resolv: Allow short error responses to match any query (bug 31890)

Reviewed-by: DJ Delorie <dj@redhat.com>

3 files changed, 134 insertions, 10 deletions

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..cf1e39876f
--- /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 a test timeout failure.  */
+  for (int i = 0;  i < 10; ++i)
+    {
+      check_hostent ("www.example", gethostbyname ("www.example"),
+                     "name: www.example\n"
+                     "address: 192.0.2.17\n");
+      check_hostent ("www.example", gethostbyname2 ("www.example", AF_INET6),
+                     "name: www.example\n"
+                     "address: 2001:db8::1\n");
+    }
+}
+
+static int
+do_test (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response,
+     });
+
+  _res.options |= RES_SNGLKUP;
+
+  rcode = 2; /* SERVFAIL.  */
+  check_one ();
+
+  rcode = 4; /* NOTIMP.  */
+  check_one ();
+
+  rcode = 5; /* REFUSED.  */
+  check_one ();
+
+  resolv_test_end (aux);
+
+  return 0;
+}
+
+#include <support/test-driver.c>