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Fixes for undefined behavior in the snmp decoder #1345

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Fixes for undefined behavior in the snmp decoder #1345

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b62bf24
Avoid undefined behavior when handling large ASN.1 integers
fenner Jul 22, 2025
2eb5203
Avoid undefined behavior when decoding OID values in print-snmp
fenner Jul 22, 2025
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18 changes: 13 additions & 5 deletions print-snmp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -531,18 +531,26 @@ asn1_parse(netdissect_options *ndo,

case INTEGER: {
uint32_t data;
uint32_t signbit = 0;
elem->type = BE_INT;
data = 0;

if (elem->asnlen == 0) {
ND_PRINT("[asnlen=0]");
goto invalid;
}
if (GET_U_1(p) & ASN_BIT8) /* negative */
if (GET_U_1(p) & ASN_BIT8) { /* negative */
data = UINT_MAX;
signbit = 0x80000000;
}
/* In order to avoid undefined behavior, we mask off the low
* 24 bits of the data before shifting up by 8 bits. Either
* way, we lose the top 8 bits, but shifting through the sign
* bit results in undefined data. If we've shifted the sign
* bit away, we will restore it below. */
for (i = elem->asnlen; i != 0; p++, i--)
data = (data << ASN_SHIFT8) | GET_U_1(p);
elem->data.integer = data;
data = ( ( data & 0x00ffffff ) << ASN_SHIFT8) | GET_U_1(p);
elem->data.integer = (int32_t)( data | signbit );
break;
}

Expand Down Expand Up @@ -748,7 +756,7 @@ asn1_print(netdissect_options *ndo,
}

for (; i != 0; p++, i--) {
o = (o << ASN_SHIFT7) + (GET_U_1(p) & ~ASN_BIT8);
o = ( ( o & 0x01ffffff ) << ASN_SHIFT7) + (GET_U_1(p) & ~ASN_BIT8);
if (GET_U_1(p) & ASN_LONGLEN)
continue;

Expand Down Expand Up @@ -898,7 +906,7 @@ smi_decode_oid(netdissect_options *ndo,
unsigned int firstval;

for (*oidlen = 0; i != 0; p++, i--) {
o = (o << ASN_SHIFT7) + (GET_U_1(p) & ~ASN_BIT8);
o = ( ( o & 0x01ffffff ) << ASN_SHIFT7) + (GET_U_1(p) & ~ASN_BIT8);
if (GET_U_1(p) & ASN_LONGLEN)
continue;

Expand Down