commit f1f00c072138af90ae6da180f260111f09afe7a3
Author: Florian Weimer <fweimer@redhat.com>
Date:   Wed Oct 14 10:54:39 2020 +0200

    resolv: Handle transaction ID collisions in parallel queries (bug 26600)
    
    If the transaction IDs are equal, the old check attributed both
    responses to the first query, not recognizing the second response.
    This fixes bug 26600.

diff --git a/resolv/Makefile b/resolv/Makefile
index 72a0f196506ac489..cee5225f8933f245 100644
--- a/resolv/Makefile
+++ b/resolv/Makefile
@@ -62,6 +62,11 @@ tests += \
   tst-resolv-search \
   tst-resolv-trailing \
 
+# This test calls __res_context_send directly, which is not exported
+# from libresolv.
+tests-internal += tst-resolv-txnid-collision
+tests-static += tst-resolv-txnid-collision
+
 # These tests need libdl.
 ifeq (yes,$(build-shared))
 tests += \
@@ -202,6 +207,8 @@ $(objpfx)tst-resolv-search: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-trailing: $(objpfx)libresolv.so $(shared-thread-library)
 $(objpfx)tst-resolv-threads: \
   $(libdl) $(objpfx)libresolv.so $(shared-thread-library)
+$(objpfx)tst-resolv-txnid-collision: $(objpfx)libresolv.a \
+  $(static-thread-library)
 $(objpfx)tst-resolv-canonname: \
   $(libdl) $(objpfx)libresolv.so $(shared-thread-library)
 
diff --git a/resolv/res_send.c b/resolv/res_send.c
index c9b02cca130bc20d..ac19627634281c2f 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -1315,15 +1315,6 @@ send_dg(res_state statp,
 			*terrno = EMSGSIZE;
 			return close_and_return_error (statp, resplen2);
 		}
-		if ((recvresp1 || hp->id != anhp->id)
-		    && (recvresp2 || hp2->id != anhp->id)) {
-			/*
-			 * response from old query, ignore it.
-			 * XXX - potential security hazard could
-			 *	 be detected here.
-			 */
-			goto wait;
-		}
 
 		/* Paranoia check.  Due to the connected UDP socket,
 		   the kernel has already filtered invalid addresses
@@ -1333,15 +1324,24 @@ send_dg(res_state statp,
 
 		/* Check for the correct header layout and a matching
 		   question.  */
-		if ((recvresp1 || !res_queriesmatch(buf, buf + buflen,
-						       *thisansp,
-						       *thisansp
-						       + *thisanssizp))
-		    && (recvresp2 || !res_queriesmatch(buf2, buf2 + buflen2,
-						       *thisansp,
-						       *thisansp
-						       + *thisanssizp)))
-		  goto wait;
+		int matching_query = 0; /* Default to no matching query.  */
+		if (!recvresp1
+		    && anhp->id == hp->id
+		    && res_queriesmatch (buf, buf + buflen,
+					 *thisansp, *thisansp + *thisanssizp))
+		  matching_query = 1;
+		if (!recvresp2
+		    && anhp->id == hp2->id
+		    && res_queriesmatch (buf2, buf2 + buflen2,
+					 *thisansp, *thisansp + *thisanssizp))
+		  matching_query = 2;
+		if (matching_query == 0)
+		  /* Spurious UDP packet.  Drop it and continue
+		     waiting.  */
+		  {
+		    need_recompute = 1;
+		    goto wait;
+		  }
 
 		if (anhp->rcode == SERVFAIL ||
 		    anhp->rcode == NOTIMP ||
@@ -1356,7 +1356,7 @@ send_dg(res_state statp,
 			    /* No data from the first reply.  */
 			    resplen = 0;
 			    /* We are waiting for a possible second reply.  */
-			    if (hp->id == anhp->id)
+			    if (matching_query == 1)
 			      recvresp1 = 1;
 			    else
 			      recvresp2 = 1;
@@ -1387,7 +1387,7 @@ send_dg(res_state statp,
 			return (1);
 		}
 		/* Mark which reply we received.  */
-		if (recvresp1 == 0 && hp->id == anhp->id)
+		if (matching_query == 1)
 			recvresp1 = 1;
 		else
 			recvresp2 = 1;
diff --git a/resolv/tst-resolv-txnid-collision.c b/resolv/tst-resolv-txnid-collision.c
new file mode 100644
index 0000000000000000..611d37362f3e5e89
--- /dev/null
+++ b/resolv/tst-resolv-txnid-collision.c
@@ -0,0 +1,329 @@
+/* Test parallel queries with transaction ID collisions.
+   Copyright (C) 2020 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 <arpa/nameser.h>
+#include <array_length.h>
+#include <resolv-internal.h>
+#include <resolv_context.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/check_nss.h>
+#include <support/resolv_test.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+
+/* Result of parsing a DNS question name.
+
+   A question name has the form reorder-N-M-rcode-C.example.net, where
+   N and M are either 0 and 1, corresponding to the reorder member,
+   and C is a number that will be stored in the rcode field.
+
+   Also see parse_qname below.  */
+struct parsed_qname
+{
+  /* The DNS response code requested from the first server.  The
+     second server always responds with RCODE zero.  */
+  int rcode;
+
+  /* Indicates whether to perform reordering in the responses from the
+     respective server.  */
+  bool reorder[2];
+};
+
+/* Fills *PARSED based on QNAME.  */
+static void
+parse_qname (struct parsed_qname *parsed, const char *qname)
+{
+  int reorder0;
+  int reorder1;
+  int rcode;
+  char *suffix;
+  if (sscanf (qname, "reorder-%d-%d.rcode-%d.%ms",
+              &reorder0, &reorder1, &rcode, &suffix) == 4)
+    {
+      if (reorder0 != 0)
+        TEST_COMPARE (reorder0, 1);
+      if (reorder1 != 0)
+        TEST_COMPARE (reorder1, 1);
+      TEST_VERIFY (rcode >= 0 && rcode <= 15);
+      TEST_COMPARE_STRING (suffix, "example.net");
+      free (suffix);
+
+      parsed->rcode = rcode;
+      parsed->reorder[0] = reorder0;
+      parsed->reorder[1] = reorder1;
+    }
+  else
+    FAIL_EXIT1 ("unexpected query: %s", qname);
+}
+
+/* Used to construct a response. The first server responds with an
+   error, the second server succeeds.  */
+static void
+build_response (const struct resolv_response_context *ctx,
+                struct resolv_response_builder *b,
+                const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  struct parsed_qname parsed;
+  parse_qname (&parsed, qname);
+
+  switch (ctx->server_index)
+    {
+    case 0:
+      {
+        struct resolv_response_flags flags = { 0 };
+        if (parsed.rcode == 0)
+          /* Simulate a delegation in case a NODATA (RCODE zero)
+             response is requested.  */
+          flags.clear_ra = true;
+        else
+          flags.rcode = parsed.rcode;
+
+        resolv_response_init (b, flags);
+        resolv_response_add_question (b, qname, qclass, qtype);
+      }
+      break;
+
+    case 1:
+      {
+        struct resolv_response_flags flags = { 0, };
+        resolv_response_init (b, 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);
+        if (qtype == T_A)
+          {
+            char ipv4[4] = { 192, 0, 2, 1 };
+            resolv_response_add_data (b, &ipv4, sizeof (ipv4));
+          }
+        else
+          {
+            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));
+          }
+        resolv_response_close_record (b);
+      }
+      break;
+    }
+}
+
+/* Used to reorder responses.  */
+struct resolv_response_context *previous_query;
+
+/* Used to keep track of the queries received.  */
+static int previous_server_index = -1;
+static uint16_t previous_qtype;
+
+/* For each server, buffer the first query and then send both answers
+   to the second query, reordered if requested.  */
+static void
+response (const struct resolv_response_context *ctx,
+          struct resolv_response_builder *b,
+          const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  TEST_VERIFY (qtype == T_A || qtype == T_AAAA);
+  if (ctx->server_index != 0)
+    TEST_COMPARE (ctx->server_index, 1);
+
+  struct parsed_qname parsed;
+  parse_qname (&parsed, qname);
+
+  if (previous_query == NULL)
+    {
+      /* No buffered query.  Record this query and do not send a
+         response.  */
+      TEST_COMPARE (previous_qtype, 0);
+      previous_query = resolv_response_context_duplicate (ctx);
+      previous_qtype = qtype;
+      resolv_response_drop (b);
+      previous_server_index = ctx->server_index;
+
+      if (test_verbose)
+        printf ("info: buffering first query for: %s\n", qname);
+    }
+  else
+    {
+      TEST_VERIFY (previous_query != 0);
+      TEST_COMPARE (ctx->server_index, previous_server_index);
+      TEST_VERIFY (previous_qtype != qtype); /* Not a duplicate.  */
+
+      /* If reordering, send a response for this query explicitly, and
+         then skip the implicit send.  */
+      if (parsed.reorder[ctx->server_index])
+        {
+          if (test_verbose)
+            printf ("info: sending reordered second response for: %s\n",
+                    qname);
+          build_response (ctx, b, qname, qclass, qtype);
+          resolv_response_send_udp (ctx, b);
+          resolv_response_drop (b);
+        }
+
+      /* Build a response for the previous query and send it, thus
+         reordering the two responses.  */
+      {
+        if (test_verbose)
+          printf ("info: sending first response for: %s\n", qname);
+        struct resolv_response_builder *btmp
+          = resolv_response_builder_allocate (previous_query->query_buffer,
+                                              previous_query->query_length);
+        build_response (ctx, btmp, qname, qclass, previous_qtype);
+        resolv_response_send_udp (ctx, btmp);
+        resolv_response_builder_free (btmp);
+      }
+
+      /* If not reordering, send the reply as usual.  */
+      if (!parsed.reorder[ctx->server_index])
+        {
+          if (test_verbose)
+            printf ("info: sending non-reordered second response for: %s\n",
+                    qname);
+          build_response (ctx, b, qname, qclass, qtype);
+        }
+
+      /* Unbuffer the response and prepare for the next query.  */
+      resolv_response_context_free (previous_query);
+      previous_query = NULL;
+      previous_qtype = 0;
+      previous_server_index = -1;
+    }
+}
+
+/* Runs a query for QNAME and checks for the expected reply.  See
+   struct parsed_qname for the expected format for QNAME.  */
+static void
+test_qname (const char *qname, int rcode)
+{
+  struct resolv_context *ctx = __resolv_context_get ();
+  TEST_VERIFY_EXIT (ctx != NULL);
+
+  unsigned char q1[512];
+  int q1len = res_mkquery (QUERY, qname, C_IN, T_A, NULL, 0, NULL,
+                           q1, sizeof (q1));
+  TEST_VERIFY_EXIT (q1len > 12);
+
+  unsigned char q2[512];
+  int q2len = res_mkquery (QUERY, qname, C_IN, T_AAAA, NULL, 0, NULL,
+                           q2, sizeof (q2));
+  TEST_VERIFY_EXIT (q2len > 12);
+
+  /* Produce a transaction ID collision.  */
+  memcpy (q2, q1, 2);
+
+  unsigned char ans1[512];
+  unsigned char *ans1p = ans1;
+  unsigned char *ans2p = NULL;
+  int nans2p = 0;
+  int resplen2 = 0;
+  int ans2p_malloced = 0;
+
+  /* Perform a parallel A/AAAA query.  */
+  int resplen1 = __res_context_send (ctx, q1, q1len, q2, q2len,
+                                     ans1, sizeof (ans1), &ans1p,
+                                     &ans2p, &nans2p,
+                                     &resplen2, &ans2p_malloced);
+
+  TEST_VERIFY (resplen1 > 12);
+  TEST_VERIFY (resplen2 > 12);
+  if (resplen1 <= 12 || resplen2 <= 12)
+    return;
+
+  if (rcode == 1 || rcode == 3)
+    {
+      /* Format Error and Name Error responses does not trigger
+         switching to the next server.  */
+      TEST_COMPARE (ans1p[3] & 0x0f, rcode);
+      TEST_COMPARE (ans2p[3] & 0x0f, rcode);
+      return;
+    }
+
+  /* The response should be successful.  */
+  TEST_COMPARE (ans1p[3] & 0x0f, 0);
+  TEST_COMPARE (ans2p[3] & 0x0f, 0);
+
+  /* Due to bug 19691, the answer may not be in the slot matching the
+     query.  Assume that the AAAA response is the longer one.  */
+  unsigned char *a_answer;
+  int a_answer_length;
+  unsigned char *aaaa_answer;
+  int aaaa_answer_length;
+  if (resplen2 > resplen1)
+    {
+      a_answer = ans1p;
+      a_answer_length = resplen1;
+      aaaa_answer = ans2p;
+      aaaa_answer_length = resplen2;
+    }
+  else
+    {
+      a_answer = ans2p;
+      a_answer_length = resplen2;
+      aaaa_answer = ans1p;
+      aaaa_answer_length = resplen1;
+    }
+
+  {
+    char *expected = xasprintf ("name: %s\n"
+                                "address: 192.0.2.1\n",
+                                qname);
+    check_dns_packet (qname, a_answer, a_answer_length, expected);
+    free (expected);
+  }
+  {
+    char *expected = xasprintf ("name: %s\n"
+                                "address: 2001:db8::1\n",
+                                qname);
+    check_dns_packet (qname, aaaa_answer, aaaa_answer_length, expected);
+    free (expected);
+  }
+
+  if (ans2p_malloced)
+    free (ans2p);
+
+  __resolv_context_put (ctx);
+}
+
+static int
+do_test (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response,
+     });
+
+  for (int rcode = 0; rcode <= 5; ++rcode)
+    for (int do_reorder_0 = 0; do_reorder_0 < 2; ++do_reorder_0)
+      for (int do_reorder_1 = 0; do_reorder_1 < 2; ++do_reorder_1)
+        {
+          char *qname = xasprintf ("reorder-%d-%d.rcode-%d.example.net",
+                                   do_reorder_0, do_reorder_1, rcode);
+          test_qname (qname, rcode);
+          free (qname);
+        }
+
+  resolv_test_end (aux);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
