CVE-2026-74621

In the Linux kernel, the following vulnerability has been resolved:

net/sched: act_ct: fix sk_buff leak when the header checks reject a packet

tcf_ct_handle_fragments() runs its header sanity checks before handing anything to the defragmentation engine:

if (family == NFPROTO_IPV4)
	err = tcf_ct_ipv4_is_fragment(skb, &frag);
else
	err = tcf_ct_ipv6_is_fragment(skb, &frag);
if (err || !frag)
	return err;

tcf_ct_ipv4_is_fragment() returns -EINVAL or -ENOMEM; tcf_ct_ipv6_is_fragment() adds -EPROTO when ipv6_find_hdr() fails. None of them frees or queues the skb, so on that path the caller still owns it.

tcf_ct_act() however funnels every non-zero return into the ownership-transfer exit:

err = tcf_ct_handle_fragments(net, skb, family, p->zone, &defrag);
if (err)
	goto out_frag;
...

out_frag: if (err != -EINPROGRESS) tcf_action_inc_drop_qstats(&c->common); return TC_ACT_CONSUMED;

TC_ACT_CONSUMED means the action took ownership of the skb, so no caller frees it - sch_handle_ingress(), sch_handle_egress() and tcf_qevent_handle() all deliberately skip the free for that verdict. The skb is therefore orphaned: one sk_buff plus its data buffer is leaked per malformed packet, unbounded. Note the drop counter is already incremented for these errors, so the statistics claim a drop that never happens.

Three different ownership states reach out_frag: today - the skb may be queued by the defrag engine (-EINPROGRESS), already freed by nf_ct_handle_fragments(), or still owned by us. Tell the caller which of those it is, and free the packet ourselves in the last case, which restores the TC_ACT_SHOT behaviour that predated the Fixes: commit.

Reproduced on v7.2-rc6 with a 54-byte frame carrying a 40-byte IPv6 header with nexthdr = 0 (hop-by-hop) and nothing after it, on a clsact ingress chain with action ct. kmemleak reports one leaked 232-byte skbuff_head_cache object plus its 704-byte data buffer per packet; with this patch it reports none.

Affected Software

References

• https://git.kernel.org/stable/c/23e97d594ddd0153020c506d5041048fbde1beb4
• https://git.kernel.org/stable/c/439d3e404f9d5e515911cc8132cde198b337c19e
• https://git.kernel.org/stable/c/47d99828591d0fe8be4b9c8992ff3b8e47968db9
• https://git.kernel.org/stable/c/737873a59905a54ca0d2d127ef882f3f88bf4379
• https://git.kernel.org/stable/c/8a7ed561671aa6a911a2de99e59ef670a4d0b1df
• https://git.kernel.org/stable/c/b47bb899e04b5407c5a63fe88d4b6676586a6e84
• https://git.kernel.org/stable/c/b5dbecc2016e1692fd1c2532af9c41ba729cb747