base64: fix decoder output buffer overrun (reads)

Was caught by base64 test with enabled ASAN.

It also caused data corruption - garbage instead of "extra bits" was
saved into state->result if there was no space in output buffer.

Decode state removed along with helper functions.

Added test for "zero-sized output buffer" case.

Fixes: #3069
(cherry picked from commit 7214add2c7f2a86265a5e08f2184029a19fc184d)

test/unit/base64.c

@@ -75,9 +75,22 @@ base64_invalid_chars_test(void)
 	check_plan();
 }
 
+static void
+base64_no_space_test(void)
+{
+	plan(1);
+
+	const char *const in = "sIIpHw==";
+	const int in_len = strlen(in);
+	const int rc = base64_decode(in, in_len, NULL, 0);
+	is(rc, 0, "no space in out buffer");
+
+	check_plan();
+}
+
 int main(int argc, char *argv[])
 {
-	plan(29);
+	plan(30);
 	header();
 
 	const char *option_tests[] = {
@@ -96,6 +109,7 @@ int main(int argc, char *argv[])
 	}
 
 	base64_invalid_chars_test();
+	base64_no_space_test();
 
 	footer();
 	return check_plan();

test/unit/base64.result

-1..29
+1..30
 	*** main ***
     1..3
     ok 1 - length
@@ -178,4 +178,7 @@ ok 28 - subtests
     1..1
     ok 1 - ignoring invalid chars
 ok 29 - subtests
+    1..1
+    ok 1 - no space in out buffer
+ok 30 - subtests
 	*** main: done ***

third_party/base64.c

@@ -202,14 +202,6 @@ base64_encode(const char *in_bin, int in_len,
 
 /* {{{ decode */
 
-enum base64_decodestep { step_a, step_b, step_c, step_d };
-
-struct base64_decodestate
-{
-	enum base64_decodestep step;
-	char result;
-};
-
 static int
 base64_decode_value(int value)
 {
@@ -231,17 +223,9 @@ base64_decode_value(int value)
 	return decoding[codepos];
 }
 
-static inline void
-base64_decodestate_init(struct base64_decodestate *state)
-{
-	state->step = step_a;
-	state->result = 0;
-}
-
-static int
-base64_decode_block(const char *in_base64, int in_len,
-		    char *out_bin, int out_len,
-		    struct base64_decodestate *state)
+int
+base64_decode(const char *in_base64, int in_len,
+	      char *out_bin, int out_len)
 {
 	const char *in_pos = in_base64;
 	const char *in_end = in_base64 + in_len;
@@ -249,76 +233,39 @@ base64_decode_block(const char *in_base64, int in_len,
 	char *out_end = out_bin + out_len;
 	int fragment;
 
-	*out_pos = state->result;
-
-	switch (state->step)
+	while (1)
 	{
-		while (1)
-		{
-	case step_a:
-			do {
-				if (in_pos == in_end || out_pos >= out_end)
-				{
-					state->step = step_a;
-					state->result = *out_pos;
-					return out_pos - out_bin;
-				}
-				fragment = base64_decode_value(*in_pos++);
-			} while (fragment < 0);
-			*out_pos    = (fragment & 0x03f) << 2;
-	case step_b:
-			do {
-				if (in_pos == in_end || out_pos >= out_end)
-				{
-					state->step = step_b;
-					state->result = *out_pos;
-					return out_pos - out_bin;
-				}
-				fragment = base64_decode_value(*in_pos++);
-			} while (fragment < 0);
-			*out_pos++ |= (fragment & 0x030) >> 4;
-			if (out_pos < out_end)
-				*out_pos = (fragment & 0x00f) << 4;
-	case step_c:
-			do {
-				if (in_pos == in_end || out_pos >= out_end)
-				{
-					state->step = step_c;
-					state->result = *out_pos;
-					return out_pos - out_bin;
-				}
-				fragment = base64_decode_value(*in_pos++);
-			} while (fragment < 0);
-			*out_pos++ |= (fragment & 0x03c) >> 2;
-			if (out_pos < out_end)
-				*out_pos = (fragment & 0x003) << 6;
-	case step_d:
-			do {
-				if (in_pos == in_end || out_pos >= out_end)
-				{
-					state->step = step_d;
-					state->result = *out_pos;
-					return out_pos - out_bin;
-				}
-				fragment = base64_decode_value(*in_pos++);
-			} while (fragment < 0);
-			*out_pos++   |= (fragment & 0x03f);
-		}
+		do {
+			if (in_pos == in_end || out_pos >= out_end)
+				return out_pos - out_bin;
+			fragment = base64_decode_value(*in_pos++);
+		} while (fragment < 0);
+		*out_pos = (fragment & 0x03f) << 2;
+		do {
+			if (in_pos == in_end || out_pos >= out_end)
+				return out_pos - out_bin;
+			fragment = base64_decode_value(*in_pos++);
+		} while (fragment < 0);
+		*out_pos++ |= (fragment & 0x030) >> 4;
+		if (out_pos < out_end)
+			*out_pos = (fragment & 0x00f) << 4;
+		do {
+			if (in_pos == in_end || out_pos >= out_end)
+				return out_pos - out_bin;
+			fragment = base64_decode_value(*in_pos++);
+		} while (fragment < 0);
+		*out_pos++ |= (fragment & 0x03c) >> 2;
+		if (out_pos < out_end)
+			*out_pos = (fragment & 0x003) << 6;
+		do {
+			if (in_pos == in_end || out_pos >= out_end)
+				return out_pos - out_bin;
+			fragment = base64_decode_value(*in_pos++);
+		} while (fragment < 0);
+		*out_pos++ |= (fragment & 0x03f);
 	}
 	/* control should not reach here */
 	return out_pos - out_bin;
 }
 
-
-
-int
-base64_decode(const char *in_base64, int in_len,
-	      char *out_bin, int out_len)
-{
-	struct base64_decodestate state;
-	base64_decodestate_init(&state);
-	return base64_decode_block(in_base64, in_len,
-				   out_bin, out_len, &state);
-}
-
 /* }}} */