In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh...

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.