linux-kernel - Re: [PATCH next] vhost_task: after freeing vhost_task it should not be accessed in vhost_task

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Date: Tue, 30 Apr 2024 20:01:11 -0500
From: Mike Christie <michael.christie@...cle.com>
To: Hillf Danton <hdanton@...a.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Edward Adam Davis
 <eadavis@...com>,
        syzbot+98edc2df894917b3431f@...kaller.appspotmail.com,
        jasowang@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        virtualization@...ts.linux.dev
Subject: Re: [PATCH next] vhost_task: after freeing vhost_task it should not
 be accessed in vhost_task_fn

On 4/30/24 7:15 PM, Hillf Danton wrote:
> On Tue, Apr 30, 2024 at 11:23:04AM -0500, Mike Christie wrote:
>> On 4/30/24 8:05 AM, Edward Adam Davis wrote:
>>>  static int vhost_task_fn(void *data)
>>>  {
>>>  	struct vhost_task *vtsk = data;
>>> @@ -51,7 +51,7 @@ static int vhost_task_fn(void *data)
>>>  			schedule();
>>>  	}
>>>  
>>> -	mutex_lock(&vtsk->exit_mutex);
>>> +	mutex_lock(&exit_mutex);
>>>  	/*
>>>  	 * If a vhost_task_stop and SIGKILL race, we can ignore the SIGKILL.
>>>  	 * When the vhost layer has called vhost_task_stop it's already stopped
>>> @@ -62,7 +62,7 @@ static int vhost_task_fn(void *data)
>>>  		vtsk->handle_sigkill(vtsk->data);
>>>  	}
>>>  	complete(&vtsk->exited);
>>> -	mutex_unlock(&vtsk->exit_mutex);
>>> +	mutex_unlock(&exit_mutex);
>>>  
>>
>> Edward, thanks for the patch. I think though I just needed to swap the
>> order of the calls above.
>>
>> Instead of:
>>
>> complete(&vtsk->exited);
>> mutex_unlock(&vtsk->exit_mutex);
>>
>> it should have been:
>>
>> mutex_unlock(&vtsk->exit_mutex);
>> complete(&vtsk->exited);
> 
> JFYI Edward did it [1]
> 
> [1] https://lore.kernel.org/lkml/tencent_546DA49414E876EEBECF2C78D26D242EE50A@qq.com/

Thanks.

I tested the code with that change and it no longer triggers the UAF.

I've fixed up the original patch that had the bug and am going to
resubmit the patchset like how Michael requested.