Code: int snmp_helper(void *context, size_t hdrlen, unsigned char tag,
 		const void *data, size_t datalen)
 {
 	struct snmp_ctx *ctx = (struct snmp_ctx *)context;
	__be32 *pdata = (__be32 *)data;
 
 	if (*pdata == ctx->from) {
 		pr_debug("%s: %pI4 to %pI4\n", __func__,
 			 (void *)&ctx->from, (void *)&ctx->to);

		if (*ctx->check)
			fast_csum(ctx, (unsigned char *)data - ctx->begin);
		*pdata = ctx->to;
	}

	return 1;
}

Vulnerability Type: 
CWE ID: CWE-129
Summary: In the Linux kernel before 4.20.12, net/ipv4/netfilter/nf_nat_snmp_basic_main.c in the SNMP NAT module has insufficient ASN.1 length checks (aka an array index error), making out-of-bounds read and write operations possible, leading to an OOPS or local privilege escalation. This affects snmp_version and snmp_helper.
Commit Message: netfilter: nf_nat_snmp_basic: add missing length checks in ASN.1 cbs

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@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>