arm64: ssbd: Restore mitigation status on CPU resume

On a system where firmware can dynamically change the state of the
mitigation, the CPU will always come up with the mitigation enabled,
including when coming back from suspend.

If the user has requested "no mitigation" via a command line option,
let's enforce it by calling into the firmware again to disable it.

Similarily, for a resume from hibernate, the mitigation could have
been disabled by the boot kernel. Let's ensure that it is set
back on in that case.

Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/hibernate.c

@@ -313,6 +313,17 @@ int swsusp_arch_suspend(void)
 
 		sleep_cpu = -EINVAL;
 		__cpu_suspend_exit();
+
+		/*
+		 * Just in case the boot kernel did turn the SSBD
+		 * mitigation off behind our back, let's set the state
+		 * to what we expect it to be.
+		 */
+		switch (arm64_get_ssbd_state()) {
+		case ARM64_SSBD_FORCE_ENABLE:
+		case ARM64_SSBD_KERNEL:
+			arm64_set_ssbd_mitigation(true);
+		}
 	}
 
 	local_daif_restore(flags);