Re: [PATCH net] net: iptunnel: fix stale transport header during tunnel decapsulation

On Thu, Aug 20, 2026 at 12:04 PM dongchenchen (A)
<[email protected]> wrote:
>
>
>
> 在 2026/8/20 17:16, Eric Dumazet 写道:
> > On Thu, Aug 20, 2026 at 10:58 AM dongchenchen (A)
> > <[email protected]> wrote:
> >>
> >>
> >>> On Thu, Aug 13, 2026 at 5:30 AM Dong Chenchen <[email protected]> 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
> >>>> <IRQ>
> >>>> 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.

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.