firmware: google: Test spinlock on panic path to avoid lockups

[ Upstream commit 3e081438b8e639cc76ef1a5ce0c1bd8a154082c7 ]

Currently the gsmi driver registers a panic notifier as well as
reboot and die notifiers. The callbacks registered are called in
atomic and very limited context - for instance, panic disables
preemption and local IRQs, also all secondary CPUs (not executing
the panic path) are shutdown.

With that said, taking a spinlock in this scenario is a dangerous
invitation for lockup scenarios. So, fix that by checking if the
spinlock is free to acquire in the panic notifier callback - if not,
bail-out and avoid a potential hang.

Fixes: 74c5b31c66 ("driver: Google EFI SMI")
Cc: Andrew Morton <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: David Gow <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Julius Werner <[email protected]>
Cc: Petr Mladek <[email protected]>
Reviewed-by: Evan Green <[email protected]>
Signed-off-by: Guilherme G. Piccoli <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>

drivers/firmware/google/gsmi.c

@@ -679,6 +679,15 @@ static struct notifier_block gsmi_die_notifier = {
 static int gsmi_panic_callback(struct notifier_block *nb,
 			       unsigned long reason, void *arg)
 {
+
+	/*
+	 * Panic callbacks are executed with all other CPUs stopped,
+	 * so we must not attempt to spin waiting for gsmi_dev.lock
+	 * to be released.
+	 */
+	if (spin_is_locked(&gsmi_dev.lock))
+		return NOTIFY_DONE;
+
 	gsmi_shutdown_reason(GSMI_SHUTDOWN_PANIC);
 	return NOTIFY_DONE;
 }