CVE-2026-64007

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.

Fix by re-deriving th from skb->data + protoff immediately after

skb_ensure_writable() succeeds, so the subsequent checksum update

targets the linear, writable header.