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Tue, 01 Sep 2026 01:33:34 -0700 (PDT) Received: from KERNELXING-MB0.tencent.com ([43.132.141.20]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3990d49e9e0sm4656501a91.11.2026.09.01.01.33.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Sep 2026 01:33:33 -0700 (PDT) From: Jason Xing To: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, aleksander.lobakin@intel.com Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing , syzbot+f16c1b285c9f63994eec@syzkaller.appspotmail.com Subject: [PATCH net v2] xsk: set network header on skb at generic transmit Date: Tue, 1 Sep 2026 16:33:25 +0800 Message-Id: <20260901083325.3445-1-kerneljasonxing@gmail.com> X-Mailer: git-send-email 2.33.0 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit When constructing an skb in the XSK generic transmit path, neither xsk_build_skb() nor xsk_build_skb_zerocopy() calls skb_reset_network_header(). After __finalize_skb_around(), the network_header offset defaults to 0 (pointing to skb->head). The subsequent skb_reserve(skb, hr) moves skb->data forward by hr bytes, but leaves network_header still at 0. This makes skb_network_offset() return a negative value (-hr). When such an skb reaches a tunnel driver like ipgre_xmit(), the call chain pskb_inet_may_pull() -> pskb_network_may_pull_reason() computes: pskb_may_pull_reason(skb, skb_network_offset(skb) + nhlen) The negative int from skb_network_offset() is implicitly promoted to unsigned int, wrapping around to a huge value. This triggers the DEBUG_NET_WARN_ON_ONCE(len > INT_MAX) in pskb_may_pull_reason(). Call trace from syzkaller: ipgre_xmit+0x958/0xcd0 net/ipv4/ip_gre.c:658 __netdev_start_xmit include/linux/netdevice.h:5400 netdev_start_xmit include/linux/netdevice.h:5409 __dev_direct_xmit+0x4b6/0x730 net/core/dev.c:4942 __xsk_generic_xmit net/xdp/xsk.c:1079 xsk_generic_xmit+0x277a/0x40a0 net/xdp/xsk.c:1120 Reset the network header so skb_network_offset() is valid (0). Note this anchors it at the start of the frame (L2 for an ethernet device), which is the same convention AF_PACKET uses on TX. Unlike AF_PACKET, AF_XDP does not need to locate the real L2/L3 boundary or probe the transport header: an AF_XDP frame is always a complete, opaque L2 blob from userspace. AF_PACKET only cares about the boundary because it can cook an L2 header itself (SOCK_DGRAM) and supports GSO/checksum offload; none of that applies here, where the frame is sent as-is via __dev_direct_xmit(). So resetting the network header is enough to restore the invariant and avoid the crash. Fixes: 3914d88f7608 ("xsk: Respect device's headroom and tailroom on generic xmit path") Reported-by: syzbot+f16c1b285c9f63994eec@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=f16c1b285c9f63994eec Signed-off-by: Jason Xing --- V2 Link: https://lore.kernel.org/all/20260826012052.51028-1-kerneljasonxing@gmail.com/ 1. add more clarification about why we don't probe the header like packet at the end of the commit message (Jakub) --- net/xdp/xsk.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c index 33475b180ea6..2077bb6299ca 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c @@ -947,6 +947,8 @@ static int xsk_skb_init_misc(struct sk_buff *skb, struct xdp_sock *xs, { int err; + skb_reset_network_header(skb); + err = xsk_skb_destructor_set_addr(skb, addr); if (unlikely(err)) return err; -- 2.43.5