[COMMITTED 2.18] Backport fixes for CVE-2015-7547

[COMMITTED 2.18] Backport fixes for CVE-2015-7547

[Paul E. Murphy]

[-- Attachment #1: Type: text/plain, Size: 490 bytes --]

I've applied the following patches to the 2.18 branch. I initially pushed them
for the ibm derivative, but they should go here too.

I ran into a conflict applying BZ #19791.  It turns out it was caused by another
bug fixed upstream, so I applied it as well.

1e18675 resolv: Always set *resplen2 out parameter in send_dg [BZ #19791]
69242db Fix invalid file descriptor reuse while sending DNS query (BZ #15946)
97eff76 CVE-2015-7547: getaddrinfo() stack-based buffer overflow (Bug 18665).

[-- Attachment #2: 0001-CVE-2015-7547-getaddrinfo-stack-based-buffer-overflo.patch --]
[-- Type: text/x-patch, Size: 26740 bytes --]

From c5ae7f9a3b543426cb186fb7b493f9d8458467a9 Mon Sep 17 00:00:00 2001
From: Carlos O'Donell <carlos@systemhalted.org>
Date: Tue, 19 Apr 2016 17:38:09 -0500
Subject: [PATCH 1/3] 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

(cherry picked from commit e9db92d3acfe1822d56d11abcea5bfc4c41cf6ca)
---
 ChangeLog                 |  15 +++
 NEWS                      |  16 ++-
 resolv/nss_dns/dns-host.c | 111 +++++++++++++++++++-
 resolv/res_query.c        |   3 +
 resolv/res_send.c         | 262 +++++++++++++++++++++++++++++++++++-----------
 5 files changed, 343 insertions(+), 64 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index d1a598c..de1bb7c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2016-04-22  Carlos O'Donell  <carlos@redhat.com>
+
+	[BZ #18665]
+	* resolv/nss_dns/dns-host.c (gaih_getanswer_slice): Always set
+	*herrno_p.
+	(gaih_getanswer): Document functional behviour. Return tryagain
+	if any result is tryagain.
+	* resolv/res_query.c (__libc_res_nsearch): Set buffer size to zero
+	when freed.
+	* resolv/res_send.c: Add copyright text.
+	(__libc_res_nsend): Document that MAXPACKET is expected.
+	(send_vc): Document. Remove buffer reuse.
+	(send_dg): Document. Remove buffer reuse. Set *thisanssizp to set the
+	size of the buffer. Add Dprint for truncated UDP buffer.
+
 2015-12-31  Leonhard Holz  <leonhard.holz@web.de>
 
 	[BZ #16009]
diff --git a/NEWS b/NEWS
index 68d5a44..16a8c15 100644
--- a/NEWS
+++ b/NEWS
@@ -10,10 +10,24 @@ Version 2.18.1
 * The following bugs are resolved with this release:
 
   15073, 15128, 15909, 15996, 16009, 16150, 16169, 16387, 16510, 16885,
-  16916, 16943, 16958, 17269, 18032, 18928, 19018.
+  16916, 16943, 16958, 17269, 18032, 18665, 18928, 19018.
 
 * The LD_POINTER_GUARD environment variable can no longer be used to
   disable the pointer guard feature.  It is always enabled.
+
+* 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)
 \f
 Version 2.18
 
diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
index 9018bb9..e30f9d9 100644
--- a/resolv/nss_dns/dns-host.c
+++ b/resolv/nss_dns/dns-host.c
@@ -1049,7 +1049,10 @@ 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)
     {
@@ -1226,7 +1229,14 @@ 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;
 }
 
 
@@ -1240,11 +1250,101 @@ 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
@@ -1260,8 +1360,15 @@ 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 1325f97..dd87e09 100644
--- a/resolv/res_query.c
+++ b/resolv/res_query.c
@@ -391,6 +391,7 @@ __libc_res_nsearch(res_state statp,
 		  {
 		    free (*answerp2);
 		    *answerp2 = NULL;
+		    *nanswerp2 = 0;
 		  }
 	}
 
@@ -431,6 +432,7 @@ __libc_res_nsearch(res_state statp,
 			  {
 			    free (*answerp2);
 			    *answerp2 = NULL;
+			    *nanswerp2 = 0;
 			  }
 
 			/*
@@ -502,6 +504,7 @@ __libc_res_nsearch(res_state statp,
 	  {
 	    free (*answerp2);
 	    *answerp2 = NULL;
+	    *nanswerp2 = 0;
 	  }
 	if (saved_herrno != -1)
 		RES_SET_H_ERRNO(statp, saved_herrno);
diff --git a/resolv/res_send.c b/resolv/res_send.c
index 60da5c9..bc98e59 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.
@@ -360,6 +377,8 @@ __libc_res_nsend(res_state statp, const u_char *buf, int buflen,
 #ifdef USE_HOOKS
 	if (__builtin_expect (statp->qhook || statp->rhook, 0)) {
 		if (anssiz < MAXPACKET && ansp) {
+			/* Always allocate MAXPACKET, callers expect
+			   this specific size.  */
 			u_char *buf = malloc (MAXPACKET);
 			if (buf == NULL)
 				return (-1);
@@ -652,6 +671,77 @@ libresolv_hidden_def (res_nsend)
 
 /* Private */
 
+/* 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,
@@ -661,11 +751,7 @@ 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_in6 *nsap = EXT(statp).nsaddrs[ns];
 	int truncating, connreset, resplen, n;
 	struct iovec iov[4];
@@ -741,6 +827,8 @@ 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:
@@ -777,33 +865,14 @@ 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.  */
-#ifdef _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;
@@ -811,10 +880,14 @@ 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 (__builtin_expect (anscp != NULL, 1)) {
+	/* 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;
@@ -824,6 +897,9 @@ send_vc(res_state statp,
 			*thisanssizp = MAXPACKET;
 			*thisansp = newp;
 			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,
@@ -987,6 +1063,66 @@ 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,
@@ -996,8 +1132,6 @@ 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;
@@ -1030,6 +1164,8 @@ 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;
@@ -1193,50 +1329,54 @@ 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.  */
-#ifdef _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)) {
+			|| *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;
 			}
 		}
+		/* 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);
-- 
2.4.11


[-- Attachment #3: 0002-Fix-invalid-file-descriptor-reuse-while-sending-DNS-.patch --]
[-- Type: text/x-patch, Size: 1686 bytes --]

From 04130e53ba685dd7138e3e928708e17923f795b5 Mon Sep 17 00:00:00 2001
From: Andreas Schwab <schwab@suse.de>
Date: Fri, 22 Apr 2016 10:35:41 -0500
Subject: [PATCH 2/3] Fix invalid file descriptor reuse while sending DNS query
 (BZ #15946)

(cherry picked from commit 45af2f6fe19b8a776373cac5a2691460179aa1a3)
---
 ChangeLog         | 6 ++++++
 NEWS              | 4 ++--
 resolv/res_send.c | 1 +
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index de1bb7c..0be4a59 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-04-22  Andreas Schwab  <schwab@suse.de>
+
+	[BZ #15946]
+	* resolv/res_send.c (send_dg): Reload file descriptor after
+	calling reopen.
+
 2016-04-22  Carlos O'Donell  <carlos@redhat.com>
 
 	[BZ #18665]
diff --git a/NEWS b/NEWS
index 16a8c15..94bcc4e 100644
--- a/NEWS
+++ b/NEWS
@@ -9,8 +9,8 @@ Version 2.18.1
 
 * The following bugs are resolved with this release:
 
-  15073, 15128, 15909, 15996, 16009, 16150, 16169, 16387, 16510, 16885,
-  16916, 16943, 16958, 17269, 18032, 18665, 18928, 19018.
+  15073, 15128, 15909, 15946, 15996, 16009, 16150, 16169, 16387, 16510,
+  16885, 16916, 16943, 16958, 17269, 18032, 18665, 18928, 19018.
 
 * The LD_POINTER_GUARD environment variable can no longer be used to
   disable the pointer guard feature.  It is always enabled.
diff --git a/resolv/res_send.c b/resolv/res_send.c
index bc98e59..e8b1809 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -1542,6 +1542,7 @@ send_dg(res_state statp,
 					retval = reopen (statp, terrno, ns);
 					if (retval <= 0)
 						return retval;
+					pfd[0].fd = EXT(statp).nssocks[ns];
 				}
 			}
 			goto wait;
-- 
2.4.11


[-- Attachment #4: 0003-resolv-Always-set-resplen2-out-parameter-in-send_dg-.patch --]
[-- Type: text/x-patch, Size: 8528 bytes --]

From 715e7fe0322004fd2e3fed853e9be3c279f5015d Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Fri, 22 Apr 2016 10:38:15 -0500
Subject: [PATCH 3/3] resolv: Always set *resplen2 out parameter in send_dg [BZ
 #19791]

Since commit 44d20bca52ace85850012b0ead37b360e3ecd96e (Implement
second fallback mode for DNS requests), there is a code path which
returns early, before *resplen2 is initialized.  This happens if the
name server address is immediately recognized as invalid (because of
lack of protocol support, or if it is a broadcast address such
255.255.255.255, or another invalid address).

If this happens and *resplen2 was non-zero (which is the case if a
previous query resulted in a failure), __libc_res_nquery would reuse
an existing second answer buffer.  This answer has been previously
identified as unusable (for example, it could be an NXDOMAIN
response).  Due to the presence of a second answer, no name server
switching will occur.  The result is a name resolution failure,
although a successful resolution would have been possible if name
servers have been switched and queries had proceeded along the search
path.

The above paragraph still simplifies the situation.  Before glibc
2.23, if the second answer needed malloc, the stub resolver would
still attempt to reuse the second answer, but this is not possible
because __libc_res_nsearch has freed it, after the unsuccessful call
to __libc_res_nquerydomain, and set the buffer pointer to NULL.  This
eventually leads to an assertion failure in __libc_res_nquery:

	/* Make sure both hp and hp2 are defined */
	assert((hp != NULL) && (hp2 != NULL));

If assertions are disabled, the consequence is a NULL pointer
dereference on the next line.

Starting with glibc 2.23, as a result of commit
e9db92d3acfe1822d56d11abcea5bfc4c41cf6ca (CVE-2015-7547: getaddrinfo()
stack-based buffer overflow (Bug 18665)), the second answer is always
allocated with malloc.  This means that the assertion failure happens
with small responses as well because there is no buffer to reuse, as
soon as there is a name resolution failure which triggers a search for
an answer along the search path.

This commit addresses the issue by ensuring that *resplen2 is
initialized before the send_dg function returns.

This commit also addresses a bug where an invalid second reply is
incorrectly returned as a valid to the caller.

(cherry picked from commit b66d837bb5398795c6b0f651bd5a5d66091d8577)
---
 ChangeLog         |  9 ++++++++
 NEWS              |  3 ++-
 resolv/res_send.c | 63 +++++++++++++++++++++++++++++++++++--------------------
 3 files changed, 51 insertions(+), 24 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 0be4a59..fb777de 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2016-04-22  Florian Weimer  <fweimer@redhat.com>
+
+	[BZ #19791]
+	* resolv/res_send.c (close_and_return_error): New function.
+	(send_dg): Initialize *resplen2 after reopen failure.  Call
+	close_and_return_error for error returns.  On error paths without
+	__res_iclose, initialze *resplen2 explicitly.  Update comment for
+	successful return.
+
 2016-04-22  Andreas Schwab  <schwab@suse.de>
 
 	[BZ #15946]
diff --git a/NEWS b/NEWS
index 94bcc4e..67a29f8 100644
--- a/NEWS
+++ b/NEWS
@@ -10,7 +10,8 @@ Version 2.18.1
 * The following bugs are resolved with this release:
 
   15073, 15128, 15909, 15946, 15996, 16009, 16150, 16169, 16387, 16510,
-  16885, 16916, 16943, 16958, 17269, 18032, 18665, 18928, 19018.
+  16885, 16916, 16943, 16958, 17269, 18032, 18665, 18928, 19018, 19791.
+
 
 * The LD_POINTER_GUARD environment variable can no longer be used to
   disable the pointer guard feature.  It is always enabled.
diff --git a/resolv/res_send.c b/resolv/res_send.c
index e8b1809..9a10ef4 100644
--- a/resolv/res_send.c
+++ b/resolv/res_send.c
@@ -671,6 +671,18 @@ libresolv_hidden_def (res_nsend)
 
 /* Private */
 
+/* Close the resolver structure, assign zero to *RESPLEN2 if RESPLEN2
+   is not NULL, and return zero.  */
+static int
+__attribute__ ((warn_unused_result))
+close_and_return_error (res_state statp, int *resplen2)
+{
+  __res_iclose(statp, false);
+  if (resplen2 != NULL)
+    *resplen2 = 0;
+  return 0;
+}
+
 /* 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
@@ -1156,7 +1168,11 @@ send_dg(res_state statp,
  retry_reopen:
 	retval = reopen (statp, terrno, ns);
 	if (retval <= 0)
-		return retval;
+	  {
+	    if (resplen2 != NULL)
+	      *resplen2 = 0;
+	    return retval;
+	  }
  retry:
 	evNowTime(&now);
 	evConsTime(&timeout, seconds, 0);
@@ -1169,8 +1185,6 @@ send_dg(res_state statp,
 	int recvresp2 = buf2 == NULL;
 	pfd[0].fd = EXT(statp).nssocks[ns];
 	pfd[0].events = POLLOUT;
-	if (resplen2 != NULL)
-	  *resplen2 = 0;
  wait:
 	if (need_recompute) {
 	recompute_resend:
@@ -1178,9 +1192,7 @@ send_dg(res_state statp,
 		if (evCmpTime(finish, now) <= 0) {
 		poll_err_out:
 			Perror(statp, stderr, "poll", errno);
-		err_out:
-			__res_iclose(statp, false);
-			return (0);
+			return close_and_return_error (statp, resplen2);
 		}
 		evSubTime(&timeout, &finish, &now);
 		need_recompute = 0;
@@ -1227,7 +1239,9 @@ send_dg(res_state statp,
 		  }
 
 		*gotsomewhere = 1;
-		return (0);
+		if (resplen2 != NULL)
+		  *resplen2 = 0;
+		return 0;
 	}
 	if (n < 0) {
 		if (errno == EINTR)
@@ -1295,7 +1309,7 @@ send_dg(res_state statp,
 
 		      fail_sendmmsg:
 			Perror(statp, stderr, "sendmmsg", errno);
-			goto err_out;
+			return close_and_return_error (statp, resplen2);
 		      }
 		  }
 		else
@@ -1313,7 +1327,7 @@ send_dg(res_state statp,
 		      if (errno == EINTR || errno == EAGAIN)
 			goto recompute_resend;
 		      Perror(statp, stderr, "send", errno);
-		      goto err_out;
+		      return close_and_return_error (statp, resplen2);
 		    }
 		  just_one:
 		    if (nwritten != 0 || buf2 == NULL || single_request)
@@ -1389,7 +1403,7 @@ send_dg(res_state statp,
 				goto wait;
 			}
 			Perror(statp, stderr, "recvfrom", errno);
-			goto err_out;
+			return close_and_return_error (statp, resplen2);
 		}
 		*gotsomewhere = 1;
 		if (__builtin_expect (*thisresplenp < HFIXEDSZ, 0)) {
@@ -1400,7 +1414,7 @@ send_dg(res_state statp,
 			       (stdout, ";; undersized: %d\n",
 				*thisresplenp));
 			*terrno = EMSGSIZE;
-			goto err_out;
+			return close_and_return_error (statp, resplen2);
 		}
 		if ((recvresp1 || hp->id != anhp->id)
 		    && (recvresp2 || hp2->id != anhp->id)) {
@@ -1449,7 +1463,7 @@ send_dg(res_state statp,
 				? *thisanssizp : *thisresplenp);
 			/* record the error */
 			statp->_flags |= RES_F_EDNS0ERR;
-			goto err_out;
+			return close_and_return_error (statp, resplen2);
 	}
 #endif
 		if (!(statp->options & RES_INSECURE2)
@@ -1501,10 +1515,10 @@ send_dg(res_state statp,
 			  }
 
 		next_ns:
-			__res_iclose(statp, false);
 			/* don't retry if called from dig */
 			if (!statp->pfcode)
-				return (0);
+			  return close_and_return_error (statp, resplen2);
+			__res_iclose(statp, false);
 		}
 		if (anhp->rcode == NOERROR && anhp->ancount == 0
 		    && anhp->aa == 0 && anhp->ra == 0 && anhp->arcount == 0) {
@@ -1526,6 +1540,8 @@ send_dg(res_state statp,
 			__res_iclose(statp, false);
 			// XXX if we have received one reply we could
 			// XXX use it and not repeat it over TCP...
+			if (resplen2 != NULL)
+			  *resplen2 = 0;
 			return (1);
 		}
 		/* Mark which reply we received.  */
@@ -1541,21 +1557,22 @@ send_dg(res_state statp,
 					__res_iclose (statp, false);
 					retval = reopen (statp, terrno, ns);
 					if (retval <= 0)
-						return retval;
+					  {
+					    if (resplen2 != NULL)
+					      *resplen2 = 0;
+					    return retval;
+					  }
 					pfd[0].fd = EXT(statp).nssocks[ns];
 				}
 			}
 			goto wait;
 		}
-		/*
-		 * All is well, or the error is fatal.  Signal that the
-		 * next nameserver ought not be tried.
-		 */
+		/* All is well.  We have received both responses (if
+		   two responses were requested).  */
 		return (resplen);
-	} else if (pfd[0].revents & (POLLERR | POLLHUP | POLLNVAL)) {
-		/* Something went wrong.  We can stop trying.  */
-		goto err_out;
-	}
+	} else if (pfd[0].revents & (POLLERR | POLLHUP | POLLNVAL))
+	  /* Something went wrong.  We can stop trying.  */
+	  return close_and_return_error (statp, resplen2);
 	else {
 		/* poll should not have returned > 0 in this case.  */
 		abort ();
-- 
2.4.11