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/include/asm/cpufeature.h

@@ -553,6 +553,12 @@ static inline int arm64_get_ssbd_state(void)
 #endif
 }
 
+#ifdef CONFIG_ARM64_SSBD
+void arm64_set_ssbd_mitigation(bool state);
+#else
+static inline void arm64_set_ssbd_mitigation(bool state) {}
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #endif

arch/arm64/kernel/cpu_errata.c

@@ -303,7 +303,7 @@ void __init arm64_enable_wa2_handling(struct alt_instr *alt,
 		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
 }
 
-static void arm64_set_ssbd_mitigation(bool state)
+void arm64_set_ssbd_mitigation(bool state)
 {
 	switch (psci_ops.conduit) {
 	case PSCI_CONDUIT_HVC:

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);

arch/arm64/kernel/suspend.c

@@ -62,6 +62,14 @@ void notrace __cpu_suspend_exit(void)
 	 */
 	if (hw_breakpoint_restore)
 		hw_breakpoint_restore(cpu);
+
+	/*
+	 * On resume, firmware implementing dynamic mitigation will
+	 * have turned the mitigation on. If the user has forcefully
+	 * disabled it, make sure their wishes are obeyed.
+	 */
+	if (arm64_get_ssbd_state() == ARM64_SSBD_FORCE_DISABLE)
+		arm64_set_ssbd_mitigation(false);
 }
 
 /*