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

[Dimitri Sivanich <sivanich@sgi.com>]

On Sat, May 26, 2012 at 12:18:21PM +0200, Thomas Gleixner wrote:
> On Fri, 25 May 2012, Suresh Siddha wrote:
> > On Thu, 2012-05-24 at 21:16 +0200, Thomas Gleixner wrote:
> > There are other (not-so common) irq desc references, like in the
> > show_interrupts() (cat /proc/interrupts path) etc, that does things like
> > this in the process context:
> > 
> >         desc = irq_to_desc(i);
> >         if (!desc)
> >                 return 0;
> > 
> >         raw_spin_lock_irqsave(&desc->lock, flags);
> > 
> > May be we should introduce something like
> > get_irq_desc_locked()/put_irq_desc_locked() that can safely access the
> > irq desc with pre-emption/irq's disabled and lock it etc. And the
> > synchronize_sched() will enable the destroy_irq()/free_desc() to free it
> > safely etc.
> 
> I want to avoid that and instead use proper refcounting. The reason is
> that we want to move the irq descriptor when the affinity changes
> nodes, and for that we need refcounting anyway.
>

While this proposal sounds good, in the meantime would there be any harm in
putting the NULL cfg check into smp_irq_move_cleanup_interrupt()?

It's a minimal change, and eliminates the panics that I've encountered thus far.

Reposting the patch.



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.

There is also a window in destroy_irq() where we've cleared the irq_cfg
pointer in free_irq_cfg(), but have not yet called irq_free_desc().  Note
that we have cleared vector_irq in __clear_irq_vector() prior to free_irq_cfg(),
but smp_irq_move_cleanup_interrupt() might've already referenced the irq_desc.

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.