From: Timo Sirainen <timo.sirainen@open-xchange.com>
Date: Tue, 7 Apr 2020 16:16:10 +0000 (+0300)
Subject: lib: Add str_equals_timing_almost_safe()
X-Git-Tag: 2.3.11.2~352
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=5a73613c7ca029fa7555e84fd1d9f802c2da956a;p=thirdparty%2Fdovecot%2Fcore.git

lib: Add str_equals_timing_almost_safe()
---

diff --git a/src/lib/strfuncs.c b/src/lib/strfuncs.c
index cf98e12753..49fe81872f 100644
--- a/src/lib/strfuncs.c
+++ b/src/lib/strfuncs.c
@@ -583,6 +583,21 @@ bool mem_equals_timing_safe(const void *p1, const void *p2, size_t size)
 	return ret == 0;
 }
 
+bool str_equals_timing_almost_safe(const char *s1, const char *s2)
+{
+	size_t i;
+	int ret = 0;
+
+	for (i = 0; s1[i] != '\0' && s2[i] != '\0'; i++)
+		ret |= s1[i] ^ s2[i];
+	ret |= s1[i] ^ s2[i];
+
+	/* make sure the compiler optimizer doesn't try to break out of the
+	   above loop early. */
+	timing_safety_unoptimization = ret;
+	return ret == 0;
+}
+
 size_t
 str_match(const char *p1, const char *p2)
 {
diff --git a/src/lib/strfuncs.h b/src/lib/strfuncs.h
index bfd2bd17b3..0fef770ac8 100644
--- a/src/lib/strfuncs.h
+++ b/src/lib/strfuncs.h
@@ -80,6 +80,11 @@ int i_strcasecmp_p(const char *const *p1, const char *const *p2) ATTR_PURE;
 /* Returns TRUE if the two memory areas are equal. This function is safe
    against timing attacks, so it compares all the bytes every time. */
 bool mem_equals_timing_safe(const void *p1, const void *p2, size_t size);
+/* Returns TRUE if the two strings are equal. Similar to
+   mem_equals_timing_safe() this function is safe against timing attacks when
+   the string lengths are the same. If not, the length of the secret string may
+   be leaked, but otherwise the contents won't be. */
+bool str_equals_timing_almost_safe(const char *s1, const char *s2);
 
 size_t str_match(const char *p1, const char *p2) ATTR_PURE;
 static inline ATTR_PURE bool str_begins(const char *haystack, const char *needle)
diff --git a/src/lib/test-strfuncs.c b/src/lib/test-strfuncs.c
index a2ea307376..5db2711037 100644
--- a/src/lib/test-strfuncs.c
+++ b/src/lib/test-strfuncs.c
@@ -387,6 +387,32 @@ static void test_mem_equals_timing_safe(void)
 	test_end();
 }
 
+static void test_str_equals_timing_almost_safe(void)
+{
+	const struct {
+		const char *a, *b;
+	} tests[] = {
+		{ "", "" },
+		{ "a", "a" },
+		{ "b", "a" },
+		{ "ab", "ab" },
+		{ "ab", "ba" },
+		{ "ab", "bc" },
+		{ "a", "" },
+		{ "a", "ab" },
+		{ "a", "abc" },
+		{ "ab", "abc" },
+	};
+	test_begin("str_equals_timing_almost_safe()");
+	for (unsigned int i = 0; i < N_ELEMENTS(tests); i++) {
+		test_assert((strcmp(tests[i].a, tests[i].b) == 0) ==
+			    str_equals_timing_almost_safe(tests[i].a, tests[i].b));
+		test_assert((strcmp(tests[i].a, tests[i].b) == 0) ==
+			    str_equals_timing_almost_safe(tests[i].b, tests[i].a));
+	}
+	test_end();
+}
+
 static void test_dec2str_buf(void)
 {
 	const uintmax_t test_input[] = {
@@ -462,6 +488,7 @@ void test_strfuncs(void)
 	test_t_strarray_join();
 	test_p_array_const_string_join();
 	test_mem_equals_timing_safe();
+	test_str_equals_timing_almost_safe();
 	test_dec2str_buf();
 	test_str_match();
 }