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Subject: Re: [PATCH v3 3/3] uio: fix crash after the device is unregistered
To:     Xiubo Li <xiubli@redhat.com>, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org
References: <1530845836-49101-1-git-send-email-xiubli@redhat.com>
 <1530845836-49101-4-git-send-email-xiubli@redhat.com>
 <5B3FB3B0.7010105@redhat.com>
 <1d3a72f6-2d48-1e36-82e4-c764c5359765@redhat.com>
Cc:     hamish.martin@alliedtelesis.co.nz, jannh@google.com,
        pkalever@redhat.com, pkarampu@redhat.com, atumball@redhat.com,
        sabose@redhat.com
From:   Mike Christie <mchristi@redhat.com>
Message-ID: <5B439618.5060800@redhat.com>
Date:   Mon, 9 Jul 2018 12:06:32 -0500
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On 07/06/2018 08:28 PM, Xiubo Li wrote:
> On 2018/7/7 2:23, Mike Christie wrote:
>> On 07/05/2018 09:57 PM, xiubli@redhat.com wrote:
>>>   static irqreturn_t uio_interrupt(int irq, void *dev_id)
>>>   {
>>>       struct uio_device *idev = (struct uio_device *)dev_id;
>>> -    irqreturn_t ret = idev->info->handler(irq, idev->info);
>>> +    irqreturn_t ret;
>>> +
>>> +    mutex_lock(&idev->info_lock);
>>> +    if (!idev->info) {
>>> +        ret = IRQ_NONE;
>>> +        goto out;
>>> +    }
>>>   +    ret = idev->info->handler(irq, idev->info);
>>>       if (ret == IRQ_HANDLED)
>>>           uio_event_notify(idev->info);
>>>   +out:
>>> +    mutex_unlock(&idev->info_lock);
>>>       return ret;
>>>   }
>>
>> Do you need the interrupt related changes in this patch and the first
>> one?
> Actually, the NULL checking is not a must, we can remove this. But the
> lock/unlock is needed.
>>   When we do uio_unregister_device -> free_irq does free_irq return
>> when there are no longer running interrupt handlers that we requested?
>>
>> If that is not the case then I think we can hit a similar bug. We do:
>>
>> __uio_register_device -> device_register -> device's refcount goes to
>> zero so we do -> uio_device_release -> kfree(idev)
>>
>> and if it is possible the interrupt handler could still run after
>> free_irq then we would end up doing:
>>
>> uio_interrupt -> mutex_lock(&idev->info_lock) -> idev access freed
>> memory.
> 
> I think this shouldn't happen. Because the free_irq function does not
> return until any executing interrupts for this IRQ have completed.
> 

If free_irq returns after executing interrupts and does not allow new
executions what is the lock protecting in uio_interrupt?