linux-kernel - Re: [PATCH] xen/events: avoid NULL pointer dereference in evtchn_from

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Message-ID: <c36de3eb-c0ad-45e1-e08b-cb7d86d197f6@oracle.com>
Date:   Thu, 21 May 2020 17:57:26 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen/events: avoid NULL pointer dereference in
 evtchn_from_irq()

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?


-boris