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

[Rich Felker <dalias@libc.org>]

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.

Rich