[PATCH] netfilter: nf_conntrack_h323: check for zero length in DecodeQ931()

[Jenny Guanni Qu <qguanni@gmail.com>]

In DecodeQ931(), the UserUserIE code path reads a 16-bit length from
the packet, then decrements it by 1 to skip the protocol discriminator
byte before passing it to DecodeH323_UserInformation(). If the encoded
length is 0, the decrement wraps to -1, which is then passed as a
large value to the decoder, leading to an out-of-bounds read.

Add a check to ensure len is positive after the decrement.

Fixes: 5e35941d9901 ("[NETFILTER]: Add H.323 conntrack/NAT helper")
Reported-by: Klaudia Kloc <klaudia@vidocsecurity.com>
Reported-by: Dawid Moczadło <dawid@vidocsecurity.com>
Tested-by: Jenny Guanni Qu <qguanni@gmail.com>
Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
---
 net/netfilter/nf_conntrack_h323_asn1.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/netfilter/nf_conntrack_h323_asn1.c b/net/netfilter/nf_conntrack_h323_asn1.c
index 540d97715bd2..ca103c946190 100644
--- a/net/netfilter/nf_conntrack_h323_asn1.c
+++ b/net/netfilter/nf_conntrack_h323_asn1.c
@@ -922,6 +922,8 @@ int DecodeQ931(unsigned char *buf, size_t sz, Q931 *q931)
 				break;
 			p++;
 			len--;
+			if (len <= 0)
+				break;
 			return DecodeH323_UserInformation(buf, p, len,
 							  &q931->UUIE);
 		}
-- 
2.34.1