[PATCH] powerpc/64: prevent replayed interrupt handlers from running softirqs

[Nicholas Piggin]

Running softirqs enables interrupts, which can then end up recursing
into the irq soft-mask code we're adjusting, including replaying
interrupts itself, which might be theoretically unbounded.

This abridged trace shows how this can occur:

! NIP replay_soft_interrupts
  LR  interrupt_exit_kernel_prepare
  Call Trace:
    interrupt_exit_kernel_prepare (unreliable)
    interrupt_return
  --- interrupt: ea0 at __rb_reserve_next
  NIP __rb_reserve_next
  LR __rb_reserve_next
  Call Trace:
    ring_buffer_lock_reserve
    trace_function
    function_trace_call
    ftrace_call
    __do_softirq
    irq_exit
    timer_interrupt
!   replay_soft_interrupts
    interrupt_exit_kernel_prepare
    interrupt_return
  --- interrupt: ea0 at arch_local_irq_restore

Fix this by disabling bhs (softirqs) around the interrupt replay.

I don't know that commit 3282a3da25bd ("powerpc/64: Implement soft
interrupt replay in C") actually introduced the problem. Prior to that
change, the interrupt replay seems like it should still be subect to
this recusion, however it's done after all the irq state has been fixed
up at the end of the replay, so it seems reasonable to fix back to this
commit.

Fixes: 3282a3da25bd ("powerpc/64: Implement soft interrupt replay in C")
Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
 arch/powerpc/kernel/irq.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index 6b1eca53e36c..7064135f9dc3 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -188,6 +188,18 @@ void replay_soft_interrupts(void)
 	unsigned char happened = local_paca->irq_happened;
 	struct pt_regs regs;
 
+	/*
+	 * Prevent softirqs from being run when an interrupt handler returns
+	 * and calls irq_exit(), because softirq processing enables interrupts.
+	 * If an interrupt is taken, it may then call replay_soft_interrupts
+	 * on its way out, which gets messy and recursive.
+	 *
+	 * softirqs created by replayed interrupts will be run at the end of
+	 * this function when bhs are enabled (if they were enabled in our
+	 * caller).
+	 */
+	local_bh_disable();
+
 	ppc_save_regs(&regs);
 	regs.softe = IRQS_ENABLED;
 
@@ -263,6 +275,8 @@ void replay_soft_interrupts(void)
 		trace_hardirqs_off();
 		goto again;
 	}
+
+	local_bh_enable();
 }
 
 notrace void arch_local_irq_restore(unsigned long mask)
-- 
2.23.0