Do not search outside of the system directory for windows DLLs

Hardens against some attacks.

evdns.c

@@ -3509,7 +3509,8 @@ load_nameservers_with_getnetworkparams(struct evdns_base *base)
 	GetNetworkParams_fn_t fn;
 
 	ASSERT_LOCKED(base);
-	if (!(handle = LoadLibrary(TEXT("iphlpapi.dll")))) {
+	if (!(handle = evutil_load_windows_system_library(
+			TEXT("iphlpapi.dll")))) {
 		log(EVDNS_LOG_WARN, "Could not open iphlpapi.dll");
 		status = -1;
 		goto done;

evutil.c

@@ -35,6 +35,7 @@
 #include <windows.h>
 #undef WIN32_LEAN_AND_MEAN
 #include <io.h>
+#include <tchar.h>
 #endif
 
 #include <sys/types.h>
@@ -2060,3 +2061,19 @@ evutil_hex_char_to_int(char c)
 	}
 	return -1;
 }
+
+#ifdef WIN32
+HANDLE
+evutil_load_windows_system_library(const TCHAR *library_name)
+{
+  TCHAR path[MAX_PATH];
+  unsigned n;
+  n = GetSystemDirectory(path, MAX_PATH);
+  if (n == 0 || n + _tcslen(library_name) + 2 >= MAX_PATH)
+    return 0;
+  _tcscat(path, TEXT("\\"));
+  _tcscat(path, library_name);
+  return LoadLibrary(path);
+}
+#endif
+

test/regress_util.c

@@ -1037,6 +1037,22 @@ test_evutil_getaddrinfo(void *arg)
 		evutil_freeaddrinfo(ai);
 }
 
+#ifdef WIN32
+static void
+test_evutil_loadsyslib(void *arg)
+{
+	HANDLE h=NULL;
+
+	h = evutil_load_windows_system_library(TEXT("kernel32.dll"));
+	tt_assert(h);
+
+end:
+	if (h)
+		CloseHandle(h);
+
+}
+#endif
+
 struct testcase_t util_testcases[] = {
 	{ "ipv4_parse", regress_ipv4_parse, 0, NULL, NULL },
 	{ "ipv6_parse", regress_ipv6_parse, 0, NULL, NULL },
@@ -1052,6 +1068,9 @@ struct testcase_t util_testcases[] = {
 	{ "integers", test_evutil_integers, 0, NULL, NULL },
 	{ "rand", test_evutil_rand, TT_FORK, NULL, NULL },
 	{ "getaddrinfo", test_evutil_getaddrinfo, TT_FORK, NULL, NULL },
+#ifdef WIN32
+	{ "loadsyslib", test_evutil_loadsyslib, TT_FORK, NULL, NULL },
+#endif
 	END_OF_TESTCASES,
 };
 

util-internal.h

@@ -224,6 +224,10 @@ long evutil_tv_to_msec(const struct timeval *tv);
 
 int evutil_hex_char_to_int(char c);
 
+#ifdef WIN32
+HANDLE evutil_load_windows_system_library(const TCHAR *library_name);
+#endif
+
 #ifdef __cplusplus
 }
 #endif