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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: cc2d586 ("netfilter: nf_nat_snmp_basic: use asn1 decoder library")
Signed-off-by: Jann Horn <>
Signed-off-by: Pablo Neira Ayuso <>
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thejh authored and ummakynes committed Feb 6, 2019
1 parent 31b58ad commit c4c07b4d6fa1f11880eab8e076d3d060ef3f55fc
Showing with 6 additions and 1 deletion.
  1. +6 −1 net/ipv4/netfilter/nf_nat_snmp_basic_main.c
@@ -105,6 +105,8 @@ static void fast_csum(struct snmp_ctx *ctx, unsigned char offset)
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;
@@ -114,8 +116,11 @@ 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;
__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);

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