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Re: [PATCH v7 3/3] xen/events: rework fifo queue locking



On 24.11.20 17:32, Jan Beulich wrote:
On 24.11.2020 15:49, Jürgen Groß wrote:
On 24.11.20 15:02, Jan Beulich wrote:
On 24.11.2020 08:01, Juergen Gross wrote:
Two cpus entering evtchn_fifo_set_pending() for the same event channel
can race in case the first one gets interrupted after setting
EVTCHN_FIFO_PENDING and when the other one manages to set
EVTCHN_FIFO_LINKED before the first one is testing that bit. This can
lead to evtchn_check_pollers() being called before the event is put
properly into the queue, resulting eventually in the guest not seeing
the event pending and thus blocking forever afterwards.

Note that commit 5f2df45ead7c1195 ("xen/evtchn: rework per event channel
lock") made the race just more obvious, while the fifo event channel
implementation had this race from the beginning when an unmask operation
was running in parallel with an event channel send operation.

Ah yes, but then also only for inter-domain channels, as it was
only in that case that the "wrong" domain's event lock was held.
IOW there was a much earlier change already where this issue
got widened (when the per-channel locking got introduced). This
then got reduced to the original scope by XSA-343's adding of
locking to evtchn_unmask(). (Not sure how much of this history
wants actually adding here. I'm writing it down not the least to
make sure I have a complete enough picture.)

I think we both agree that this race was possible for quite some time.
And I even think one customer bug I've been looking into recently
might be exactly this problem (a dom0 was occasionally hanging in
cross-cpu function calls, but switching to 2-level events made the
problem disappear).

IPIs weren't affected earlier on (i.e. in any released version),
if my analysis above is correct.

I don't think it is correct.

An unmask operation in parallel with set_pending will have had the
same race for IPIs.


Additionally when an
event channel needs to change queues both queues need to be locked
initially.

Since this was (afaict) intentionally not the case before, I
think I would want to see a word spent on the "why", perhaps
better in a code comment than here. Even more so that you
delete a respective comment justifying the possible race as
permissible. And I have to admit right now I'm still uncertain
both ways, i.e. I neither have a clear understanding of why it
would have been considered fine the other way around before,
nor why the double locking is strictly needed.

I need the double locking to avoid someone entering the locked region
when dropping the lock for the old queue and taking the one for the
new queue, as this would open the same race window again.

Well, that's what have already said. Thing is that the code
prior to your change gives the impression as if this race was
benign.

The race regarding a queue change, yes. But not the race I'm fixing with
this patch. I need to make sure that only one caller is inside the big
if clause for a specific event. And dropping the lock inside this clause
would violate that assumption.


+        lastq.raw = read_atomic(&evtchn->fifo_lastq);
+        old_v = d->vcpu[lastq.last_vcpu_id];
+        if ( q == &v->evtchn_fifo->queue[evtchn->priority] &&
+             old_q == &old_v->evtchn_fifo->queue[lastq.last_priority] )
+            break;
+
+        if ( q != old_q )
+            spin_unlock(&old_q->lock);
+        spin_unlock_irqrestore(&q->lock, flags);
+
+        if ( try == 3 )
+        {
+            gprintk(XENLOG_WARNING,
+                    "dom%d port %d lost event (too many queue changes)\n",
+                    d->domain_id, evtchn->port);
+            return;

Originally evtchn_check_pollers() would still have been called
in this case. Wouldn't you better retain this, or else justify
the possibly observable change in behavior?

I could retain it, but without having set the event to be pending
I don't see the value in doing so.

But that's part of my concern - you now don't set PENDING when
bailing here.

Hmm, I'm not sure this will really help, as the event still won't be
LINKED, what is necessary for the guest to recognize the event to be
pending.

OTOH if the event is masked setting the PENDING bit would ensure
proper handling later, so it is better to do that, even with the risk
to have the same old race again in this case (which might have the
same effect as not setting PENDING, but with a much lower probability).

I'll change the locking to handle that properly.


Juergen

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