Re: [musl] [PATCH] dns: check length field in tcp response message

[Alexey Izbyshev <izbyshev@ispras.ru>]

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On 2023-03-22 16:16, Rich Felker wrote:
> On Wed, Mar 22, 2023 at 03:29:16PM +0300, Alexey Kodanev wrote:
>> The received length field in the message may be greater than the
>> size of the 'answer' buffer in which the message resides. Currently,
>> ABUF_SIZE is 768. And if we get a larger 'alen', it will result
>> in an out-of-bounds reading during parsing, because 'alen' will
>> be passed to __dns_parse() later:
>> 
>>     __dns_parse(abuf[i], alens[i], dns_parse_callback, &ctx);
>> 
>> To fix this, limit 'alen' to the size of the received buffer.
>> ---
>>  src/network/res_msend.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/src/network/res_msend.c b/src/network/res_msend.c
>> index fef7e3a2..291853de 100644
>> --- a/src/network/res_msend.c
>> +++ b/src/network/res_msend.c
>> @@ -297,6 +297,7 @@ int __res_msend_rc(int nqueries, const unsigned 
>> char *const *queries,
>>  			int rcode = answers[i][3] & 15;
>>  			if (rcode != 0 && rcode != 3)
>>  				goto out;
>> +			if (alen > asize) alen = asize;
>> 
>>  			/* Storing the length here commits the accepted answer.
>>  			 * Immediately close TCP socket so as not to consume
>> --
>> 2.25.1
> 
> This is incorrect. It breaks res_send, whose contract is to return the
> full answer length even if it did not fit, so that the caller can
> retry with the appropriate size.
> 
> Instead, name_from_dns just needs to clamp the value before passing it
> to __dns_parse.
> 
Not only name_from_dns, but also getnameinfo. The patch is attached.

Alexey

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From 3073d26361bcb9e9f1e9ab998440ae3b321fe830 Mon Sep 17 00:00:00 2001
From: Alexey Izbyshev <izbyshev@ispras.ru>
Date: Mon, 8 May 2023 19:03:46 +0300
Subject: [PATCH] getnameinfo: fix calling __dns_parse with potentially too
 large rlen
Mail-Followup-To: musl@lists.openwall.com

__res_send returns the full answer length even if it didn't fit the
buffer, but __dns_parse expects the length of the filled part of the
buffer.

This is analogous to commit 77327ed064bd57b0e1865cd0e0364057ff4a53b4,
which fixed the only other __dns_parse call site.
---
 src/network/getnameinfo.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/network/getnameinfo.c b/src/network/getnameinfo.c
index 7abe0fa9..133c15b3 100644
--- a/src/network/getnameinfo.c
+++ b/src/network/getnameinfo.c
@@ -162,8 +162,10 @@ int getnameinfo(const struct sockaddr *restrict sa, socklen_t sl,
 			query[3] = 0; /* don't need AD flag */
 			int rlen = __res_send(query, qlen, reply, sizeof reply);
 			buf[0] = 0;
-			if (rlen > 0)
+			if (rlen > 0) {
+				if (rlen > sizeof reply) rlen = sizeof reply;
 				__dns_parse(reply, rlen, dns_parse_callback, buf);
+			}
 		}
 		if (!*buf) {
 			if (flags & NI_NAMEREQD) return EAI_NONAME;
-- 
2.39.2