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

["dongchenchen (A)" <dongchenchen2@huawei.com>]

在 2026/8/25 17:59, Eric Dumazet 写道:
> 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>
> 
> Thanks.

Thanks for review!
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