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Re: [PATCH v4 01/10] evtchn: use per-channel lock where possible

Hi Jan,

On 11/01/2021 10:14, Jan Beulich wrote:
On 08.01.2021 21:32, Julien Grall wrote:
Hi Jan,

On 05/01/2021 13:09, Jan Beulich wrote:
Neither evtchn_status() nor domain_dump_evtchn_info() nor
flask_get_peer_sid() need to hold the per-domain lock - they all only
read a single channel's state (at a time, in the dump case).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
v4: New.

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -968,15 +968,16 @@ int evtchn_status(evtchn_status_t *statu
       if ( d == NULL )
           return -ESRCH;
- spin_lock(&d->event_lock);
       if ( !port_is_valid(d, port) )

There is one issue that is now becoming more apparent. To be clear, the
problem is not in this patch, but I think it is the best place to
discuss it as d->event_lock may be part of the solution.

After XSA-344, evtchn_destroy() will end up to decrement d->valid_evtchns.

Given that evtchn_status() can work on the non-current domain, it would
be possible to run it concurrently with evtchn_destroy(). As a
consequence, port_is_valid() will be unstable as a valid event channel
may turn invalid.

AFAICT, we are getting away so far, as the memory is not freed until the
domain is fully destroyed. However, we re-introduced XSA-338 in a
different way.

To be clear this is not the fault of this patch. But I don't think this
is sane to re-introduce a behavior that lead us to an XSA.

I'm getting confused, I'm afraid, from the varying statements above:
Are you suggesting this patch does re-introduce bad behavior or not?

No. I am pointing out that this is widening the bad behavior (again).

Yes, the decrementing of ->valid_evtchns has a similar effect, but
I'm not convinced it gets us into XSA territory again. The problem
wasn't the reducing of ->max_evtchns as such, but the derived
assumptions elsewhere in the code. If there were any such again, I
suppose we'd have reason to issue another XSA.

I don't think it get us to the XSA territory yet. However, the locking/interaction in the event channel code is quite complex.

To give a concrete example, below the current implementation of free_xen_event_channel():

    if ( !port_is_valid(d, port) )
         * Make sure ->is_dying is read /after/ ->valid_evtchns, pairing
         * with the spin_barrier() and BUG_ON() in evtchn_destroy().

    evtchn_close(d, port, 0);

It would be fair for a developer to assume that after the check above, port_is_valid() would return true. However, this is not the case...

I am not aware of any issue so far... But I am not ready to be this is not going to be missed out. How about you?

> If there were any such again, I
> suppose we'd have reason to issue another XSA.

The point of my e-mail is to prevent this XSA to happen. I am pretty sure you want the same.

Furthermore there are other paths already using port_is_valid()
without holding the domain's event lock; I've not been able to spot
a problem with this though, so far.

Right. Most of the fine are fine because d == current. Therefore, the domain must be running and evtchn_destroy() couldn't happen concurrently.

I can see two solutions:
    1) Use d->event_lock to protect port_is_valid() when d !=
current->domain. This would require evtchn_destroy() to grab the lock
when updating d->valid_evtchns.
    2) Never decrement d->valid_evtchns and use a different field for
closing ports

I am not a big fan of 1) because this is muddying the already complex
locking situation in the event channel code. But I suggested it because
I wasn't sure whether you would be happy with 2).

I agree 1) wouldn't be very nice, and you're right in assuming I
wouldn't like 2) very much. For the moment I'm not (yet) convinced
we need to do anything at all - as you say yourself, while the
result of port_is_valid() is potentially unstable when a domain is
in the process of being cleaned up, the state guarded by such
checks remains usable in (I think) a race free manner.

It remains usable *today*, the question is how long this will last?

All the recent XSAs in the event channel taught me that the locking/interaction is extremely complex. This series is another proof.

We would save us quite a bit of trouble by making port_is_valid() stable no matter the state of the domain.

I think an extra field (option 2) is quite a good compromise with space use, maintenance, speed.

I am would be interested to hear from others.


Julien Grall



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