linux-kernel - Re: [PATCH v2] media: em28xx: add missing em28xx_close

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Date:   Thu, 29 Jul 2021 15:40:54 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     mchehab@...nel.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+a6969ef522a36d3344c9@...kaller.appspotmail.com
Subject: Re: [PATCH v2] media: em28xx: add missing em28xx_close_extension

On 29/07/2021 14:45, Pavel Skripkin wrote:
> On Thu, 29 Jul 2021 11:45:19 +0200
> Hans Verkuil <hverkuil@...all.nl> wrote:
> 
>> On 21/07/2021 21:43, Pavel Skripkin wrote:
>>> If em28xx dev has ->dev_next pointer, we need to delete dev_next
>>> list node from em28xx_extension_devlist on disconnect to avoid UAF
>>> bugs and corrupted list bugs, since driver frees this pointer on
>>> disconnect.
>>>
>>> Fixes: 1a23f81b7dc3 ("V4L/DVB (9979): em28xx: move usb probe code
>>> to a proper place") Reported-and-tested-by:
>>> syzbot+a6969ef522a36d3344c9@...kaller.appspotmail.com
>>> Signed-off-by: Pavel Skripkin <paskripkin@...il.com> ---
>>>
>>> Changes in v2:
>>> 	Previous patch was completely broken. I've done some
>>> debugging again and found true root case of the reported bug.
>>>
>>> ---
>>>  drivers/media/usb/em28xx/em28xx-cards.c | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/media/usb/em28xx/em28xx-cards.c
>>> b/drivers/media/usb/em28xx/em28xx-cards.c index
>>> c1e0dccb7408..d56b040e1bd7 100644 ---
>>> a/drivers/media/usb/em28xx/em28xx-cards.c +++
>>> b/drivers/media/usb/em28xx/em28xx-cards.c @@ -4139,8 +4139,10 @@
>>> static void em28xx_usb_disconnect(struct usb_interface *intf) 
>>>  	em28xx_close_extension(dev);
>>>  
>>> -	if (dev->dev_next)
>>> +	if (dev->dev_next) {
>>>  		em28xx_release_resources(dev->dev_next);
>>> +		em28xx_close_extension(dev->dev_next);
>>
>> Wouldn't it be better to swap these two?
>>
>> That order is also used for em28xx_close_extension(dev) and
>> em28xx_release_resources(dev).
>>
>> You do need to store dev->dev_next in a temp variable, though.
>>
> 
> Hi, Hans!
> 
> I don't understand why I need to store dev->dev_next in a temp
> variable. I don't see code in em28xx_release_resources() or
> em28xx_close_extension() that zeroes this pointer.

Apologies for the confusion, just ignore that bit. I misunderstood
what dev_next was for.

So check if swapping these two lines passes syzbot, and then that's
the final version.

Regards,

	Hans

> 
> 
> 
> With regards,
> Pavel Skripkin
> 
>