Harden putpwent, putgrent, putspent, putspent against injection [BZ #18724]

This prevents injection of ':' and '\n' into output functions which
use the NSS files database syntax.  Critical fields (user/group names
and file system paths) are checked strictly.  For backwards
compatibility, the GECOS field is rewritten instead.

The getent program is adjusted to use the put*ent functions in libc,
instead of local copies.  This changes the behavior of getent if user
names start with '-' or '+'.

nss/rewrite_field.c

@@ -0,0 +1,51 @@
+/* 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 <nss.h>
+#include <string.h>
+
+/* Rewrite VALUE to a valid field value in the NSS database.  Invalid
+   characters are replaced with a single space character ' '.  If
+   VALUE is NULL, the empty string is returned.  *TO_BE_FREED is
+   overwritten with a pointer the caller has to free if the function
+   returns successfully.  On failure, return NULL.  */
+const char *
+__nss_rewrite_field (const char *value, char **to_be_freed)
+{
+  *to_be_freed = NULL;
+  if (value == NULL)
+    return "";
+
+  /* Search for non-allowed characters.  */
+  const char *p = strpbrk (value, __nss_invalid_field_characters);
+  if (p == NULL)
+    return value;
+  *to_be_freed = __strdup (value);
+  if (*to_be_freed == NULL)
+    return NULL;
+
+  /* Switch pointer to freshly-allocated buffer.  */
+  char *bad = *to_be_freed + (p - value);
+  do
+    {
+      *bad = ' ';
+      bad = strpbrk (bad + 1, __nss_invalid_field_characters);
+    }
+  while (bad != NULL);
+
+  return *to_be_freed;
+}