Re: [PATCH] dmaengine: plx_dma: Fix potential double free in plx_dma_create

[Logan Gunthorpe]

On 2022-12-23 03:42, Dan Carpenter wrote:
> On Tue, Dec 20, 2022 at 09:35:38AM -0700, Logan Gunthorpe wrote:
>>
>>
>> On 2022-12-19 23:17, Miaoqian Lin wrote:
>>> When all references are dropped, callback function plx_dma_release()
>>> for put_device() will call kfree(plxdev) to release memory.
>>> Fix the error path to fix the possible double free.
>>>
>>> Fixes: 07503e6aefe4 ("dmaengine: plx_dma: add a missing put_device() on error path")
>>> Signed-off-by: Miaoqian Lin <linmq006@xxxxxxxxx>
>>> ---
>>> Please correct me if I make mistakes, thanks for your review.
>>> ---
>>>  drivers/dma/plx_dma.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/dma/plx_dma.c b/drivers/dma/plx_dma.c
>>> index 12725fa1655f..bce724ff4e16 100644
>>> --- a/drivers/dma/plx_dma.c
>>> +++ b/drivers/dma/plx_dma.c
>>> @@ -546,8 +546,9 @@ static int plx_dma_create(struct pci_dev *pdev)
>>>  	return 0;
>>>  
>>>  put_device:
>>> -	put_device(&pdev->dev);
>>>  	free_irq(pci_irq_vector(pdev, 0),  plxdev);
>>> +	put_device(&pdev->dev);
>>> +	return rc;
>>>  free_plx:
>>>  	kfree(plxdev);
>>>  
>>
>>
>> Sorry, after reviewing, I don't think this patch is correct.
>>
>> plx_dma_release() is called from dma_async_device_unregister() which
>> won't be called if dma_async_device_register() fails. It does not get
>> freed when the pci device is put. So I don't think this is a possible
>> double free.
> 
> I think instead of "double free" it Miaoqian meant "use after free".  It
> does look like a use after free to me.  Certainly there is no harm from
> applying the patch and it makes the code more obviously correct.

Yeah, moving the put_device() up would certainly look more correct.

But I think it's actually not a bug either because plx_dma_create() is
called from the pci probe function, so the pci device cannot go away.

Logan