wireguard: netlink: avoid variable-sized memcpy on sockaddr

Doing a variable-sized memcpy is slower, and the compiler isn't smart
enough to turn this into a constant-size assignment.

Further, Kees' latest fortified memcpy will actually bark, because the
destination pointer is type sockaddr, not explicitly sockaddr_in or
sockaddr_in6, so it thinks there's an overflow:

    memcpy: detected field-spanning write (size 28) of single field
    "&endpoint.addr" at drivers/net/wireguard/netlink.c:446 (size 16)

Fix this by just assigning by using explicit casts for each checked
case.

Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reported-by: syzbot+a448cda4dba2dac50de5@syzkaller.appspotmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/wireguard/netlink.c

@@ -436,14 +436,13 @@ static int set_peer(struct wg_device *wg, struct nlattr **attrs)
 	if (attrs[WGPEER_A_ENDPOINT]) {
 		struct sockaddr *addr = nla_data(attrs[WGPEER_A_ENDPOINT]);
 		size_t len = nla_len(attrs[WGPEER_A_ENDPOINT]);
+		struct endpoint endpoint = { { { 0 } } };
 
-		if ((len == sizeof(struct sockaddr_in) &&
-		     addr->sa_family == AF_INET) ||
-		    (len == sizeof(struct sockaddr_in6) &&
-		     addr->sa_family == AF_INET6)) {
-			struct endpoint endpoint = { { { 0 } } };
-
-			memcpy(&endpoint.addr, addr, len);
+		if (len == sizeof(struct sockaddr_in) && addr->sa_family == AF_INET) {
+			endpoint.addr4 = *(struct sockaddr_in *)addr;
+			wg_socket_set_peer_endpoint(peer, &endpoint);
+		} else if (len == sizeof(struct sockaddr_in6) && addr->sa_family == AF_INET6) {
+			endpoint.addr6 = *(struct sockaddr_in6 *)addr;
 			wg_socket_set_peer_endpoint(peer, &endpoint);
 		}
 	}