[PATCH 1/4] resolv: Allow short error responses to match any query (bug 31890)

[Sam James]

Florian Weimer <fweimer@redhat.com> writes:

> ---
>  resolv/Makefile                    |   3 +
>  resolv/res_send.c                  |  29 +++++---
>  resolv/tst-resolv-short-response.c | 112 +++++++++++++++++++++++++++++
>  3 files changed, 134 insertions(+), 10 deletions(-)
>  create mode 100644 resolv/tst-resolv-short-response.c
>
> diff --git a/resolv/Makefile b/resolv/Makefile
> index 5f44f5896b..d927e337d9 100644
> --- a/resolv/Makefile
> +++ b/resolv/Makefile
> @@ -106,6 +106,7 @@ tests += \
>    tst-resolv-nondecimal \
>    tst-resolv-res_init-multi \
>    tst-resolv-search \
> +  tst-resolv-short-response \
>    tst-resolv-trailing \
>  
>  # This test calls __res_context_send directly, which is not exported
> @@ -299,6 +300,8 @@ $(objpfx)tst-resolv-nondecimal: $(objpfx)libresolv.so $(shared-thread-library)
>  $(objpfx)tst-resolv-qtypes: $(objpfx)libresolv.so $(shared-thread-library)
>  $(objpfx)tst-resolv-rotate: $(objpfx)libresolv.so $(shared-thread-library)
>  $(objpfx)tst-resolv-search: $(objpfx)libresolv.so $(shared-thread-library)
> +$(objpfx)tst-resolv-short-response: $(objpfx)libresolv.so \
> +  $(shared-thread-library)
>  $(objpfx)tst-resolv-trailing: $(objpfx)libresolv.so $(shared-thread-library)
>  $(objpfx)tst-resolv-threads: $(objpfx)libresolv.so $(shared-thread-library)
>  $(objpfx)tst-resolv-txnid-collision: $(objpfx)libresolv.a \
> diff --git a/resolv/res_send.c b/resolv/res_send.c
> index ea7cf192b2..572e72c32f 100644
> --- a/resolv/res_send.c
> +++ b/resolv/res_send.c
> @@ -1199,19 +1199,30 @@ send_dg(res_state statp,
>  		}
>  
>  		/* Check for the correct header layout and a matching
> -		   question.  */
> +		   question.  Some recursive resolvers send REFUSED
> +		   without copying back the question section
> +		   (producing a response that is only HFIXEDSZ bytes
> +		   long).  Skip query matching in this case.  */
> +		bool thisansp_error = (anhp->rcode == SERVFAIL ||
> +				       anhp->rcode == NOTIMP ||
> +				       anhp->rcode == REFUSED);
> +		bool skip_query_match = (*thisresplenp == HFIXEDSZ
> +					 && ntohs (anhp->qdcount) == 0
> +					 && thisansp_error);
>  		int matching_query = 0; /* Default to no matching query.  */
>  		if (!recvresp1
>  		    && anhp->id == hp->id
> -		    && __libc_res_queriesmatch (buf, buf + buflen,
> -						*thisansp,
> -						*thisansp + *thisanssizp))
> +		    && (skip_query_match
> +			|| __libc_res_queriesmatch (buf, buf + buflen,
> +						    *thisansp,
> +						    *thisansp + *thisanssizp)))
>  		  matching_query = 1;
>  		if (!recvresp2
>  		    && anhp->id == hp2->id
> -		    && __libc_res_queriesmatch (buf2, buf2 + buflen2,
> -						*thisansp,
> -						*thisansp + *thisanssizp))
> +		    && (skip_query_match
> +			|| __libc_res_queriesmatch (buf2, buf2 + buflen2,
> +						    *thisansp,
> +						    *thisansp + *thisanssizp)))
>  		  matching_query = 2;
>  		if (matching_query == 0)
>  		  /* Spurious UDP packet.  Drop it and continue
> @@ -1221,9 +1232,7 @@ send_dg(res_state statp,
>  		    goto wait;
>  		  }
>  
> -		if (anhp->rcode == SERVFAIL ||
> -		    anhp->rcode == NOTIMP ||
> -		    anhp->rcode == REFUSED) {
> +		if (thisansp_error) {
>  		next_ns:
>  			if (recvresp1 || (buf2 != NULL && recvresp2)) {
>  			  *resplen2 = 0;
> diff --git a/resolv/tst-resolv-short-response.c b/resolv/tst-resolv-short-response.c
> new file mode 100644
> index 0000000000..0647de0704
> --- /dev/null
> +++ b/resolv/tst-resolv-short-response.c
> @@ -0,0 +1,112 @@
> +/* Test for spurious timeouts with short 12-byte responses (bug 31890).
> +   Copyright (C) 2024 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <resolv.h>
> +#include <support/check.h>
> +#include <support/resolv_test.h>
> +#include <support/check_nss.h>
> +
> +/* The rcode in the initial response.  */
> +static volatile int rcode;
> +
> +static void
> +response (const struct resolv_response_context *ctx,
> +          struct resolv_response_builder *b,
> +          const char *qname, uint16_t qclass, uint16_t qtype)
> +{
> +  switch (ctx->server_index)
> +    {
> +    case 0:
> +      /* First server times out.  */
> +      struct resolv_response_flags flags = {.rcode = rcode};
> +      resolv_response_init (b, flags);
> +      break;
> +    case 1:
> +      /* Second server sends reply.  */
> +      resolv_response_init (b, (struct resolv_response_flags) {});
> +      resolv_response_add_question (b, qname, qclass, qtype);
> +      resolv_response_section (b, ns_s_an);
> +      resolv_response_open_record (b, qname, qclass, qtype, 0);
> +      switch (qtype)
> +        {
> +        case T_A:
> +          {
> +            char ipv4[4] = {192, 0, 2, 17};
> +            resolv_response_add_data (b, &ipv4, sizeof (ipv4));
> +          }
> +          break;
> +        case T_AAAA:
> +          {
> +            char ipv6[16]
> +              = {0x20, 0x01, 0xd, 0xb8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1};
> +            resolv_response_add_data (b, &ipv6, sizeof (ipv6));
> +          }
> +          break;
> +        default:
> +          FAIL_EXIT1 ("unexpected TYPE%d query", qtype);
> +        }
> +      resolv_response_close_record (b);
> +      break;
> +    default:
> +      FAIL_EXIT1 ("unexpected query to server %d", ctx->server_index);
> +    }
> +}
> +
> +static void
> +check_one (void)
> +{
> +
> +  /* The buggy 1-second query timeout results in 30 seconds of delay,
> +     which triggers are test timeout failure.  */

"which triggers a test timeout failure"?

> [...]