netdev - [PATCH net 3/4] netfilter: nf_conntrack_irc: Tighten matching on DCC message

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Message-Id: <20220908095757.1755-4-fw@strlen.de>
Date:   Thu,  8 Sep 2022 11:57:56 +0200
From:   Florian Westphal <fw@...len.de>
To:     <netdev@...r.kernel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        <netfilter-devel@...r.kernel.org>, David Leadbeater <dgl@....cx>,
        Florian Westphal <fw@...len.de>
Subject: [PATCH net 3/4] netfilter: nf_conntrack_irc: Tighten matching on DCC message

From: David Leadbeater <dgl@....cx>

CTCP messages should only be at the start of an IRC message, not
anywhere within it.

While the helper only decodes packes in the ORIGINAL direction, its
possible to make a client send a CTCP message back by empedding one into
a PING request.  As-is, thats enough to make the helper believe that it
saw a CTCP message.

Fixes: 869f37d8e48f ("[NETFILTER]: nf_conntrack/nf_nat: add IRC helper port")
Signed-off-by: David Leadbeater <dgl@....cx>
Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/netfilter/nf_conntrack_irc.c | 34 ++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/net/netfilter/nf_conntrack_irc.c b/net/netfilter/nf_conntrack_irc.c
index 992decbcaa5c..5703846bea3b 100644
--- a/net/netfilter/nf_conntrack_irc.c
+++ b/net/netfilter/nf_conntrack_irc.c
@@ -157,15 +157,37 @@ static int help(struct sk_buff *skb, unsigned int protoff,
 	data = ib_ptr;
 	data_limit = ib_ptr + datalen;
 
-	/* strlen("\1DCC SENT t AAAAAAAA P\1\n")=24
-	 * 5+MINMATCHLEN+strlen("t AAAAAAAA P\1\n")=14 */
-	while (data < data_limit - (19 + MINMATCHLEN)) {
-		if (memcmp(data, "\1DCC ", 5)) {
+	/* Skip any whitespace */
+	while (data < data_limit - 10) {
+		if (*data == ' ' || *data == '\r' || *data == '\n')
+			data++;
+		else
+			break;
+	}
+
+	/* strlen("PRIVMSG x ")=10 */
+	if (data < data_limit - 10) {
+		if (strncasecmp("PRIVMSG ", data, 8))
+			goto out;
+		data += 8;
+	}
+
+	/* strlen(" :\1DCC SENT t AAAAAAAA P\1\n")=26
+	 * 7+MINMATCHLEN+strlen("t AAAAAAAA P\1\n")=26
+	 */
+	while (data < data_limit - (21 + MINMATCHLEN)) {
+		/* Find first " :", the start of message */
+		if (memcmp(data, " :", 2)) {
 			data++;
 			continue;
 		}
+		data += 2;
+
+		/* then check that place only for the DCC command */
+		if (memcmp(data, "\1DCC ", 5))
+			goto out;
 		data += 5;
-		/* we have at least (19+MINMATCHLEN)-5 bytes valid data left */
+		/* we have at least (21+MINMATCHLEN)-(2+5) bytes valid data left */
 
 		iph = ip_hdr(skb);
 		pr_debug("DCC found in master %pI4:%u %pI4:%u\n",
@@ -181,7 +203,7 @@ static int help(struct sk_buff *skb, unsigned int protoff,
 			pr_debug("DCC %s detected\n", dccprotos[i]);
 
 			/* we have at least
-			 * (19+MINMATCHLEN)-5-dccprotos[i].matchlen bytes valid
+			 * (21+MINMATCHLEN)-7-dccprotos[i].matchlen bytes valid
 			 * data left (== 14/13 bytes) */
 			if (parse_dcc(data, data_limit, &dcc_ip,
 				       &dcc_port, &addr_beg_p, &addr_end_p)) {
-- 
2.35.1