CVE-2023-4527: Stack read overflow with large TCP responses in no-aaaa mode

Without passing alt_dns_packet_buffer, __res_context_search can only
store 2048 bytes (what fits into dns_packet_buffer).  However,
the function returns the total packet size, and the subsequent
DNS parsing code in _nss_dns_gethostbyname4_r reads beyond the end
of the stack-allocated buffer.

Fixes commit f282cdbe7f ("resolv: Implement no-aaaa
stub resolver option") and bug 30842.

resolv/Makefile

@@ -102,6 +102,7 @@ tests += \
   tst-resolv-invalid-cname \
   tst-resolv-network \
   tst-resolv-noaaaa \
+  tst-resolv-noaaaa-vc \
   tst-resolv-nondecimal \
   tst-resolv-res_init-multi \
   tst-resolv-search \
@@ -293,6 +294,7 @@ $(objpfx)tst-resolv-res_init-thread: $(objpfx)libresolv.so \
 $(objpfx)tst-resolv-invalid-cname: $(objpfx)libresolv.so \
   $(shared-thread-library)
 $(objpfx)tst-resolv-noaaaa: $(objpfx)libresolv.so $(shared-thread-library)
+$(objpfx)tst-resolv-noaaaa-vc: $(objpfx)libresolv.so $(shared-thread-library)
 $(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)

resolv/nss_dns/dns-host.c

@@ -427,7 +427,7 @@ _nss_dns_gethostbyname4_r (const char *name, struct gaih_addrtuple **pat,
     {
       n = __res_context_search (ctx, name, C_IN, T_A,
 				dns_packet_buffer, sizeof (dns_packet_buffer),
-				NULL, NULL, NULL, NULL, NULL);
+				&alt_dns_packet_buffer, NULL, NULL, NULL, NULL);
       if (n >= 0)
 	status = gaih_getanswer_noaaaa (alt_dns_packet_buffer, n,
 					&abuf, pat, errnop, herrnop, ttlp);

resolv/tst-resolv-noaaaa-vc.c

@@ -0,0 +1,129 @@
+/* Test the RES_NOAAAA resolver option with a large response.
+   Copyright (C) 2022-2023 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 <errno.h>
+#include <netdb.h>
+#include <resolv.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/check_nss.h>
+#include <support/resolv_test.h>
+#include <support/support.h>
+#include <support/xmemstream.h>
+
+/* Used to keep track of the number of queries.  */
+static volatile unsigned int queries;
+
+/* If true, add a large TXT record at the start of the answer section.  */
+static volatile bool stuff_txt;
+
+static void
+response (const struct resolv_response_context *ctx,
+          struct resolv_response_builder *b,
+          const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  /* If not using TCP, just force its use.  */
+  if (!ctx->tcp)
+    {
+      struct resolv_response_flags flags = {.tc = true};
+      resolv_response_init (b, flags);
+      resolv_response_add_question (b, qname, qclass, qtype);
+      return;
+    }
+
+  /* The test needs to send four queries, the first three are used to
+     grow the NSS buffer via the ERANGE handshake.  */
+  ++queries;
+  TEST_VERIFY (queries <= 4);
+
+  /* AAAA queries are supposed to be disabled.  */
+  TEST_COMPARE (qtype, T_A);
+  TEST_COMPARE (qclass, C_IN);
+  TEST_COMPARE_STRING (qname, "example.com");
+
+  struct resolv_response_flags flags = {};
+  resolv_response_init (b, flags);
+  resolv_response_add_question (b, qname, qclass, qtype);
+
+  resolv_response_section (b, ns_s_an);
+
+  if (stuff_txt)
+    {
+      resolv_response_open_record (b, qname, qclass, T_TXT, 60);
+      int zero = 0;
+      for (int i = 0; i <= 15000; ++i)
+        resolv_response_add_data (b, &zero, sizeof (zero));
+      resolv_response_close_record (b);
+    }
+
+  for (int i = 0; i < 200; ++i)
+    {
+      resolv_response_open_record (b, qname, qclass, qtype, 60);
+      char ipv4[4] = {192, 0, 2, i + 1};
+      resolv_response_add_data (b, &ipv4, sizeof (ipv4));
+      resolv_response_close_record (b);
+    }
+}
+
+static int
+do_test (void)
+{
+  struct resolv_test *obj = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response
+     });
+
+  _res.options |= RES_NOAAAA;
+
+  for (int do_stuff_txt = 0; do_stuff_txt < 2; ++do_stuff_txt)
+    {
+      queries = 0;
+      stuff_txt = do_stuff_txt;
+
+      struct addrinfo *ai = NULL;
+      int ret;
+      ret = getaddrinfo ("example.com", "80",
+                         &(struct addrinfo)
+                         {
+                           .ai_family = AF_UNSPEC,
+                           .ai_socktype = SOCK_STREAM,
+                         }, &ai);
+
+      char *expected_result;
+      {
+        struct xmemstream mem;
+        xopen_memstream (&mem);
+        for (int i = 0; i < 200; ++i)
+          fprintf (mem.out, "address: STREAM/TCP 192.0.2.%d 80\n", i + 1);
+        xfclose_memstream (&mem);
+        expected_result = mem.buffer;
+      }
+
+      check_addrinfo ("example.com", ai, ret, expected_result);
+
+      free (expected_result);
+      freeaddrinfo (ai);
+    }
+
+  resolv_test_end (obj);
+  return 0;
+}
+
+#include <support/test-driver.c>