High [CVE-2026-64007] refresh tcphdr after skb_ensure_writable

In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh tcphdr after skb_ensure_writable synproxy_tstamp_adjust() rewrites the TCP timestamp option in place and then patches the TCP checksum via inet_proto_csum_replace4() on the caller-supplied tcphdr pointer.

Both ipv4_synproxy_hook() and ipv6_synproxy_hook() obtain that pointer with skb_header_pointer() before calling in, so it may either alias skb->head directly or point at the caller's on-stack _tcph buffer.

Between obtaining the pointer and using it, the function calls skb_ensure_writable(skb, optend), which on a cloned or non-linear skb invokes pskb_expand_head() and frees the old skb->head.

After that point the cached th is stale: caller (ipv[46]_synproxy_hook) th = skb_header_pointer(skb,..., &_tcph) synproxy_tstamp_adjust(skb, protoff, th,...) skb_ensure_writable(skb, optend) pskb_expand_head() /* kfree(old skb->head) */... inet_proto_csum_replace4(&th->check,...) /* writes into freed head, or into the caller's stack copy leaving the on-wire checksum stale */ The option bytes are written through skb->data and are fine; only the checksum update goes through th and so lands in the wrong place.

The result is either a write into freed slab memory or a packet leaving with a checksum that does not match its payload.