powerpc/64s/interrupt: Fix interrupt exit race with security mitigation switch

The RFI and STF security mitigation options can flip the interrupt_exit_not_reentrant static branch condition concurrently with the interrupt exit code which tests that branch.

Interrupt exit tests this condition to set MSR[EE|RI] for exit, then again in the case a soft-masked interrupt is found pending, to recover the MSR so the interrupt can be replayed before attempting to exit again. If the condition changes between these two tests, the MSR and irq soft-mask state will become corrupted, leading to warnings and possible crashes. For example, if the branch is initially true then false, MSR[EE] will be 0 but PACA_IRQ_HARD_DIS clear and EE may not get enabled, leading to warnings in irq_64.c.

---
The static_branch condition should not be evaluated multiple times by the caller when coded like this AFAIKS because if code patching is disabled then it becomes an atomic_read of the key, and if it is enabled then it should be done with asm volatile so the compiler should be unable to expand it again.

Thanks,
Nick

 arch/powerpc/kernel/interrupt.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

index fc6631a80527..0ec1581619db 100644
@@ -50,16 +50,18 @@ static inline bool exit_must_hard_disable(void)
  */
 static notrace __always_inline bool prep_irq_for_enabled_exit(bool restartable)
 {
+	bool must_hard_disable = (exit_must_hard_disable() || !restartable);
+
 	/* This must be done with RI=1 because tracing may touch vmaps */
 	trace_hardirqs_on();
 
-	if (exit_must_hard_disable() || !restartable)
+	if (must_hard_disable)
 		__hard_EE_RI_disable();
 
 #ifdef CONFIG_PPC64
 	/* This pattern matches prep_irq_for_idle */
 	if (unlikely(lazy_irq_pending_nocheck())) {
-		if (exit_must_hard_disable() || !restartable) {
+		if (must_hard_disable) {
 			local_paca->irq_happened |= PACA_IRQ_HARD_DIS;
 			__hard_RI_enable();
 		}

2.37.2

Applied to powerpc/fixes.

[1/1] powerpc/64s/interrupt: Fix interrupt exit race with security mitigation switch

      https://git.kernel.org/powerpc/c/2ea31e2e62bbc4d11c411eeb36f1b02841dbcab1

cheers