[PATCH net] net/sched: sch_frag: reject a negative L2 length in sch_fragment()

sch_fragment() bounds the L2 header length only from above:

	if (skb_network_offset(skb) > VLAN_ETH_HLEN)

skb_network_offset() returns int and the comparison is signed, so a
negative offset passes. sch_frag_prepare_frag() then stores it in an
unsigned int and uses it as a memcpy() length into the 18-byte per-CPU
l2_data buffer:

	unsigned int hlen = skb_network_offset(skb);
	memcpy(&data->l2_data, skb->data, hlen);

The offset is negative whenever the network header sits behind
skb->data. ipv6_srh_rcv() runs in that state, because
ip6_protocol_deliver_rcu() pulls each extension header as it walks the
chain and the segments_left > 0 branch pushes back only
sizeof(struct ipv6hdr). act_ct enables the static key that gates
sch_frag_xmit_hook() and sets tc_skb_cb(skb)->mru, which survives
receive and forward, so an act_mirred redirect from a root qdisc can
reach sch_fragment() with the offset still negative.

A negative L2 header length is invalid, so reject it through the existing
"L2 header too long to fragment" path. A non-negative offset behaves
exactly as before.

Reproduced on v7.2 by an unprivileged user with only
unshare(CLONE_NEWUSER|CLONE_NEWNET). RDX is the truncated length,
(unsigned int)(-42):

  RIP: 0010:memcpy+0x8/0x20
  RDX: 00000000ffffffd6
   sch_frag_prepare_frag+0x364/0x440   net/sched/sch_frag.c:74
   sch_fragment+0x1c7/0x7a0            net/sched/sch_frag.c:122
   tcf_mirred_to_dev+0x7a9/0xf10
   ip6_forward+0x114b/0x31b0
   ipv6_rthdr_rcv+0x40d7/0x5e10

With the patch applied the drop path fires and the packet is discarded.

Fixes: c129412f74e9 ("net/sched: sch_frag: add generic packet fragment support.")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <redacted>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Fourie Zhang <redacted>
---
 net/sched/sch_frag.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_frag.c b/net/sched/sch_frag.c
index 75ee52750919..b68a198b1646 100644
--- a/net/sched/sch_frag.c
+++ b/net/sched/sch_frag.c

@@ -89,9 +89,10 @@ static struct dst_ops sch_frag_dst_ops = {
 static int sch_fragment(struct net *net, struct sk_buff *skb,
 			u16 mru, int (*xmit)(struct sk_buff *skb))
 {
+	int hlen = skb_network_offset(skb);
 	int ret = -1;
 
-	if (skb_network_offset(skb) > VLAN_ETH_HLEN) {
+	if (hlen < 0 || hlen > VLAN_ETH_HLEN) {
 		net_warn_ratelimited("L2 header too long to fragment\n");
 		goto err;
 	}

-- 
2.43.7