linux-kernel - Re: [PATCH] drivers: usb: gadget: udc: remove pointer dereference after free

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Message-ID: <20170208153322.Horde.mdD6qAMhN7aTL6u10JtoBcQ@gator4166.hostgator.com>
Date:   Wed, 08 Feb 2017 15:33:22 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     gregkh@...uxfoundation.org, balbi@...nel.org, bhelgaas@...gle.com,
        heikki.krogerus@...ux.intel.com, mail@...oabal.eu,
        mina86@...a86.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Senna Tschudin <peter.senna@...il.com>
Subject: Re: [PATCH] drivers: usb: gadget: udc: remove pointer dereference
 after free

Hi Andy,

Quoting Andy Shevchenko <andriy.shevchenko@...ux.intel.com>:

> On Wed, 2017-02-08 at 13:15 -0600, Gustavo A. R. Silva wrote:
>> Remove pointer dereference and write after free.
>
> It's wrong description. There is no write after free. The memory is
> still in pool and one may access it. Though the access is *formally*
> illegal.
>
> Code itself looks interesting.
>
>>
>> Addresses-Coverity-ID: 1091173
>> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
>> ---
>>  drivers/usb/gadget/udc/pch_udc.c | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/usb/gadget/udc/pch_udc.c
>> b/drivers/usb/gadget/udc/pch_udc.c
>> index a97da64..8a365aa 100644
>> --- a/drivers/usb/gadget/udc/pch_udc.c
>> +++ b/drivers/usb/gadget/udc/pch_udc.c
>> @@ -1523,7 +1523,6 @@ static void pch_udc_free_dma_chain(struct
>> pch_udc_dev *dev,
>>  		td = phys_to_virt(addr);
>>  		addr2 = (dma_addr_t)td->next;
>>  		pci_pool_free(dev->data_requests, td, addr);
>> -		td->next = 0x00;
>
> I think the better fix is to move this line before pci_pool_free() call.
> I dunno those td->next = 0x00; make any sense there.
>

Yeah, I think it is better to do as you say.
It seems to me that the intention of that line of code is to get rid  
of the address 'next' points to, after it has been backed up into  
'addr2'.

I will also change the description.

> Is it done under some lock / serialization?
>
>>  		addr = addr2;
>>  	}
>>  	req->chain_len = 1;
>
> --
> Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> Intel Finland Oy

Thanks
--
Gustavo A. R. Silva