[PATCH net v3] seg6: Fix unset mac header in SRv6 End.DT VRF decapsulation

[Boyan Liu <yymhvert@gmail.com>]

SRv6 End.DT4/End.DT6 VRF decapsulation unsets the mac header in
end_dt_vrf_rcv() so that the VRF device can rebuild it for packet
sniffers. The VRF rx handler only performs the rebuild when a
sniffer is actually attached to the VRF device (ptype_all
non-empty); otherwise the packet continues up the stack with
mac_header == 0xffff.

Every other receive path delivers packets with a valid mac header:
NIC drivers set it in eth_type_trans(), and tunnels without an L2
header (ipip, sit, gre, tun) reset it to the network header after
decapsulation. skb_mac_header() even warns on the unset state with
DEBUG_NET_WARN_ON_ONCE().

If the decapsulated packet is an unexpected UDP GSO packet,
udp_queue_rcv_skb() re-segments it after restoring skb->data to the
mac header with __skb_push(skb, -skb_mac_offset(skb)). With the mac
header unset, skb_mac_offset() returns a large positive value,
which the unsigned __skb_push() argument turns into a huge length:
skb->data moves far outside the buffer and skb->len is corrupted,
so segmentation reads out of bounds:

    BUG: KASAN: slab-out-of-bounds in inet_gso_segment (net/ipv4/af_inet.c:1383)
    Read of size 1 at addr ffff88800da7ffff by task exploit/149
    Call Trace:
     inet_gso_segment (net/ipv4/af_inet.c:1383)
     skb_mac_gso_segment (net/core/gso.c:53)
     __skb_gso_segment (net/core/gso.c:124)
     udp_queue_rcv_skb (net/ipv4/udp.c:2476 net/ipv4/udp.c:2466)
     udp_unicast_rcv_skb (net/ipv4/udp.c:2620)
     ip_local_deliver (net/ipv4/ip_input.c:262)
     seg6_local_input_core (net/ipv6/seg6_local.c:1640)
     lwtunnel_input (net/core/lwtunnel.c:465)
     ipv6_rcv (net/ipv6/ip6_input.c:351)

Restore a valid mac header once the VRF rx handler returns: point
it at the network header, and reset mac_len, which still holds the
ingress frame's L2 header length. The sniffer rebuild happens
inside the handler while the header is still unset, so tap behavior
is unchanged; when no rebuild happened, the packet now carries the
same mac header state as any packet received on an L3 device.

Fixes: 664d6f86868b ("seg6: add support for the SRv6 End.DT4 behavior")
Cc: stable@vger.kernel.org
Reported-by: co+98d7de09cd94fec5@bugs.sh
Signed-off-by: Boyan Liu <yymhvert@gmail.com>
---
v3:
- Also reset skb->mac_len with skb_reset_mac_len(), as suggested by
  Andrea Mayer: it still held the ingress frame's L2 header length.
v2: https://lore.kernel.org/netdev/20261017120144.3059-1-yymhvert@gmail.com/
- Move the fix from the UDP GSO path to seg6local.
- Restore the mac header after the VRF rx handler returns, preserving
  the VRF sniffer behavior.
- Verify the original reproducer, datagram delivery, and VRF sniffer
  output under QEMU.
v1: https://lore.kernel.org/netdev/20261010073015.7665-1-yymhvert@gmail.com/

 net/ipv6/seg6_local.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 7b5212220185..4ef9d32907e5 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c
@@ -1113,6 +1113,16 @@ static struct sk_buff *end_dt_vrf_rcv(struct sk_buff *skb, u16 family,
 		/* the skb buffer was consumed by the handler */
 		return NULL;
 
+	/* the VRF device had its chance to rebuild the mac header for
+	 * sniffers; restore a valid one pointing at the network header,
+	 * like any L3 device without an L2 header, before the packet
+	 * continues up the stack.
+	 */
+	if (!skb_mac_header_was_set(skb)) {
+		skb_reset_mac_header(skb);
+		skb_reset_mac_len(skb);
+	}
+
 	/* when a packet is received by a VRF or by one of its slaves, the
 	 * master device reference is set into the skb.
 	 */
-- 
2.43.0