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Message-ID: <4c0cc3b7-7b2b-f9c3-6a1a-6bb44c28d71d@bell-sw.com>
Date: Wed, 22 Mar 2023 16:48:00 +0300
From: Alexey Kodanev <aleksei.kodanev@...l-sw.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] dns: check length field in tcp response message
Hi Rich,
On 22.03.2023 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.
>
OK, I see, something like this or better with sizeof *abuf?
diff --git a/src/network/lookup_name.c b/src/network/lookup_name.c
index 5f6867cb..65b3e8fb 100644
--- a/src/network/lookup_name.c
+++ b/src/network/lookup_name.c
@@ -175,6 +175,7 @@ static int name_from_dns(struct address buf[static MAXADDRS], char canon[static
if (alens[i] < 4 || (abuf[i][3] & 15) == 2) return EAI_AGAIN;
if ((abuf[i][3] & 15) == 3) return 0;
if ((abuf[i][3] & 15) != 0) return EAI_FAIL;
+ if (alens[i] > ABUF_SIZE) alens[i] = ABUF_SIZE;
}
for (i=nq-1; i>=0; i--) {
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