Subject: Re: [PATCH] brcmfmac: unlink URB when request timed out

From: Mathy Vanhoef <[email-masked]>
Date: Tue, 11 Nov 2014 20:02:27 -0500

On 11/10/2014 04:08 AM, Oliver Neukum wrote:
> On Sun, 2014-11-09 at 13:10 -0500, Mathy Vanhoef wrote:
>> From: Mathy Vanhoef <[email-masked]>
>>
>> Unlink the submitted URB in brcmf_usb_dl_cmd if the request timed out. This
>> assures the URB is never submitted twice, preventing a driver crash.
>
> Hi,
>
> I am afrad this patch is no good. The diagnosis is good,
> but the fix introduces serious problems.
>
>> diff --git a/drivers/net/wireless/brcm80211/brcmfmac/usb.c b/drivers/net/wireless/brcm80211/brcmfmac/usb.c
>> index 5265aa7..1bc7858 100644
>> --- a/drivers/net/wireless/brcm80211/brcmfmac/usb.c
>> +++ b/drivers/net/wireless/brcm80211/brcmfmac/usb.c
>> @@ -738,10 +738,12 @@ static int brcmf_usb_dl_cmd(struct brcmf_usbdev_info *devinfo, u8 cmd,
>>   goto finalize;
>>   }
>>  
>> - if (!brcmf_usb_ioctl_resp_wait(devinfo))
>> + if (!brcmf_usb_ioctl_resp_wait(devinfo)) {
>> + usb_unlink_urb(devinfo->ctl_urb);
>
> This is the asynchronous unlink. You have no guarantee it is finished
> after this point.
>
>>   ret = -ETIMEDOUT;
>> - else
>> + } else {
>>   memcpy(buffer, tmpbuf, buflen);
>> + }
>>  
>>  finalize:
>>   kfree(tmpbuf);
>
> Which means that you are freeing memory that may still be used by DMA
> at this time.
> In addition you have no guarantee that the unlink is indeed finished
> by the time the URB is reused.
> If you wish to take this approach you better forget about this URB
> and allocate a new one and free the buffer from the callback.

Hi Oliver,

Good catch. I think the DMA issue is also present in the current driver: it
frees the buffer without unlinking/killing the URB at all. Can a malicious USB
device force a timeout to occur (i.e. delay the call to the completion
handler)? If so this might be a use-after-free vulnerability.

It seems using usb_kill_urb instead of usb_unlink_urb in the patch prevents any
possible use-after-free. Can someone double check?

Kind regards,
Mathy

> Regards
> Oliver