wg: use key_is_zero for comparing to zeros

Maybe an attacker on the system could use the infoleak in /proc to gauge
how long a wg(8) process takes to complete and determine the number of
leading zeros. This is somewhat ridiculous, but it's possible somebody
somewhere might at somepoint care in the future, so alright.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/config.c

@@ -387,17 +387,11 @@ bool config_read_init(struct config_ctx *ctx, struct wgdevice **device, bool app
 	return true;
 }
 
-static inline bool key_is_valid(uint8_t key[WG_KEY_LEN])
-{
-	static const uint8_t zero[WG_KEY_LEN] = { 0 };
-	return !!memcmp(key, zero, WG_KEY_LEN);
-}
-
 bool config_read_finish(struct config_ctx *ctx)
 {
 	size_t i;
 	struct wgpeer *peer;
-	if (ctx->buf.dev->flags & WGDEVICE_REPLACE_PEERS && !key_is_valid(ctx->buf.dev->private_key)) {
+	if (ctx->buf.dev->flags & WGDEVICE_REPLACE_PEERS && key_is_zero(ctx->buf.dev->private_key)) {
 		fprintf(stderr, "No private key configured\n");
 		goto err;
 	}
@@ -405,7 +399,7 @@ bool config_read_finish(struct config_ctx *ctx)
 		ctx->buf.dev->flags |= WGDEVICE_REMOVE_FWMARK;
 
 	for_each_wgpeer(ctx->buf.dev, peer, i) {
-		if (!key_is_valid(peer->public_key)) {
+		if (key_is_zero(peer->public_key)) {
 			fprintf(stderr, "A peer is missing a public key\n");
 			goto err;
 		}

src/encoding.c

@@ -77,12 +77,12 @@ void key_to_hex(char hex[static WG_KEY_LEN_HEX], const uint8_t key[static WG_KEY
 
 bool key_from_hex(uint8_t key[static WG_KEY_LEN], const char *hex)
 {
-	uint8_t i, c, c_acc = 0, c_alpha0, c_alpha, c_num0, c_num, c_val, state = 0;
+	uint8_t c, c_acc = 0, c_alpha0, c_alpha, c_num0, c_num, c_val, state = 0;
 
 	if (strlen(hex) != WG_KEY_LEN_HEX - 1)
 		return false;
 
-	for (i = 0; i < WG_KEY_LEN_HEX - 1; ++i) {
+	for (unsigned int i = 0; i < WG_KEY_LEN_HEX - 1; ++i) {
 		c = (uint8_t)hex[i];
 		c_num = c ^ 48U;
 		c_num0 = (c_num - 10U) >> 8;
@@ -99,3 +99,13 @@ bool key_from_hex(uint8_t key[static WG_KEY_LEN], const char *hex)
 	}
 	return true;
 }
+
+bool key_is_zero(const uint8_t key[static WG_KEY_LEN])
+{
+	uint8_t acc = 0;
+	for (unsigned int i = 0; i < WG_KEY_LEN; ++i) {
+		acc |= key[i];
+		__asm__ ("" : "=r" (acc) : "0" (acc));
+	}
+	return acc == 0;
+}

src/encoding.h

@@ -16,4 +16,6 @@ bool key_from_base64(uint8_t key[static WG_KEY_LEN], const char *base64);
 void key_to_hex(char hex[static WG_KEY_LEN_HEX], const uint8_t key[static WG_KEY_LEN]);
 bool key_from_hex(uint8_t key[static WG_KEY_LEN], const char *hex);
 
+bool key_is_zero(const uint8_t key[static WG_KEY_LEN]);
+
 #endif

src/ipc.c

@@ -170,7 +170,6 @@ out:
 
 static int userspace_set_device(struct wgdevice *dev)
 {
-	static const uint8_t zero[WG_KEY_LEN] = { 0 };
 	char hex[WG_KEY_LEN_HEX], ip[INET6_ADDRSTRLEN], host[4096 + 1], service[512 + 1];
 	struct wgpeer *peer;
 	struct wgipmask *ipmask;
@@ -186,7 +185,7 @@ static int userspace_set_device(struct wgdevice *dev)
 
 	if (dev->flags & WGDEVICE_REMOVE_PRIVATE_KEY)
 		fprintf(f, "private_key=\n");
-	else if (memcmp(dev->private_key, zero, WG_KEY_LEN)) {
+	else if (!key_is_zero(dev->private_key)) {
 		key_to_hex(hex, dev->private_key);
 		fprintf(f, "private_key=%s\n", hex);
 	}
@@ -208,7 +207,7 @@ static int userspace_set_device(struct wgdevice *dev)
 		}
 		if (peer->flags & WGPEER_REMOVE_PRESHARED_KEY)
 			fprintf(f, "preshared_key=\n");
-		else if (memcmp(peer->preshared_key, zero, WG_KEY_LEN)) {
+		else if (!key_is_zero(peer->preshared_key)) {
 			key_to_hex(hex, peer->preshared_key);
 			fprintf(f, "preshared_key=%s\n", hex);
 		}

src/show.c

@@ -75,12 +75,10 @@ static void sort_peers(struct wgdevice *device)
 	free(new_device);
 }
 
-static const uint8_t zero[WG_KEY_LEN] = { 0 };
-
 static char *key(const uint8_t key[static WG_KEY_LEN])
 {
 	static char base64[WG_KEY_LEN_BASE64];
-	if (!memcmp(key, zero, WG_KEY_LEN))
+	if (key_is_zero(key))
 		return "(none)";
 	key_to_base64(base64, key);
 	return base64;
@@ -212,9 +210,9 @@ static void pretty_print(struct wgdevice *device)
 
 	terminal_printf(TERMINAL_RESET);
 	terminal_printf(TERMINAL_FG_GREEN TERMINAL_BOLD "interface" TERMINAL_RESET ": " TERMINAL_FG_GREEN "%s" TERMINAL_RESET "\n", device->interface);
-	if (memcmp(device->public_key, zero, WG_KEY_LEN))
+	if (!key_is_zero(device->public_key))
 		terminal_printf("  " TERMINAL_BOLD "public key" TERMINAL_RESET ": %s\n", key(device->public_key));
-	if (memcmp(device->private_key, zero, WG_KEY_LEN))
+	if (!key_is_zero(device->private_key))
 		terminal_printf("  " TERMINAL_BOLD "private key" TERMINAL_RESET ": %s\n", masked_key(device->private_key));
 	if (device->port)
 		terminal_printf("  " TERMINAL_BOLD "listening port" TERMINAL_RESET ": %u\n", device->port);
@@ -226,7 +224,7 @@ static void pretty_print(struct wgdevice *device)
 	}
 	for_each_wgpeer(device, peer, i) {
 		terminal_printf(TERMINAL_FG_YELLOW TERMINAL_BOLD "peer" TERMINAL_RESET ": " TERMINAL_FG_YELLOW "%s" TERMINAL_RESET "\n", key(peer->public_key));
-		if (memcmp(peer->preshared_key, zero, WG_KEY_LEN))
+		if (!key_is_zero(peer->preshared_key))
 			terminal_printf("  " TERMINAL_BOLD "preshared key" TERMINAL_RESET ": %s\n", masked_key(peer->preshared_key));
 		if (peer->endpoint.addr.sa_family == AF_INET || peer->endpoint.addr.sa_family == AF_INET6)
 			terminal_printf("  " TERMINAL_BOLD "endpoint" TERMINAL_RESET ": %s\n", endpoint(&peer->endpoint.addr));

src/showconf.c

@@ -16,7 +16,6 @@
 
 int showconf_main(int argc, char *argv[])
 {
-	static const uint8_t zero[WG_KEY_LEN] = { 0 };
 	char base64[WG_KEY_LEN_BASE64];
 	char ip[INET6_ADDRSTRLEN];
 	struct wgdevice *device = NULL;
@@ -46,7 +45,7 @@ int showconf_main(int argc, char *argv[])
 		printf("ListenPort = %u\n", device->port);
 	if (device->fwmark)
 		printf("FwMark = 0x%x\n", device->fwmark);
-	if (memcmp(device->private_key, zero, WG_KEY_LEN)) {
+	if (!key_is_zero(device->private_key)) {
 		key_to_base64(base64, device->private_key);
 		printf("PrivateKey = %s\n", base64);
 	}
@@ -54,7 +53,7 @@ int showconf_main(int argc, char *argv[])
 	for_each_wgpeer(device, peer, i) {
 		key_to_base64(base64, peer->public_key);
 		printf("[Peer]\nPublicKey = %s\n", base64);
-		if (memcmp(peer->preshared_key, zero, WG_KEY_LEN)) {
+		if (!key_is_zero(peer->preshared_key)) {
 			key_to_base64(base64, peer->preshared_key);
 			printf("PresharedKey = %s\n", base64);
 		}