From 35ea4ea680d6794f01d54eec2b1e507a45f422cf Mon Sep 17 00:00:00 2001
From: Florian Westphal <fwestpha@redhat.com>
Date: Tue, 26 May 2026 13:22:20 +0200
Subject: [PATCH] netfilter: synproxy: refresh tcphdr after skb_ensure_writable

JIRA: https://redhat.atlassian.net/browse/RHEL-168848
Upstream Status: commit 92170e6afe92

commit 92170e6afe927ab2792a3f71902845789c8e31b1
Author: Chris Mason <clm@meta.com>
Date:   Tue May 19 12:36:14 2026 -0700

    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.

    Fixes: 48b1de4c110a ("netfilter: add SYNPROXY core/target")
    Assisted-by: kres (claude-opus-4-7)
    Signed-off-by: Chris Mason <clm@meta.com>
    Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
    Signed-off-by: Florian Westphal <fw@strlen.de>

Signed-off-by: Florian Westphal <fwestpha@redhat.com>

diff --git a/net/netfilter/nf_synproxy_core.c b/net/netfilter/nf_synproxy_core.c
index 3fa3f5d..6a851ac 100644
--- a/net/netfilter/nf_synproxy_core.c
+++ b/net/netfilter/nf_synproxy_core.c
@@ -199,6 +199,8 @@ 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;