[PATCH] netfilter: nf_conntrack_sip: fix out-of-bounds read in ct_sip_parse_transport()

[Duc Anh Luu <luuducanh05051995@gmail.com>]

ct_sip_parse_transport() compares the "transport=" parameter value against
"TCP" and "UDP" with a fixed length of 3, without first checking that the
parsed value is that long:

	if (ct_sip_parse_param(ct, dptr, dataoff, datalen, "transport=",
			       &matchoff, &matchlen)) {
		if (!strncasecmp(dptr + matchoff, "TCP", strlen("TCP")))
		...

ct_sip_parse_param() sets the value end to the surrounding limit when no
';' delimiter follows, and that limit can be the end of the SIP message
(dptr + datalen) when no ',' is present either. A Contact/Via parameter
such as "transport=tc" placed at the very end of the payload therefore
yields matchlen < 3 with matchoff + matchlen == datalen, and the
strncasecmp() reads up to two bytes past the end of the message buffer.

The value of the over-read bytes can also flip the detected transport,
so this is not purely a read: a crafted short value can make the helper
classify the transport using out-of-bounds data.

Both call sites (process_register_request() and
process_register_response()) are reached from attacker-supplied SIP
traffic when the nf_conntrack_sip helper is active.

Bound each comparison by matchlen. A value shorter than three characters
can never equal "TCP"/"UDP", so this only rejects already-malformed input
and leaves valid transports (TCP, UDP, TLS, SCTP, ...) unchanged.

Reachability and behavioural equivalence were verified with a userspace
ASan harness built from the verbatim parser functions: the over-read is
confirmed as a read past dptr + datalen, and the fix removes it while
producing identical results for all well-formed transport values.

Fixes: f5b321bd37fb ("netfilter: nf_conntrack_sip: add TCP support")
Signed-off-by: Duc Anh Luu <luuducanh05051995@gmail.com>
---
 net/netfilter/nf_conntrack_sip.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index 64bc440b1181..1f9cf75c7bd5 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -701,9 +701,11 @@ static int ct_sip_parse_transport(struct nf_conn *ct, const char *dptr,
 
 	if (ct_sip_parse_param(ct, dptr, dataoff, datalen, "transport=",
 			       &matchoff, &matchlen)) {
-		if (!strncasecmp(dptr + matchoff, "TCP", strlen("TCP")))
+		if (matchlen >= 3 &&
+		    !strncasecmp(dptr + matchoff, "TCP", strlen("TCP")))
 			*proto = IPPROTO_TCP;
-		else if (!strncasecmp(dptr + matchoff, "UDP", strlen("UDP")))
+		else if (matchlen >= 3 &&
+			 !strncasecmp(dptr + matchoff, "UDP", strlen("UDP")))
 			*proto = IPPROTO_UDP;
 		else
 			return 0;
-- 
2.43.0