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Subject[PATCH 4.20 29/32] netfilter: nf_nat_snmp_basic: add missing length checks in ASN.1 cbs
4.20-stable review patch.  If anyone has any objections, please let me know.


From: Jann Horn <>

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 <>
Signed-off-by: Pablo Neira Ayuso <>
Signed-off-by: Greg Kroah-Hartman <>

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
@@ -105,6 +105,8 @@ 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;
@@ -114,8 +116,11 @@ 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);

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