linux-kernel - [PATCH 4.19 27/30] netfilter: nf_nat_snmp_basic: add missing length checks in ASN.1 cbs

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Date:   Thu, 21 Feb 2019 15:36:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: [PATCH 4.19 27/30] netfilter: nf_nat_snmp_basic: add missing length checks in ASN.1 cbs

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@...gle.com>

commit c4c07b4d6fa1f11880eab8e076d3d060ef3f55fc upstream.

The generic ASN.1 decoder infrastructure doesn't guarantee that callbacks
will get as much data as they expect; callbacks have to check the `datalen`
parameter before looking at `data`. Make sure that snmp_version() and
snmp_helper() don't read/write beyond the end of the packet data.

(Also move the assignment to `pdata` down below the check to make it clear
that it isn't necessarily a pointer we can use before the `datalen` check.)

Fixes: cc2d58634e0f ("netfilter: nf_nat_snmp_basic: use asn1 decoder library")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 net/ipv4/netfilter/nf_nat_snmp_basic_main.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/net/ipv4/netfilter/nf_nat_snmp_basic_main.c
+++ b/net/ipv4/netfilter/nf_nat_snmp_basic_main.c
@@ -104,6 +104,8 @@ static void fast_csum(struct snmp_ctx *c
 int snmp_version(void *context, size_t hdrlen, unsigned char tag,
 		 const void *data, size_t datalen)
 {
+	if (datalen != 1)
+		return -EINVAL;
 	if (*(unsigned char *)data > 1)
 		return -ENOTSUPP;
 	return 1;
@@ -113,8 +115,11 @@ int snmp_helper(void *context, size_t hd
 		const void *data, size_t datalen)
 {
 	struct snmp_ctx *ctx = (struct snmp_ctx *)context;
-	__be32 *pdata = (__be32 *)data;
+	__be32 *pdata;
 
+	if (datalen != 4)
+		return -EINVAL;
+	pdata = (__be32 *)data;
 	if (*pdata == ctx->from) {
 		pr_debug("%s: %pI4 to %pI4\n", __func__,
 			 (void *)&ctx->from, (void *)&ctx->to);