From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout06.his.huawei.com (canpmsgout06.his.huawei.com [113.46.200.221]) (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 55029476043 for ; Fri, 21 Aug 2026 09:54:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.221 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787306109; cv=none; b=fiKsO5iomf4t4rkIl4xYXf23thfL/YP6OmGPqC+RtZ1DwBNO687jeqCH7FFADb1VqspEfL3He/nwlP+V57pcuPQV6xHsS5/pIFua83nN4bB8I3L86X3MEtN9kcNlitS+FDBWvAecqtvcmdX1tiWQKEI9wj722VWPzA1skVgCj6U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787306109; c=relaxed/simple; bh=PISbZtkoGKpn3jjzrMxvDB9DCUzpT7qIaCjhNi0GprM=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=DpfXQA1Z3aai4p9PVa9Q7A60/DWMsKVOi4pS75B6bTYobRQFXVwosnmEt1cqXUxQa8ScPeSHytwws6zSlmwC4AoYOc1wP1Ps18wO6TbPNH2G7QvtecCOXQVKoX6hLWIgS2tYjsaaqXK4X2+c4Jaf9NPzsYBNU18PF+K5SjBNt6c= ARC-Authentication-Results:i=1; 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Fri, 21 Aug 2026 17:44:09 +0800 (CST) Received: from whupemk200012.china.huawei.com (unknown [7.152.185.169]) by mail.maildlp.com (Postfix) with ESMTPS id 760F94056D; Fri, 21 Aug 2026 17:54:50 +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 17:54:48 +0800 Message-ID: Date: Fri, 21 Aug 2026 17:54:47 +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: kwepems500001.china.huawei.com (7.221.188.70) To whupemk200012.china.huawei.com (7.152.185.169) >>>>>>>> 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; >> >> Certainly not. Do not add defensive code in the fast path for GTP that >> nobody uses. >> >> Add everything you think of in GTP itself, I do not care. >> >> I suggest you look at skb_gso_reset(), this might be missing in GTP. > > Or simply drop GSO packets in GTP they must be evil: we do not support > GSO in GTP and never will. Indeed, dropping these abnormal packets in gtp directly is the best option. diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c index 1e3013d49713..7f2eaa8635fc 100644 --- a/drivers/net/gtp.c +++ b/drivers/net/gtp.c @@ -312,6 +312,10 @@ static int gtp_inner_proto(struct sk_buff *skb, unsigned int hdrlen, static int gtp_rx(struct pdp_ctx *pctx, struct sk_buff *skb, unsigned int hdrlen, unsigned int role, __u16 inner_proto) { + /* gso isnt support in GTP, drop it outright */ + if (skb_is_gso(skb)) + goto err; + if (!gtp_check_ms(skb, pctx, hdrlen, role, inner_proto)) { netdev_dbg(pctx->dev, "No PDP ctx for this MS\n"); return 1; Thanks for your suggestion. v2 will be sent Best Regards Dong Chenchen