[PATCH nf] netfilter: nf_conntrack_sip: only honour Via: port in original direction

[Joas Antonio dos Santos <joasantonio108@gmail.com>]

The Cisco phone workaround in process_sip_request() records the port
from the topmost Via: header in ct_sip_info->forced_dport whenever the
Via address matches the sender of the current request and the port
differs from the sender's source port.  nf_nat_sip() then rewrites the
UDP destination port of every reply-direction packet to that value.

The check is performed for requests in both directions.  For a request
arriving in the reply direction, ct->tuplehash[dir].tuple.src is the
remote peer's own address and port, so the remote side can trivially
satisfy the condition with a Via: line of the form

  Via: SIP/2.0/UDP <own-address>:<any-port>

and force forced_dport to a port of its choosing.  From then on,
packets it sends on this flow are delivered to the NATed host at that
port instead of the port the NAT mapping was created for, which bypasses
the port restriction normally enforced by the NAT mapping.

The workaround is only meant for phones behind the NAT router, i.e.
requests in the original direction.  Restrict it accordingly.

Fixes: 7266507d8999 ("netfilter: nf_ct_sip: support Cisco 7941/7945 IP phones")
Signed-off-by: Joas Antonio dos Santos <joasantonio108@gmail.com>
Assisted-by: Claude:claude-opus-5-5
---
Tested on 602042bf29f6 (arm64, QEMU) with client -> router -> server
network namespaces; the router runs nft masquerade and a "sip" ct helper
on udp/5060.  The client sends an OPTIONS request from port 5060; the
server answers with a request carrying "Via: SIP/2.0/UDP <server>:4444"
and then a plain UDP payload on the same flow.

  before: client receives both datagrams on port 4444
  after:  client receives both datagrams on port 5060

 net/netfilter/nf_conntrack_sip.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index 64bc440b1..ef655a0bb 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -1586,8 +1586,14 @@ static int process_sip_request(struct sk_buff *skb, unsigned int protoff,
 	 * router for one of these phones, save the port number from the
 	 * Via: header so that nf_nat_sip can redirect the responses to
 	 * the correct port.
+	 *
+	 * Only honour the Via: port for requests coming from the phone,
+	 * i.e. in the original direction.  The remote peer must not be
+	 * able to pick the destination port of packets delivered to the
+	 * NATed host.
 	 */
-	if (ct_sip_parse_header_uri(ct, *dptr, NULL, *datalen,
+	if (dir == IP_CT_DIR_ORIGINAL &&
+	    ct_sip_parse_header_uri(ct, *dptr, NULL, *datalen,
 				    SIP_HDR_VIA_UDP, NULL, &matchoff,
 				    &matchlen, &addr, &port) > 0 &&
 	    port != ct->tuplehash[dir].tuple.src.u.udp.port &&