Subject: [PATCH net 02/10] netfilter: synproxy: refresh tcphdr after skb_ensure_writable

From: Florian Westphal <[email-masked]>
Date: Fri, 22 May 2026 12:42:49 +0200

From: Chris Mason <[email-masked]>

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.

Fixes: 48b1de4c110a ("netfilter: add SYNPROXY core/target")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <[email-masked]>
Reviewed-by: Fernando Fernandez Mancera <[email-masked]>
Signed-off-by: Florian Westphal <[email-masked]>
---
 net/netfilter/nf_synproxy_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/netfilter/nf_synproxy_core.c b/net/netfilter/nf_synproxy_core.c
index 57f57e2fc80a..036c8586f49b 100644
--- a/net/netfilter/nf_synproxy_core.c
+++ b/net/netfilter/nf_synproxy_core.c
@@ -200,6 +200,8 @@ synproxy_tstamp_adjust(struct sk_buff *skb, unsigned int protoff,
 	if (skb_ensure_writable(skb, optend))
 		return 0;
 
+	th = (struct tcphdr *)(skb->data + protoff);
+
 	while (optoff < optend) {
 		unsigned char *op = skb->data + optoff;
 
-- 
2.53.0