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:
- 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.
- 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.
- 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.
- 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!