[PATCH] powerpc/64s: Fix irq tracing corruption in interrupt/syscall return caused by perf interrupts

[Nicholas Piggin <npiggin@gmail.com>]

Alexey reports lockdep_assert_irqs_enabled() warnings when stress testing perf, e.g.,

WARNING: CPU: 0 PID: 1556 at kernel/softirq.c:169 __local_bh_enable_ip+0x258/0x270
CPU: 0 PID: 1556 Comm: syz-executor
NIP:  c0000000001ec888 LR: c0000000001ec884 CTR: c000000000ef0610
REGS: c000000022d4f8a0 TRAP: 0700   Not tainted  (5.8.0-rc3-x)
MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 28008844  XER: 20040000
CFAR: c0000000001dc1d0 IRQMASK: 0

The interesting thing is MSR[EE] and IRQMASK shows interrupts are enabled,
suggesting the current->hardirqs_enabled irq tracing state is going out of sync
with the actual interrupt enable state.

The cause is a window in interrupt/syscall return where irq tracing state is being
adjusted for an irqs-enabled return while MSR[EE] is still enabled. A perf
interrupt hits and ends up calling trace_hardirqs_off() when restoring
interrupt flags to a disable state.

Fix this by disabling perf interrupts as well while adjusting irq tracing state.

Add a debug check that catches the condition sooner.

Fixes: 68b34588e202 ("powerpc/64/sycall: Implement syscall entry/exit logic in C")
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---

I can reproduce similar symptoms and this patch fixes my test case,
still trying to confirm Alexey's test case or whether there's another
similar bug causing it.

 arch/powerpc/kernel/syscall_64.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/syscall_64.c b/arch/powerpc/kernel/syscall_64.c
index 79edba3ab312..6c6f88eff915 100644
--- a/arch/powerpc/kernel/syscall_64.c
+++ b/arch/powerpc/kernel/syscall_64.c
@@ -107,8 +107,13 @@ notrace long system_call_exception(long r3, long r4, long r5,
  */
 static notrace inline bool prep_irq_for_enabled_exit(void)
 {
-	/* This must be done with RI=1 because tracing may touch vmaps */
-	trace_hardirqs_on();
+	if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS)) {
+		/* Prevent perf interrupts hitting and messing up the trace_hardirqs state */
+		irq_soft_mask_set(IRQS_ALL_DISABLED);
+
+		/* This must be done with RI=1 because tracing may touch vmaps */
+		trace_hardirqs_on();
+	}
 
 	/* This pattern matches prep_irq_for_idle */
 	__hard_EE_RI_disable();
@@ -123,6 +128,8 @@ static notrace inline bool prep_irq_for_enabled_exit(void)
 	local_paca->irq_happened = 0;
 	irq_soft_mask_set(IRQS_ENABLED);
 
+	lockdep_assert_irqs_enabled();
+
 	return true;
 }
 
-- 
2.23.0