CVE-2015-7547: getaddrinfo() stack-based buffer overflow (Bug 18665).

* A stack-based buffer overflow was found in libresolv when invoked from
  libnss_dns, allowing specially crafted DNS responses to seize control
  of execution flow in the DNS client.  The buffer overflow occurs in
  the functions send_dg (send datagram) and send_vc (send TCP) for the
  NSS module libnss_dns.so.2 when calling getaddrinfo with AF_UNSPEC
  family.  The use of AF_UNSPEC triggers the low-level resolver code to
  send out two parallel queries for A and AAAA.  A mismanagement of the
  buffers used for those queries could result in the response of a query
  writing beyond the alloca allocated buffer created by
  _nss_dns_gethostbyname4_r.  Buffer management is simplified to remove
  the overflow.  Thanks to the Google Security Team and Red Hat for
  reporting the security impact of this issue, and Robert Holiday of
  Ciena for reporting the related bug 18665. (CVE-2015-7547)

See also:
https://sourceware.org/ml/libc-alpha/2016-02/msg00416.html
https://sourceware.org/ml/libc-alpha/2016-02/msg00418.html

3 files changed, 309 insertions, 65 deletions

diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
index a255d5e..8599f4c 100644
--- a/resolv/nss_dns/dns-host.c
+++ b/resolv/nss_dns/dns-host.c
@@ -1031,7 +1031,10 @@ gaih_getanswer_slice (const querybuf *answer, int anslen, const char *qname,
   int h_namelen = 0;
 
   if (ancount == 0)
-    return NSS_STATUS_NOTFOUND;
+    {
+      *h_errnop = HOST_NOT_FOUND;
+      return NSS_STATUS_NOTFOUND;
+    }
 
   while (ancount-- > 0 && cp < end_of_message && had_error == 0)
     {
@@ -1208,7 +1211,14 @@ gaih_getanswer_slice (const querybuf *answer, int anslen, const char *qname,
   /* Special case here: if the resolver sent a result but it only
      contains a CNAME while we are looking for a T_A or T_AAAA record,
      we fail with NOTFOUND instead of TRYAGAIN.  */
-  return canon == NULL ? NSS_STATUS_TRYAGAIN : NSS_STATUS_NOTFOUND;
+  if (canon != NULL)
+    {
+      *h_errnop = HOST_NOT_FOUND;
+      return NSS_STATUS_NOTFOUND;
+    }
+
+  *h_errnop = NETDB_INTERNAL;
+  return NSS_STATUS_TRYAGAIN;
 }
 
 
@@ -1222,11 +1232,101 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 
   enum nss_status status = NSS_STATUS_NOTFOUND;
 
+  /* Combining the NSS status of two distinct queries requires some
+     compromise and attention to symmetry (A or AAAA queries can be
+     returned in any order).  What follows is a breakdown of how this
+     code is expected to work and why. We discuss only SUCCESS,
+     TRYAGAIN, NOTFOUND and UNAVAIL, since they are the only returns
+     that apply (though RETURN and MERGE exist).  We make a distinction
+     between TRYAGAIN (recoverable) and TRYAGAIN' (not-recoverable).
+     A recoverable TRYAGAIN is almost always due to buffer size issues
+     and returns ERANGE in errno and the caller is expected to retry
+     with a larger buffer.
+
+     Lastly, you may be tempted to make significant changes to the
+     conditions in this code to bring about symmetry between responses.
+     Please don't change anything without due consideration for
+     expected application behaviour.  Some of the synthesized responses
+     aren't very well thought out and sometimes appear to imply that
+     IPv4 responses are always answer 1, and IPv6 responses are always
+     answer 2, but that's not true (see the implementation of send_dg
+     and send_vc to see response can arrive in any order, particularly
+     for UDP). However, we expect it holds roughly enough of the time
+     that this code works, but certainly needs to be fixed to make this
+     a more robust implementation.
+
+     ----------------------------------------------
+     | Answer 1 Status /   | Synthesized | Reason |
+     | Answer 2 Status     | Status      |        |
+     |--------------------------------------------|
+     | SUCCESS/SUCCESS     | SUCCESS     | [1]    |
+     | SUCCESS/TRYAGAIN    | TRYAGAIN    | [5]    |
+     | SUCCESS/TRYAGAIN'   | SUCCESS     | [1]    |
+     | SUCCESS/NOTFOUND    | SUCCESS     | [1]    |
+     | SUCCESS/UNAVAIL     | SUCCESS     | [1]    |
+     | TRYAGAIN/SUCCESS    | TRYAGAIN    | [2]    |
+     | TRYAGAIN/TRYAGAIN   | TRYAGAIN    | [2]    |
+     | TRYAGAIN/TRYAGAIN'  | TRYAGAIN    | [2]    |
+     | TRYAGAIN/NOTFOUND   | TRYAGAIN    | [2]    |
+     | TRYAGAIN/UNAVAIL    | TRYAGAIN    | [2]    |
+     | TRYAGAIN'/SUCCESS   | SUCCESS     | [3]    |
+     | TRYAGAIN'/TRYAGAIN  | TRYAGAIN    | [3]    |
+     | TRYAGAIN'/TRYAGAIN' | TRYAGAIN'   | [3]    |
+     | TRYAGAIN'/NOTFOUND  | TRYAGAIN'   | [3]    |
+     | TRYAGAIN'/UNAVAIL   | UNAVAIL     | [3]    |
+     | NOTFOUND/SUCCESS    | SUCCESS     | [3]    |
+     | NOTFOUND/TRYAGAIN   | TRYAGAIN    | [3]    |
+     | NOTFOUND/TRYAGAIN'  | TRYAGAIN'   | [3]    |
+     | NOTFOUND/NOTFOUND   | NOTFOUND    | [3]    |
+     | NOTFOUND/UNAVAIL    | UNAVAIL     | [3]    |
+     | UNAVAIL/SUCCESS     | UNAVAIL     | [4]    |
+     | UNAVAIL/TRYAGAIN    | UNAVAIL     | [4]    |
+     | UNAVAIL/TRYAGAIN'   | UNAVAIL     | [4]    |
+     | UNAVAIL/NOTFOUND    | UNAVAIL     | [4]    |
+     | UNAVAIL/UNAVAIL     | UNAVAIL     | [4]    |
+     ----------------------------------------------
+
+     [1] If the first response is a success we return success.
+	 This ignores the state of the second answer and in fact
+	 incorrectly sets errno and h_errno to that of the second
+	 answer.  However because the response is a success we ignore
+	 *errnop and *h_errnop (though that means you touched errno on
+	 success).  We are being conservative here and returning the
+	 likely IPv4 response in the first answer as a success.
+
+     [2] If the first response is a recoverable TRYAGAIN we return
+	 that instead of looking at the second response.  The
+	 expectation here is that we have failed to get an IPv4 response
+	 and should retry both queries.
+
+     [3] If the first response was not a SUCCESS and the second
+	 response is not NOTFOUND (had a SUCCESS, need to TRYAGAIN,
+	 or failed entirely e.g. TRYAGAIN' and UNAVAIL) then use the
+	 result from the second response, otherwise the first responses
+	 status is used.  Again we have some odd side-effects when the
+	 second response is NOTFOUND because we overwrite *errnop and
+	 *h_errnop that means that a first answer of NOTFOUND might see
+	 its *errnop and *h_errnop values altered.  Whether it matters
+	 in practice that a first response NOTFOUND has the wrong
+	 *errnop and *h_errnop is undecided.
+
+     [4] If the first response is UNAVAIL we return that instead of
+	 looking at the second response.  The expectation here is that
+	 it will have failed similarly e.g. configuration failure.
+
+     [5] Testing this code is complicated by the fact that truncated
+	 second response buffers might be returned as SUCCESS if the
+	 first answer is a SUCCESS.  To fix this we add symmetry to
+	 TRYAGAIN with the second response.  If the second response
+	 is a recoverable error we now return TRYAGIN even if the first
+	 response was SUCCESS.  */
+
   if (anslen1 > 0)
     status = gaih_getanswer_slice(answer1, anslen1, qname,
 				  &pat, &buffer, &buflen,
 				  errnop, h_errnop, ttlp,
 				  &first);
+
   if ((status == NSS_STATUS_SUCCESS || status == NSS_STATUS_NOTFOUND
        || (status == NSS_STATUS_TRYAGAIN
 	   /* We want to look at the second answer in case of an
@@ -1242,8 +1342,15 @@ gaih_getanswer (const querybuf *answer1, int anslen1, const querybuf *answer2,
 						     &pat, &buffer, &buflen,
 						     errnop, h_errnop, ttlp,
 						     &first);
+      /* Use the second response status in some cases.  */
       if (status != NSS_STATUS_SUCCESS && status2 != NSS_STATUS_NOTFOUND)
 	status = status2;
+      /* Do not return a truncated second response (unless it was
+	 unavoidable e.g. unrecoverable TRYAGAIN).  */
+      if (status == NSS_STATUS_SUCCESS
+	  && (status2 == NSS_STATUS_TRYAGAIN
+	      && *errnop == ERANGE && *h_errnop != NO_RECOVERY))
+	status = NSS_STATUS_TRYAGAIN;
     }
 
   return status;
diff --git a/resolv/res_query.c b/resolv/res_query.c
index 4a9b3b3..95470a9 100644
--- a/resolv/res_query.c
+++ b/resolv/res_query.c
@@ -396,6 +396,7 @@ __libc_res_nsearch(res_state statp,
 		  {
 		    free (*answerp2);
 		    *answerp2 = NULL;
+		    *nanswerp2 = 0;
 		    *answerp2_malloced = 0;
 		  }
 	}
@@ -447,6 +448,7 @@ __libc_res_nsearch(res_state statp,
 			  {
 			    free (*answerp2);
 			    *answerp2 = NULL;
+			    *nanswerp2 = 0;
 			    *answerp2_malloced = 0;
 			  }
 
@@ -521,6 +523,7 @@ __libc_res_nsearch(res_state statp,
 	  {
 	    free (*answerp2);
 	    *answerp2 = NULL;
+	    *nanswerp2 = 0;
 	    *answerp2_malloced = 0;
 	  }
 	if (saved_herrno != -1)
diff --git a/resolv/res_send.c b/resolv/res_send.c
index a968b95..25c19f1 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -1,3 +1,20 @@
+/* Copyright (C) 2016 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
+   <http://www.gnu.org/licenses/>.  */
+
 /*
  * Copyright (c) 1985, 1989, 1993
  *    The Regents of the University of California.  All rights reserved.
@@ -355,6 +372,8 @@ __libc_res_nsend(res_state statp, const u_char *buf, int buflen,
 #ifdef USE_HOOKS
 	if (__glibc_unlikely (statp->qhook || statp->rhook))       {
 		if (anssiz < MAXPACKET && ansp) {
+			/* Always allocate MAXPACKET, callers expect
+			   this specific size.  */
 			u_char *buf = malloc (MAXPACKET);
 			if (buf == NULL)
 				return (-1);
@@ -630,6 +649,77 @@ get_nsaddr (res_state statp, int n)
     return (struct sockaddr *) (void *) &statp->nsaddr_list[n];
 }
 
+/* The send_vc function is responsible for sending a DNS query over TCP
+   to the nameserver numbered NS from the res_state STATP i.e.
+   EXT(statp).nssocks[ns].  The function supports sending both IPv4 and
+   IPv6 queries at the same serially on the same socket.
+
+   Please note that for TCP there is no way to disable sending both
+   queries, unlike UDP, which honours RES_SNGLKUP and RES_SNGLKUPREOP
+   and sends the queries serially and waits for the result after each
+   sent query.  This implemetnation should be corrected to honour these
+   options.
+
+   Please also note that for TCP we send both queries over the same
+   socket one after another.  This technically violates best practice
+   since the server is allowed to read the first query, respond, and
+   then close the socket (to service another client).  If the server
+   does this, then the remaining second query in the socket data buffer
+   will cause the server to send the client an RST which will arrive
+   asynchronously and the client's OS will likely tear down the socket
+   receive buffer resulting in a potentially short read and lost
+   response data.  This will force the client to retry the query again,
+   and this process may repeat until all servers and connection resets
+   are exhausted and then the query will fail.  It's not known if this
+   happens with any frequency in real DNS server implementations.  This
+   implementation should be corrected to use two sockets by default for
+   parallel queries.
+
+   The query stored in BUF of BUFLEN length is sent first followed by
+   the query stored in BUF2 of BUFLEN2 length.  Queries are sent
+   serially on the same socket.
+
+   Answers to the query are stored firstly in *ANSP up to a max of
+   *ANSSIZP bytes.  If more than *ANSSIZP bytes are needed and ANSCP
+   is non-NULL (to indicate that modifying the answer buffer is allowed)
+   then malloc is used to allocate a new response buffer and ANSCP and
+   ANSP will both point to the new buffer.  If more than *ANSSIZP bytes
+   are needed but ANSCP is NULL, then as much of the response as
+   possible is read into the buffer, but the results will be truncated.
+   When truncation happens because of a small answer buffer the DNS
+   packets header field TC will bet set to 1, indicating a truncated
+   message and the rest of the socket data will be read and discarded.
+
+   Answers to the query are stored secondly in *ANSP2 up to a max of
+   *ANSSIZP2 bytes, with the actual response length stored in
+   *RESPLEN2.  If more than *ANSSIZP bytes are needed and ANSP2
+   is non-NULL (required for a second query) then malloc is used to
+   allocate a new response buffer, *ANSSIZP2 is set to the new buffer
+   size and *ANSP2_MALLOCED is set to 1.
+
+   The ANSP2_MALLOCED argument will eventually be removed as the
+   change in buffer pointer can be used to detect the buffer has
+   changed and that the caller should use free on the new buffer.
+
+   Note that the answers may arrive in any order from the server and
+   therefore the first and second answer buffers may not correspond to
+   the first and second queries.
+
+   It is not supported to call this function with a non-NULL ANSP2
+   but a NULL ANSCP.  Put another way, you can call send_vc with a
+   single unmodifiable buffer or two modifiable buffers, but no other
+   combination is supported.
+
+   It is the caller's responsibility to free the malloc allocated
+   buffers by detecting that the pointers have changed from their
+   original values i.e. *ANSCP or *ANSP2 has changed.
+
+   If errors are encountered then *TERRNO is set to an appropriate
+   errno value and a zero result is returned for a recoverable error,
+   and a less-than zero result is returned for a non-recoverable error.
+
+   If no errors are encountered then *TERRNO is left unmodified and
+   a the length of the first response in bytes is returned.  */
 static int
 send_vc(res_state statp,
 	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
@@ -639,11 +729,7 @@ send_vc(res_state statp,
 {
 	const HEADER *hp = (HEADER *) buf;
 	const HEADER *hp2 = (HEADER *) buf2;
-	u_char *ans = *ansp;
-	int orig_anssizp = *anssizp;
-	// XXX REMOVE
-	// int anssiz = *anssizp;
-	HEADER *anhp = (HEADER *) ans;
+	HEADER *anhp = (HEADER *) *ansp;
 	struct sockaddr *nsap = get_nsaddr (statp, ns);
 	int truncating, connreset, n;
 	/* On some architectures compiler might emit a warning indicating
@@ -731,6 +817,8 @@ send_vc(res_state statp,
 	 * Receive length & response
 	 */
 	int recvresp1 = 0;
+	/* Skip the second response if there is no second query.
+	   To do that we mark the second response as received.  */
 	int recvresp2 = buf2 == NULL;
 	uint16_t rlen16;
  read_len:
@@ -767,36 +855,14 @@ send_vc(res_state statp,
 	u_char **thisansp;
 	int *thisresplenp;
 	if ((recvresp1 | recvresp2) == 0 || buf2 == NULL) {
+		/* We have not received any responses
+		   yet or we only have one response to
+		   receive.  */
 		thisanssizp = anssizp;
 		thisansp = anscp ?: ansp;
 		assert (anscp != NULL || ansp2 == NULL);
 		thisresplenp = &resplen;
 	} else {
-		if (*anssizp != MAXPACKET) {
-			/* No buffer allocated for the first
-			   reply.  We can try to use the rest
-			   of the user-provided buffer.  */
-			DIAG_PUSH_NEEDS_COMMENT;
-			DIAG_IGNORE_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
-#if _STRING_ARCH_unaligned
-			*anssizp2 = orig_anssizp - resplen;
-			*ansp2 = *ansp + resplen;
-#else
-			int aligned_resplen
-			  = ((resplen + __alignof__ (HEADER) - 1)
-			     & ~(__alignof__ (HEADER) - 1));
-			*anssizp2 = orig_anssizp - aligned_resplen;
-			*ansp2 = *ansp + aligned_resplen;
-#endif
-			DIAG_POP_NEEDS_COMMENT;
-		} else {
-			/* The first reply did not fit into the
-			   user-provided buffer.  Maybe the second
-			   answer will.  */
-			*anssizp2 = orig_anssizp;
-			*ansp2 = *ansp;
-		}
-
 		thisanssizp = anssizp2;
 		thisansp = ansp2;
 		thisresplenp = resplen2;
@@ -804,10 +870,14 @@ send_vc(res_state statp,
 	anhp = (HEADER *) *thisansp;
 
 	*thisresplenp = rlen;
-	if (rlen > *thisanssizp) {
-		/* Yes, we test ANSCP here.  If we have two buffers
-		   both will be allocatable.  */
-		if (__glibc_likely (anscp != NULL))       {
+	/* Is the answer buffer too small?  */
+	if (*thisanssizp < rlen) {
+		/* If the current buffer is not the the static
+		   user-supplied buffer then we can reallocate
+		   it.  */
+		if (thisansp != NULL && thisansp != ansp) {
+			/* Always allocate MAXPACKET, callers expect
+			   this specific size.  */
 			u_char *newp = malloc (MAXPACKET);
 			if (newp == NULL) {
 				*terrno = ENOMEM;
@@ -819,6 +889,9 @@ send_vc(res_state statp,
 			if (thisansp == ansp2)
 			  *ansp2_malloced = 1;
 			anhp = (HEADER *) newp;
+			/* A uint16_t can't be larger than MAXPACKET
+			   thus it's safe to allocate MAXPACKET but
+			   read RLEN bytes instead.  */
 			len = rlen;
 		} else {
 			Dprint(statp->options & RES_DEBUG,
@@ -948,6 +1021,66 @@ reopen (res_state statp, int *terrno, int ns)
 	return 1;
 }
 
+/* The send_dg function is responsible for sending a DNS query over UDP
+   to the nameserver numbered NS from the res_state STATP i.e.
+   EXT(statp).nssocks[ns].  The function supports IPv4 and IPv6 queries
+   along with the ability to send the query in parallel for both stacks
+   (default) or serially (RES_SINGLKUP).  It also supports serial lookup
+   with a close and reopen of the socket used to talk to the server
+   (RES_SNGLKUPREOP) to work around broken name servers.
+
+   The query stored in BUF of BUFLEN length is sent first followed by
+   the query stored in BUF2 of BUFLEN2 length.  Queries are sent
+   in parallel (default) or serially (RES_SINGLKUP or RES_SNGLKUPREOP).
+
+   Answers to the query are stored firstly in *ANSP up to a max of
+   *ANSSIZP bytes.  If more than *ANSSIZP bytes are needed and ANSCP
+   is non-NULL (to indicate that modifying the answer buffer is allowed)
+   then malloc is used to allocate a new response buffer and ANSCP and
+   ANSP will both point to the new buffer.  If more than *ANSSIZP bytes
+   are needed but ANSCP is NULL, then as much of the response as
+   possible is read into the buffer, but the results will be truncated.
+   When truncation happens because of a small answer buffer the DNS
+   packets header field TC will bet set to 1, indicating a truncated
+   message, while the rest of the UDP packet is discarded.
+
+   Answers to the query are stored secondly in *ANSP2 up to a max of
+   *ANSSIZP2 bytes, with the actual response length stored in
+   *RESPLEN2.  If more than *ANSSIZP bytes are needed and ANSP2
+   is non-NULL (required for a second query) then malloc is used to
+   allocate a new response buffer, *ANSSIZP2 is set to the new buffer
+   size and *ANSP2_MALLOCED is set to 1.
+
+   The ANSP2_MALLOCED argument will eventually be removed as the
+   change in buffer pointer can be used to detect the buffer has
+   changed and that the caller should use free on the new buffer.
+
+   Note that the answers may arrive in any order from the server and
+   therefore the first and second answer buffers may not correspond to
+   the first and second queries.
+
+   It is not supported to call this function with a non-NULL ANSP2
+   but a NULL ANSCP.  Put another way, you can call send_vc with a
+   single unmodifiable buffer or two modifiable buffers, but no other
+   combination is supported.
+
+   It is the caller's responsibility to free the malloc allocated
+   buffers by detecting that the pointers have changed from their
+   original values i.e. *ANSCP or *ANSP2 has changed.
+
+   If an answer is truncated because of UDP datagram DNS limits then
+   *V_CIRCUIT is set to 1 and the return value non-zero to indicate to
+   the caller to retry with TCP.  The value *GOTSOMEWHERE is set to 1
+   if any progress was made reading a response from the nameserver and
+   is used by the caller to distinguish between ECONNREFUSED and
+   ETIMEDOUT (the latter if *GOTSOMEWHERE is 1).
+
+   If errors are encountered then *TERRNO is set to an appropriate
+   errno value and a zero result is returned for a recoverable error,
+   and a less-than zero result is returned for a non-recoverable error.
+
+   If no errors are encountered then *TERRNO is left unmodified and
+   a the length of the first response in bytes is returned.  */
 static int
 send_dg(res_state statp,
 	const u_char *buf, int buflen, const u_char *buf2, int buflen2,
@@ -957,8 +1090,6 @@ send_dg(res_state statp,
 {
 	const HEADER *hp = (HEADER *) buf;
 	const HEADER *hp2 = (HEADER *) buf2;
-	u_char *ans = *ansp;
-	int orig_anssizp = *anssizp;
 	struct timespec now, timeout, finish;
 	struct pollfd pfd[1];
 	int ptimeout;
@@ -991,6 +1122,8 @@ send_dg(res_state statp,
 	int need_recompute = 0;
 	int nwritten = 0;
 	int recvresp1 = 0;
+	/* Skip the second response if there is no second query.
+	   To do that we mark the second response as received.  */
 	int recvresp2 = buf2 == NULL;
 	pfd[0].fd = EXT(statp).nssocks[ns];
 	pfd[0].events = POLLOUT;
@@ -1154,55 +1287,56 @@ send_dg(res_state statp,
 		int *thisresplenp;
 
 		if ((recvresp1 | recvresp2) == 0 || buf2 == NULL) {
+			/* We have not received any responses
+			   yet or we only have one response to
+			   receive.  */
 			thisanssizp = anssizp;
 			thisansp = anscp ?: ansp;
 			assert (anscp != NULL || ansp2 == NULL);
 			thisresplenp = &resplen;
 		} else {
-			if (*anssizp != MAXPACKET) {
-				/* No buffer allocated for the first
-				   reply.  We can try to use the rest
-				   of the user-provided buffer.  */
-#if _STRING_ARCH_unaligned
-				*anssizp2 = orig_anssizp - resplen;
-				*ansp2 = *ansp + resplen;
-#else
-				int aligned_resplen
-				  = ((resplen + __alignof__ (HEADER) - 1)
-				     & ~(__alignof__ (HEADER) - 1));
-				*anssizp2 = orig_anssizp - aligned_resplen;
-				*ansp2 = *ansp + aligned_resplen;
-#endif
-			} else {
-				/* The first reply did not fit into the
-				   user-provided buffer.  Maybe the second
-				   answer will.  */
-				*anssizp2 = orig_anssizp;
-				*ansp2 = *ansp;
-			}
-
 			thisanssizp = anssizp2;
 			thisansp = ansp2;
 			thisresplenp = resplen2;
 		}
 
 		if (*thisanssizp < MAXPACKET
-		    /* Yes, we test ANSCP here.  If we have two buffers
-		       both will be allocatable.  */
-		    && anscp
+		    /* If the current buffer is not the the static
+		       user-supplied buffer then we can reallocate
+		       it.  */
+		    && (thisansp != NULL && thisansp != ansp)
 #ifdef FIONREAD
+		    /* Is the size too small?  */
 		    && (ioctl (pfd[0].fd, FIONREAD, thisresplenp) < 0
 			|| *thisanssizp < *thisresplenp)
 #endif
                     ) {
+			/* Always allocate MAXPACKET, callers expect
+			   this specific size.  */
 			u_char *newp = malloc (MAXPACKET);
 			if (newp != NULL) {
-				*anssizp = MAXPACKET;
-				*thisansp = ans = newp;
+				*thisanssizp = MAXPACKET;
+				*thisansp = newp;
 				if (thisansp == ansp2)
 				  *ansp2_malloced = 1;
 			}
 		}
+		/* We could end up with truncation if anscp was NULL
+		   (not allowed to change caller's buffer) and the
+		   response buffer size is too small.  This isn't a
+		   reliable way to detect truncation because the ioctl
+		   may be an inaccurate report of the UDP message size.
+		   Therefore we use this only to issue debug output.
+		   To do truncation accurately with UDP we need
+		   MSG_TRUNC which is only available on Linux.  We
+		   can abstract out the Linux-specific feature in the
+		   future to detect truncation.  */
+		if (__glibc_unlikely (*thisanssizp < *thisresplenp)) {
+			Dprint(statp->options & RES_DEBUG,
+			       (stdout, ";; response may be truncated (UDP)\n")
+			);
+		}
+
 		HEADER *anhp = (HEADER *) *thisansp;
 		socklen_t fromlen = sizeof(struct sockaddr_in6);
 		assert (sizeof(from) <= fromlen);