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Re: [Xen-devel] [RFC PATCH 22/49] ARM: new VGIC: Implement virtual IRQ injection

On 27/02/18 10:17, Andre Przywara wrote:

Hi Andre,

On 12/02/18 18:59, Julien Grall wrote:
On 09/02/18 14:39, Andre Przywara wrote:
    * Iterate over the VM's list of mapped LPIs to find the one with a
    * matching interrupt ID and return a reference to the IRQ structure.
@@ -97,6 +123,204 @@ void vgic_put_irq(struct domain *d, struct
vgic_irq *irq)
+ * vgic_target_oracle - compute the target vcpu for an irq
+ *
+ * @irq:    The irq to route. Must be already locked.
+ *
+ * Based on the current state of the interrupt (enabled, pending,
+ * active, vcpu and target_vcpu), compute the next vcpu this should be
+ * given to. Return NULL if this shouldn't be injected at all.
+ *
+ * Requires the IRQ lock to be held.
+ */
+static struct vcpu *vgic_target_oracle(struct vgic_irq *irq)
+    ASSERT(spin_is_locked(&irq->irq_lock));
+    /* If the interrupt is active, it must stay on the current vcpu */
+    if ( irq->active )
+        return irq->vcpu ? : irq->target_vcpu;
I am not sure to understand why you check whether irq->vcpu is NULL. If
the interrupt is active, then irq->vcpu should be NULL. Did I miss
                                                not           you mean?

Yes not NULL.


Not if it has been explicitly activated via ISACTIVER. This is not
implemented in Xen at the moment, but would be in the future. So I like
to keep this in.

Oh, I missed that case. Thank you for the explanation :).


The list is not sorted on insertion because this is not necessary most
of the time. In fact the hardware VGIC will do the sorting (kind of)
within the LRs. So as long as we don't have more than <number of LRS>
IRQs in the list, sorting is a waste of time. Experiments in the past
showed that the number of used LRs is less than 4 almost every time. And
since 4 is the mostly used number of implemented LRs, we virtually never
need the sorting. So we avoid doing that on every insertion, instead
doing that only if it's necessary.

I can foresee quite a few issues with this choice on Xen:
     1) You compute the size of ap list in vgic_flush_lr_state() and take
lock on every IRQ one by one. A guest could be nasty and make that list
quite big by make IRQs pending but never "active" them (i.e read IAR).

Yeah, we could try to shortcut a bit here.

     2) This might be an issue while checking whether you need to deliver
an interrupt (vgic_vcpu_pending_irq) because the list is not sorted.

Most of the time the list is very short, storing one, two or actually no
IRQs. In the function where we check for pending IRQs we bail out as
soon as we found the first eligible interrupt. So sorting does not help
in the majority of cases.

As you say "most of the time". Malicious guest are unusual but just enough to keep busy both the hypervisor and the security team.

If you are really concerned about that list growing too long, we could
think about mitigations:
1) Try to avoid iterating the whole list while checking whether it needs
to be sorted.
2) Store a flag that notes if the list has already been sorted. As long
as we don't change anything, we don't need to sort again. Would be good
to test whether this is actually helpful. But we would need to keep this
flag up-to-date, which sounds a bit fragile to get right.
3) Switch to sort-on-insertion once we reached a certain number of IRQs
on the list, to mitigate DOS attacks from the guest. This should avoid
list iterations in hot paths, with IRQs disabled.

But all of these sound a bit hackish to me and just would spoil the very
clean and robust code we have today. Also I am not sure we can avoid
list iterations every time (for instance in prune_ap_list()). There is
an upper limit today (number of SPIs), so we might be happy with that
for now.
To be honest I would very much dislike changing the code at this point.
I believe a patch series afterwards would be better, also to actually
have some numbers on the impact of this.

More robust than the current vGIC yes. However, clean code is not an excuse to dismiss valid (but unusual) use case. So even if I quite like the new vGIC, I really don't want to deal with yet another security.

Thankfully passthrough case is not currently security supported (see SUPPORT.md). So we probably can defer, although I would like to keep track of known pitfalls of the new vGIC.


Julien Grall

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