allow ClearSession purely via confirm button

Holding confirm button at home screen asks user whether they wish to
lock the TREZOR (clear the cached PIN and passphrase and show the
screensaver). This is identical behaviour to the ClearSession message.

firmware/layout2.h

@@ -24,6 +24,8 @@
 #include "types.pb.h"
 #include "bitmaps.h"
 
+extern void *layoutLast;
+
 void layoutDialogSwipe(const BITMAP *icon, const char *btnNo, const char *btnYes, const char *desc, const char *line1, const char *line2, const char *line3, const char *line4, const char *line5, const char *line6);
 void layoutProgressSwipe(const char *desc, int permil);
 

firmware/trezor.c

@@ -27,6 +27,7 @@
 #include "layout.h"
 #include "layout2.h"
 #include "rng.h"
+#include "buttons.h"
 
 uint32_t __stack_chk_guard;
 
@@ -36,6 +37,46 @@ void __attribute__((noreturn)) __stack_chk_fail(void)
 	for (;;) {} // loop forever
 }
 
+void check_lock_screen(void)
+{
+	buttonUpdate();
+
+	// wake from screensaver on any button
+	if (layoutLast == layoutScreensaver && (button.NoUp || button.YesUp)) {
+		layoutHome();
+		return;
+	}
+
+	// button held for long enough
+	if (layoutLast == layoutHome && button.NoDown >= 500000) {
+
+		layoutDialog(&bmp_icon_question, "Cancel", "Lock Device", NULL, "Do you really want to", "lock your TREZOR?", NULL, NULL, NULL, NULL);
+
+		// wait until NoButton is released
+		usbTiny(1);
+		do {
+			usbDelay(3300);
+			buttonUpdate();
+		} while (!button.NoUp);
+
+		// wait for confirmation/cancellation of the dialog
+		do {
+			usbDelay(3300);
+			buttonUpdate();
+		} while (!button.YesUp && !button.NoUp);
+		usbTiny(0);
+
+		if (button.YesUp) {
+			// lock the screen
+			session_clear(true);
+			layoutScreensaver();
+		} else {
+			// resume homescreen
+			layoutHome();
+		}
+	}
+}
+
 int main(void)
 {
 	__stack_chk_guard = random32();
@@ -61,6 +102,7 @@ int main(void)
 	usbInit();
 	for (;;) {
 		usbPoll();
+		check_lock_screen();
 	}
 
 	return 0;