_init(). That function computes the GSO header length >>>>>> from skb_transport_offset(skb). Because the offset is negative, the >>>>>> unsigned cast overflows and pskb_may_pull(skb, hdr_len + >>>>>> sizeof(struct tcphdr)) reads past the end of the skb, triggering a >>>>>> KASAN fault or page fault. >>>>>> >>>>>> Fix this by clearing skb->transport_header to the ~0U sentinel at the >>>>>> tunnel decapsulation boundary, after each tunnel receive function has >>>>>> finished all processing that needs the outer L4 header and before the skb >>>>>> is handed to GRO or the stack. The IP/GRO receive paths then set the >>>>>> transport header correctly when they parse the inner packet. >>>>>> >>>>>> Fixes: 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()") >>>>> >>>>> This Fixes: tag seems pretty random to me. Issue was present long >>>>> before that commit. >>>>> Next time, please be precise with the `Fixes:` tag; it's not decoration. >>>>> >>>> Thanks for your review. Sorry for the late reply. >>>> The stale transport header has been present since 1da177e4c3f4 >>>> ("Linux-2.6.12-rc2"), which is completely latent until >>>> 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()" >>>> was merged. >>>> >>>> From the perspective of the root cause introduced by the issue, >>>> we can use 1da177e4c3f4 ("Linux-2.6.12-rc2") as fixtag. >>>> >>>>> Also, calling skb_unset_transport_header(skb) directly from >>>>> gro_cells_receive() would make more sense and >>>>> your patch would be less intrusive. >>>>> >>>> >>>> gtp，sit use netif_rx to receive pkt. so modifying only gro_cells_receive >>>> cannot solve all the problems. netif_rx is widely >>>> used and is not only invoked by the tunnel driver, and serval path >>>> (such as genve encap) will reset transport header before netif_rx. >>>> Therefore, should we retain unset in the GTP/SIT tunnel? >>> >>> Hi Dong, >>> >>> 1) Regarding SIT: >>> >>> SIT decapsulates IPv6 over IPv4 (IPPROTO_IPV6). There is no outer L4 transport >>> header (no UDP/TCP), so skb->transport_header is not pointing to an >>> outer L4 header. >>> >>> 2) Regarding GTP: >>> >>> GTP is an L3-only point-to-point device (ARPHRD_NONE). It cannot be >>> enslaved by a bridge >>> or forwarded at L2; its packets always go through ip_rcv()/ip6_rcv() >>> where the headers >>> are properly parsed/reset, >>> >> Hi, Eric. Thanks for the review! >> >> I missed the rcv path in the commit message. >> With an ingress qdisc attached to gtp, the stale transport header >> will been consumed in sch_handle_ingress() and its not covered by >> gro_cells_receive() either. So maybe gtp is not safe by >> construction. >> >> gtp_encap_rcv >> gtp_rx //stale transport header >> __netif_rx >> __netif_receive_skb_core >> sch_handle_ingress >> qdisc_pkt_len_segs_init >> > > Let's step back a bit :) > > For non-GSO packets, qdisc_pkt_len_segs_init() returns immediately: > > qdisc_skb_cb(skb)->pkt_len = skb->len; > if (!shinfo->gso_size) { > qdisc_skb_cb(skb)->pkt_segs = 1; > return SKB_NOT_DROPPED_YET; > } > > It does not inspect skb->transport_header at all. It will be set > eventually later if needed, > > GTP does not implement GRO on RX and does not use gro_cells, so incoming > packets in gtp_rx() have gso_size == 0. > Hi, Eric. Thanks for your patient explanation. Normal GTP traffic indeed cannot trigger this issue and pkt without SKB_GSO_UDP_TUNNEL will be caught by udp_unexpected_gso. The only way to reproduce this for gtp is by constructing and injecting packets, such as TAP device injects a GSO UDP-tunnel packet (with GSO_TCPV4 and GSO_UDP_TUNNEL flag, Ethernet/IPv4/UDP/GTP-U/IPV4/TCP). Should we add interception for such illegal pkts? diff --git a/net/core/dev.c b/net/core/dev.c index af260ff5462a..8620428b950f 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -4138,6 +4138,10 @@ static enum skb_drop_reason qdisc_pkt_len_segs_init(struct sk_buff *skb) if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))) { const struct tcphdr *th; + /* Defensive: stale offsets from buggy tunnel decap can underflow. */ + if (unlikely(hdr_len > INT_MAX)) + return 0; + if (!pskb_may_pull(skb, hdr_len + sizeof(struct tcphdr))) return SKB_DROP_REASON_SKB_BAD_GSO; ------------- Best Regards Dong Chenchen > The issue specifically requires GSO packets (shinfo->gso_size != 0), which are > processed/aggregated through gro_cells. Fixing this in > gro_cells_receive() covers > all the relevant tunnel drivers in one place. > > Thanks.