Hi Stefano,
On 30/05/17 22:39, Stefano Stabellini wrote:
On Tue, 30 May 2017, Julien Grall wrote:
Hi Andre,
On 26/05/17 18:35, Andre Przywara wrote:
When reading the priority value of a virtual interrupt, we were taking
the respective rank lock so far.
However for forwarded interrupts (Dom0 only so far) this may lead to a
deadlock with the following call chain:
- MMIO access to change the IRQ affinity, calling the ITARGETSR handler
- this handler takes the appropriate rank lock and calls
vgic_store_itargetsr()
- vgic_store_itargetsr() will eventually call vgic_migrate_irq()
- if this IRQ is already in-flight, it will remove it from the old
VCPU and inject it into the new one, by calling vgic_vcpu_inject_irq()
- vgic_vcpu_inject_irq will call vgic_get_virq_priority()
- vgic_get_virq_priority() tries to take the rank lock - again!
It seems like this code path has never been exercised before.
Fix this by avoiding taking the lock in vgic_get_virq_priority() (like we
do in vgic_get_target_vcpu()).
Actually we are just reading one byte, and priority changes while
interrupts are handled are a benign race that can happen on real hardware
too. So it looks safe to just use read_atomic() instead.
Signed-off-by: Andre Przywara <andre.przyw...@arm.com>
---
xen/arch/arm/vgic.c | 8 +-------
1 file changed, 1 insertion(+), 7 deletions(-)
diff --git a/xen/arch/arm/vgic.c b/xen/arch/arm/vgic.c
index 83569b0..54b2aad 100644
--- a/xen/arch/arm/vgic.c
+++ b/xen/arch/arm/vgic.c
@@ -227,14 +227,8 @@ struct vcpu *vgic_get_target_vcpu(struct vcpu *v,
unsigned int virq)
static int vgic_get_virq_priority(struct vcpu *v, unsigned int virq)
{
struct vgic_irq_rank *rank = vgic_rank_irq(v, virq);
- unsigned long flags;
- int priority;
-
- vgic_lock_rank(v, rank, flags);
- priority = rank->priority[virq & INTERRUPT_RANK_MASK];
- vgic_unlock_rank(v, rank, flags);
- return priority;
+ return read_atomic(&rank->priority[virq & INTERRUPT_RANK_MASK]);
The write in rank->priority will not be atomic (see vgic_reg_update
implementation): the register is first masked, the the priority set.
So you may end up to read 0 (which is the higher priority) by mistake.
We should probably think to make vgic_reg_* helper atomic.
Right! That's why I wrote
alpine.DEB.2.10.1705231135130.18759@sstabellini-ThinkPad-X260
It was not obvious from this e-mail why you wanted this. It looked more
a request for improvement rather than a potential bug. Hence my
suggestion to delay it.
Cheers,
--
Julien Grall
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