In the Linux kernel, the following vulnerability has been resolved: netfilter: synproxy: refresh tcphdr after skb_ensure_writable synproxy

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:

 1caller (ipv[46]_synproxy_hook)
 2  th = skb_header_pointer(skb, ..., &_tcph)
 3  synproxy_tstamp_adjust(skb, protoff, th, ...)
 4    skb_ensure_writable(skb, optend)
 5      pskb_expand_head()        /* kfree(old skb->head) */
 6    ...
 7    inet_proto_csum_replace4(&th->check, ...)
 8                                /* writes into freed head, or
 9                                   into the caller's stack copy
10                                   leaving the on-wire checksum
11                                   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.