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ares_create_query: avoid single-byte buffer overwrite

... when the name ends with an escaped dot.

CVE-2016-5180

Bug: https://c-ares.haxx.se/adv_20160929.html
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bagder committed Sep 23, 2016
1 parent 51fbb47 commit 65c71be1cbe587f290432bef2f669ee6cb8ac137
Showing with 39 additions and 45 deletions.
  1. +39 −45 ares_create_query.c
@@ -85,57 +85,31 @@
*/
int ares_create_query(const char *name, int dnsclass, int type,
unsigned short id, int rd, unsigned char **buf,
int *buflen, int max_udp_size)
unsigned short id, int rd, unsigned char **bufp,
int *buflenp, int max_udp_size)
{
int len;
size_t len;
unsigned char *q;
const char *p;
size_t buflen;
unsigned char *buf;
/* Set our results early, in case we bail out early with an error. */
*buflen = 0;
*buf = NULL;
*buflenp = 0;
*bufp = NULL;
/* Compute the length of the encoded name so we can check buflen.
* Start counting at 1 for the zero-length label at the end. */
len = 1;
for (p = name; *p; p++)
{
if (*p == '\\' && *(p + 1) != 0)
p++;
len++;
}
/* If there are n periods in the name, there are n + 1 labels, and
* thus n + 1 length fields, unless the name is empty or ends with a
* period. So add 1 unless name is empty or ends with a period.
/* Allocate a memory area for the maximum size this packet might need. +2
* is for the length byte and zero termination if no dots or ecscaping is
* used.
*/
if (*name && *(p - 1) != '.')
len++;
/* Immediately reject names that are longer than the maximum of 255
* bytes that's specified in RFC 1035 ("To simplify implementations,
* the total length of a domain name (i.e., label octets and label
* length octets) is restricted to 255 octets or less."). We aren't
* doing this just to be a stickler about RFCs. For names that are
* too long, 'dnscache' closes its TCP connection to us immediately
* (when using TCP) and ignores the request when using UDP, and
* BIND's named returns ServFail (TCP or UDP). Sending a request
* that we know will cause 'dnscache' to close the TCP connection is
* painful, since that makes any other outstanding requests on that
* connection fail. And sending a UDP request that we know
* 'dnscache' will ignore is bad because resources will be tied up
* until we time-out the request.
*/
if (len > MAXCDNAME)
return ARES_EBADNAME;
*buflen = len + HFIXEDSZ + QFIXEDSZ + (max_udp_size ? EDNSFIXEDSZ : 0);
*buf = ares_malloc(*buflen);
if (!*buf)
return ARES_ENOMEM;
len = strlen(name) + 2 + HFIXEDSZ + QFIXEDSZ +
(max_udp_size ? EDNSFIXEDSZ : 0);
buf = ares_malloc(len);
if (!buf)
return ARES_ENOMEM;
/* Set up the header. */
q = *buf;
q = buf;
memset(q, 0, HFIXEDSZ);
DNS_HEADER_SET_QID(q, id);
DNS_HEADER_SET_OPCODE(q, QUERY);
@@ -159,8 +133,10 @@ int ares_create_query(const char *name, int dnsclass, int type,
q += HFIXEDSZ;
while (*name)
{
if (*name == '.')
if (*name == '.') {
free (buf);
return ARES_EBADNAME;
}
/* Count the number of bytes in this label. */
len = 0;
@@ -170,8 +146,10 @@ int ares_create_query(const char *name, int dnsclass, int type,
p++;
len++;
}
if (len > MAXLABEL)
if (len > MAXLABEL) {
free (buf);
return ARES_EBADNAME;
}
/* Encode the length and copy the data. */
*q++ = (unsigned char)len;
@@ -195,14 +173,30 @@ int ares_create_query(const char *name, int dnsclass, int type,
DNS_QUESTION_SET_TYPE(q, type);
DNS_QUESTION_SET_CLASS(q, dnsclass);
q += QFIXEDSZ;
if (max_udp_size)
{
q += QFIXEDSZ;
memset(q, 0, EDNSFIXEDSZ);
q++;
DNS_RR_SET_TYPE(q, T_OPT);
DNS_RR_SET_CLASS(q, max_udp_size);
q += (EDNSFIXEDSZ-1);
}
buflen = (q - buf);
/* Reject names that are longer than the maximum of 255 bytes that's
* specified in RFC 1035 ("To simplify implementations, the total length of
* a domain name (i.e., label octets and label length octets) is restricted
* to 255 octets or less."). */
if (buflen > (MAXCDNAME + HFIXEDSZ + QFIXEDSZ +
(max_udp_size ? EDNSFIXEDSZ : 0))) {
free (buf);
return ARES_EBADNAME;
}
/* we know this fits in an int at this point */
*buflenp = (int) buflen;
*bufp = buf;
return ARES_SUCCESS;
}

3 comments on commit 65c71be

@sumitb

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sumitb replied Feb 9, 2017

I'm getting following warnings after this patch, are these ignorable?

c-ares/ares_create_query.c: In function ‘ares_create_query’:
c-ares/ares_create_query.c:107:3: warning: implicit declaration of function ‘ares_malloc’ [-Wimplicit-function-declaration]
c-ares/ares_create_query.c:107:7: warning: assignment makes pointer from integer without a cast
@bagder

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bagder replied Feb 9, 2017

This commit didn't introduce the use of ares_malloc so surely it happened already before then?

This source file includes ares_private.h where the ares_malloc function pointer is extern-declared. I get no warnings with current git.

@sumitb

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sumitb replied Feb 10, 2017

I apologize I had an earlier version of this file which was missing few commits.

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