Terminate process on invalid netlink response from kernel [BZ #12926]

The recvmsg system calls for netlink sockets have been particularly
prone to picking up unrelated data after a file descriptor race
(where the descriptor is closed and reopened concurrently in a
multi-threaded process, as the result of a file descriptor
management issue elsewhere).  This commit adds additional error
checking and aborts the process if a datagram of unexpected length
(without the netlink header) is received, or an error code which
cannot happen due to the way the netlink socket is used.

	[BZ #12926]
	Terminate process on invalid netlink response.
	* sysdeps/unix/sysv/linux/netlinkaccess.h
	(__netlink_assert_response): Declare.
	* sysdeps/unix/sysv/linux/netlink_assert_response.c: New file.
	* sysdeps/unix/sysv/linux/Makefile [$(subdir) == inet]
	(sysdep_routines): Add netlink_assert_response.
	* sysdeps/unix/sysv/linux/check_native.c (__check_native): Call
	__netlink_assert_response.
	* sysdeps/unix/sysv/linux/check_pf.c (make_request): Likewise.
	* sysdeps/unix/sysv/linux/ifaddrs.c (__netlink_request): Likewise.
	* sysdeps/unix/sysv/linux/Versions (GLIBC_PRIVATE): Add
	__netlink_assert_response.

sysdeps/unix/sysv/linux/netlink_assert_response.c

@@ -0,0 +1,106 @@
+/* Check recvmsg results for netlink sockets.
+   Copyright (C) 2015 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 <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/socket.h>
+
+#include "netlinkaccess.h"
+
+static int
+get_address_family (int fd)
+{
+  struct sockaddr_storage sa;
+  socklen_t sa_len = sizeof (sa);
+  if (__getsockname (fd, (struct sockaddr *) &sa, &sa_len) < 0)
+    return -1;
+  /* Check that the socket family number is preserved despite in-band
+     signaling.  */
+  _Static_assert (sizeof (sa.ss_family) < sizeof (int), "address family size");
+  _Static_assert (0 < (__typeof__ (sa.ss_family)) -1,
+                  "address family unsigned");
+  return sa.ss_family;
+}
+
+void
+internal_function
+__netlink_assert_response (int fd, ssize_t result)
+{
+  if (result < 0)
+    {
+      /* Check if the error is unexpected.  */
+      bool terminate = false;
+      int error_code = errno;
+      int family = get_address_family (fd);
+      if (family != AF_NETLINK)
+        /* If the address family does not match (or getsockname
+           failed), report the original error.  */
+        terminate = true;
+      else if (error_code == EBADF
+          || error_code == ENOTCONN
+          || error_code == ENOTSOCK
+          || error_code == ECONNREFUSED)
+        /* These errors indicate that the descriptor is not a
+           connected socket.  */
+        terminate = true;
+      else if (error_code == EAGAIN || error_code == EWOULDBLOCK)
+        {
+          /* The kernel might return EAGAIN for other reasons than a
+             non-blocking socket.  But if the socket is not blocking,
+             it is not ours, so report the error.  */
+          int mode = __fcntl (fd, F_GETFL, 0);
+          if (mode < 0 || (mode & O_NONBLOCK) != 0)
+            terminate = true;
+        }
+      if (terminate)
+        {
+          char message[200];
+          if (family < 0)
+            __snprintf (message, sizeof (message),
+                        "Unexpected error %d on netlink descriptor %d",
+                        error_code, fd);
+          else
+            __snprintf (message, sizeof (message),
+                        "Unexpected error %d on netlink descriptor %d"
+                        " (address family %d)",
+                        error_code, fd, family);
+          __libc_fatal (message);
+        }
+      else
+        /* Restore orignal errno value.  */
+        __set_errno (error_code);
+    }
+  else if (result < sizeof (struct nlmsghdr))
+    {
+      char message[200];
+      int family = get_address_family (fd);
+      if (family < 0)
+          __snprintf (message, sizeof (message),
+                      "Unexpected netlink response of size %zd"
+                      " on descriptor %d",
+                      result, fd);
+      else
+          __snprintf (message, sizeof (message),
+                      "Unexpected netlink response of size %zd"
+                      " on descriptor %d (address family %d)",
+                      result, fd, family);
+      __libc_fatal (message);
+    }
+}
+libc_hidden_def (__netlink_assert_response)