Re: [PATCH net v2] net: iptunnel: fix stale transport header during tunnel decapsulation

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Aug 25, 2026 at 11:59:53AM +0200, Eric Dumazet wrote:
> On Tue, Aug 25, 2026 at 11:41 AM Dong Chenchen <dongchenchen2@huawei.com> wrote:
> >
> > Syzbot reported a crash in qdisc_pkt_len_segs_init() caused by a stale
> > transport_header offset after tunnel decapsulation.
> >
> >
> > The issue is completely latent until qdisc read transport header in
> > commit 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()").
> > 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.
> >
> > The issue specifically requires GSO packets (shinfo->gso_size != 0), which
> > are processed/aggregated through gro_cells. Fix this by clearing
> > transport_header to the ~0U sentinel in gro_cell for all tunnnel driver.
> > GTP does not support GRO/GSO, drop the evil GSO packets in GTP directly.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Reported-by: syzbot+83181a31faf9455499c5@syzkaller.appspotmail.com
> > Closes: https://lore.kernel.org/all/69de2bee.a00a0220.475f0.0041.GAE@google.com/T/
> > Suggested-by: Eric Dumazet <edumazet@google.com>
> > Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>
> > ---
> >  drivers/net/gtp.c      | 5 +++++
> >  include/linux/skbuff.h | 5 +++++
> >  net/core/gro_cells.c   | 2 ++
> >  3 files changed, 12 insertions(+)
> >
> > diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
> > index 9a12cc53da00..fbf617b1acc9 100644
> > --- a/drivers/net/gtp.c
> > +++ b/drivers/net/gtp.c
> > @@ -312,6 +312,11 @@ 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)
> >  {
> > +       if (skb_is_gso(skb)) {
> > +               netdev_dbg(pctx->dev, "GSO is not support in GTP\n");
> 
> Patch looks good to me but there is a small typo here.
> 
>                     netdev_dbg(pctx->dev, "GSO is not supported in GTP\n");
> 
> Reviewed-by: Eric Dumazet <edumazet@google.com>

Ouch, may I have a chance to fix GSO in this driver?

Thanks!