sunrpc: Avoid use-after-free read access in clntudp_call [BZ #21115]

After commit bc779a1a5b3035133024b21e2f339fe4219fb11c
(CVE-2016-4429: sunrpc: Do not use alloca in clntudp_call
[BZ #20112]), ancillary data is stored on the heap,
but it is accessed after it has been freed.

The test case must be run under a heap debugger such as valgrind
to observe the invalid access.  A malloc implementation which
immediately calls munmap on free would catch this bug as well.

(cherry picked from commit d42eed4a044e5e10dfb885cf9891c2518a72a491)
(cherry picked from commit 045e368799cd253ddbf8bdec42ed92e8ebb3ce67)

3 files changed, 65 insertions, 2 deletions

diff --git a/sunrpc/Makefile b/sunrpc/Makefile
index 0c1e6124ff..daf8a28f1a 100644
--- a/sunrpc/Makefile
+++ b/sunrpc/Makefile
@@ -93,7 +93,7 @@ rpcgen-objs = rpc_main.o rpc_hout.o rpc_cout.o rpc_parse.o \
 extra-objs = $(rpcgen-objs) $(addprefix cross-,$(rpcgen-objs))
 others += rpcgen
 
-tests = tst-xdrmem tst-xdrmem2 test-rpcent
+tests = tst-xdrmem tst-xdrmem2 test-rpcent tst-udp-error
 xtests := tst-getmyaddr
 
 ifeq ($(have-thread-library),yes)
@@ -155,6 +155,7 @@ BUILD_CPPFLAGS += $(sunrpc-CPPFLAGS)
 $(objpfx)tst-getmyaddr: $(common-objpfx)linkobj/libc.so
 $(objpfx)tst-xdrmem: $(common-objpfx)linkobj/libc.so
 $(objpfx)tst-xdrmem2: $(common-objpfx)linkobj/libc.so
+$(objpfx)tst-udp-error: $(common-objpfx)linkobj/libc.so
 
 $(objpfx)rpcgen: $(addprefix $(objpfx),$(rpcgen-objs))
 
diff --git a/sunrpc/clnt_udp.c b/sunrpc/clnt_udp.c
index 4d9acb1e6a..1de25cb771 100644
--- a/sunrpc/clnt_udp.c
+++ b/sunrpc/clnt_udp.c
@@ -421,9 +421,9 @@ send_again:
 		 cmsg = CMSG_NXTHDR (&msg, cmsg))
 	      if (cmsg->cmsg_level == SOL_IP && cmsg->cmsg_type == IP_RECVERR)
 		{
-		  free (cbuf);
 		  e = (struct sock_extended_err *) CMSG_DATA(cmsg);
 		  cu->cu_error.re_errno = e->ee_errno;
+		  free (cbuf);
 		  return (cu->cu_error.re_status = RPC_CANTRECV);
 		}
 	  free (cbuf);
diff --git a/sunrpc/tst-udp-error.c b/sunrpc/tst-udp-error.c
new file mode 100644
index 0000000000..1efc02f5c6
--- /dev/null
+++ b/sunrpc/tst-udp-error.c
@@ -0,0 +1,62 @@
+/* Check for use-after-free in clntudp_call (bug 21115).
+   Copyright (C) 2017 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/>.  */
+
+#include <netinet/in.h>
+#include <rpc/clnt.h>
+#include <rpc/svc.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/xsocket.h>
+#include <unistd.h>
+
+static int
+do_test (void)
+{
+  support_become_root ();
+  support_enter_network_namespace ();
+
+  /* Obtain a likely-unused port number.  */
+  struct sockaddr_in sin =
+    {
+      .sin_family = AF_INET,
+      .sin_addr.s_addr = htonl (INADDR_LOOPBACK),
+    };
+  {
+    int fd = xsocket (AF_INET, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    xbind (fd, (struct sockaddr *) &sin, sizeof (sin));
+    socklen_t sinlen = sizeof (sin);
+    xgetsockname (fd, (struct sockaddr *) &sin, &sinlen);
+    /* Close the socket, so that we will receive an error below.  */
+    close (fd);
+  }
+
+  int sock = RPC_ANYSOCK;
+  CLIENT *clnt = clntudp_create
+    (&sin, 1, 2, (struct timeval) { 1, 0 }, &sock);
+  TEST_VERIFY_EXIT (clnt != NULL);
+  TEST_VERIFY (clnt_call (clnt, 3,
+                          (xdrproc_t) xdr_void, NULL,
+                          (xdrproc_t) xdr_void, NULL,
+                          ((struct timeval) { 3, 0 }))
+               == RPC_CANTRECV);
+  clnt_destroy (clnt);
+
+  return 0;
+}
+
+#include <support/test-driver.c>