Re: [PATCH] x86: check for valid irq_cfg pointer in smp_irq_move_cleanup_interrupt

[Dimitri Sivanich <sivanich@sgi.com>]

On Mon, May 21, 2012 at 11:34:32PM +0200, Thomas Gleixner wrote:
> Don't misunderstand me. The patch is correct, just the explanation
> sucks.
> 
Hopefully this explanation is descriptive enough.



A NULL pointer dereference can occur in smp_irq_move_cleanup_interrupt() if
we haven't yet setup the irq_cfg pointer in the irq_desc.irq_data.chip_data.

In create_irq_nr() there is a window where we have set vector_irq in
__assign_irq_vector(), but not yet called irq_set_chip_data() to set the
irq_cfg pointer.

Should an IRQ_MOVE_CLEANUP_VECTOR hit the cpu in question during this time,
smp_irq_move_cleanup_interrupt() will attempt to process the aforementioned
irq, but panic when accessing irq_cfg.

Only continue processing the irq if irq_cfg is non-NULL.

Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
---
 arch/x86/kernel/apic/io_apic.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux/arch/x86/kernel/apic/io_apic.c
===================================================================
--- linux.orig/arch/x86/kernel/apic/io_apic.c
+++ linux/arch/x86/kernel/apic/io_apic.c
@@ -2478,9 +2478,12 @@ asmlinkage void smp_irq_move_cleanup_int
 		if (!desc)
 			continue;
 
-		cfg = irq_cfg(irq);
 		raw_spin_lock(&desc->lock);
 
+		cfg = irq_cfg(irq);
+		if (!cfg)
+			goto unlock;
+
 		/*
 		 * Check if the irq migration is in progress. If so, we
 		 * haven't received the cleanup request yet for this irq.