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dns: reject messages with lengths larger than DNSHeader with prejudice

This also includes when decompressing name entries.
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commit 9aa28f3730fb3dd69c1e06f78bb2bbc43d36c684 1 parent a6a07de
@kaniini kaniini authored
Showing with 33 additions and 10 deletions.
  1. +33 −10 src/dns.cpp
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43 src/dns.cpp
@@ -38,6 +38,8 @@ looks like this, walks like this or tastes like this.
#include "configreader.h"
#include "socket.h"
+#define DN_COMP_BITMASK 0xC000 /* highest 6 bits in a DN label header */
+
/** Masks to mask off the responses we get from the DNSRequest methods
*/
enum QueryInfo
@@ -161,7 +163,10 @@ int CachedQuery::CalcTTLRemaining()
/* Allocate the processing buffer */
DNSRequest::DNSRequest(DNS* dns, int rid, const std::string &original) : dnsobj(dns)
{
- res = new unsigned char[512];
+ /* hardening against overflow here: make our work buffer twice the theoretical
+ * maximum size so that hostile input doesn't screw us over.
+ */
+ res = new unsigned char[sizeof(DNSHeader) * 2];
*res = 0;
orig = original;
RequestTimeout* RT = new RequestTimeout(ServerInstance->Config->dns_timeout ? ServerInstance->Config->dns_timeout : 5, this, rid);
@@ -690,9 +695,9 @@ DNSResult DNS::GetResult()
/** A result is ready, process it */
DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
{
- unsigned i = 0;
+ unsigned i = 0, o;
int q = 0;
- int curanswer, o;
+ int curanswer;
ResourceRecord rr;
unsigned short ptr;
@@ -717,7 +722,7 @@ DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
/* Subtract the length of the header from the length of the packet */
length -= 12;
- while ((unsigned int)q < header.qdcount && i < length)
+ while ((unsigned int)q < header.qdcount && i < (unsigned) length)
{
if (header.payload[i] > 63)
{
@@ -738,7 +743,7 @@ DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
while ((unsigned)curanswer < header.ancount)
{
q = 0;
- while (q == 0 && i < length)
+ while (q == 0 && i < (unsigned) length)
{
if (header.payload[i] > 63)
{
@@ -755,7 +760,7 @@ DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
else i += header.payload[i] + 1; /* skip length and label */
}
}
- if (length - i < 10)
+ if ((unsigned) length - i < 10)
return std::make_pair((unsigned char*)NULL,"Incorrectly sized DNS reply");
/* XXX: We actually initialise 'rr' here including its ttl field */
@@ -790,17 +795,31 @@ DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
switch (rr.type)
{
+ /*
+ * CNAME and PTR are compressed. We need to decompress them.
+ */
case DNS_QUERY_CNAME:
- /* CNAME and PTR have the same processing code */
case DNS_QUERY_PTR:
o = 0;
q = 0;
- while (q == 0 && i < length && o + 256 < 1023)
+ while (q == 0 && i < (unsigned) length && o + 256 < 1023)
{
+ /* DN label found (byte over 63) */
if (header.payload[i] > 63)
{
memcpy(&ptr,&header.payload[i],2);
- i = ntohs(ptr) - 0xC000 - 12;
+
+ i = ntohs(ptr);
+
+ /* check that highest two bits are set. if not, we've been had */
+ if (!(i & DN_COMP_BITMASK))
+ return std::make_pair((unsigned char *) NULL, "DN label decompression header is bogus");
+
+ /* mask away the two highest bits. */
+ i &= ~DN_COMP_BITMASK;
+
+ /* and decrease length by 12 bytes. */
+ i =- 12;
}
else
{
@@ -813,7 +832,11 @@ DNSInfo DNSRequest::ResultIsReady(DNSHeader &header, int length)
res[o] = 0;
if (o != 0)
res[o++] = '.';
- memcpy(&res[o],&header.payload[i + 1],header.payload[i]);
+
+ if (o + header.payload[i] > sizeof(DNSHeader))
+ return std::make_pair((unsigned char *) NULL, "DN label decompression is impossible -- malformed/hostile packet?");
+
+ memcpy(&res[o], &header.payload[i + 1], header.payload[i]);
o += header.payload[i];
i += header.payload[i] + 1;
}
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