[PATCH net v2] xsk: set network header on skb at generic transmit

[Jason Xing <kerneljasonxing@gmail.com>]

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 <kerneljasonxing@gmail.com>
---
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