Re: [PATCH] xen/events: avoid NULL pointer dereference in evtchn_from_irq()

[Jürgen Groß <jgross@xxxxxxxx>]

On 21.05.20 23:57, Boris Ostrovsky wrote:
> On 5/21/20 2:46 PM, Marek Marczykowski-Górecki wrote:
> > On Thu, May 21, 2020 at 01:22:03PM -0400, Boris Ostrovsky wrote:
> > > On 3/19/20 3:14 AM, Juergen Gross wrote:
> > > > There have been reports of races in evtchn_from_irq() where the info
> > > > pointer has been NULL.
> > > >
> > > > Avoid that case by testing info before dereferencing it.
> > > >
> > > > In order to avoid accessing a just freed info structure do the kfree()
> > > > via kfree_rcu().
> > > Looks like noone ever responded to this.
> > >
> > >
> > > This change looks fine but is there a background on the problem? I
> > > looked in the archives and didn't find the relevant discussion.
> > Here is the original bug report:
> > https://xen.markmail.org/thread/44apwkwzeme4uavo
>
> Thanks. Do we know what the race is? Is it because an event is being
> delivered from a dying guest who is in the process of tearing down event
> channels?
Missing synchronization between event channel (de-)allocation and
handling.

I have a patch sitting here, just didn't have the time to do some proper
testing and sending it out.


Juergen