linux-kernel - Re: [PATCH] media: em28xx: fix corrupted list

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Date:   Wed, 21 Jul 2021 14:25:34 +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] media: em28xx: fix corrupted list

On 21/07/2021 14:21, Pavel Skripkin wrote:
> On Tue, 20 Jul 2021 14:13:13 +0200
> Hans Verkuil <hverkuil@...all.nl> wrote:
> 
>> On 06/07/2021 16:50, Pavel Skripkin wrote:
>>> Syzbot reported corrupted list in em28xx driver. The problem was in
>>> non-reinitialized lists on disconnect. Since all 2 lists are global
>>> variables and driver can be connected and disconnected many times we
>>> should call INIT_LIST_HEAD() in .disconnect method to prevent
>>> corrupted list entries.
>>>
>>> Fixes: 1a23f81b7dc3 ("V4L/DVB (9979): em28xx: move usb probe code
>>> to a proper place") Reported-by:
>>> syzbot+a6969ef522a36d3344c9@...kaller.appspotmail.com
>>> Signed-off-by: Pavel Skripkin <paskripkin@...il.com> ---
>>>  drivers/media/usb/em28xx/em28xx-cards.c | 2 ++
>>>  drivers/media/usb/em28xx/em28xx-core.c  | 6 ++++++
>>>  drivers/media/usb/em28xx/em28xx.h       | 1 +
>>>  3 files changed, 9 insertions(+)
>>>
>>> diff --git a/drivers/media/usb/em28xx/em28xx-cards.c
>>> b/drivers/media/usb/em28xx/em28xx-cards.c index
>>> ba9292e2a587..8b1ff79c37a0 100644 ---
>>> a/drivers/media/usb/em28xx/em28xx-cards.c +++
>>> b/drivers/media/usb/em28xx/em28xx-cards.c @@ -4148,6 +4148,8 @@
>>> static void em28xx_usb_disconnect(struct usb_interface *intf)
>>> dev->dev_next = NULL; }
>>>  	kref_put(&dev->ref, em28xx_free_device);
>>> +
>>> +	em28xx_reset_lists();
>>>  }
>>>  
>>>  static int em28xx_usb_suspend(struct usb_interface *intf,
>>> diff --git a/drivers/media/usb/em28xx/em28xx-core.c
>>> b/drivers/media/usb/em28xx/em28xx-core.c index
>>> 584fa400cd7d..03970ed00dba 100644 ---
>>> a/drivers/media/usb/em28xx/em28xx-core.c +++
>>> b/drivers/media/usb/em28xx/em28xx-core.c @@ -1131,6 +1131,12 @@
>>> void em28xx_init_extension(struct em28xx *dev)
>>> mutex_unlock(&em28xx_devlist_mutex); }
>>>  
>>> +void em28xx_reset_lists(void)
>>> +{
>>> +	INIT_LIST_HEAD(&em28xx_devlist);
>>> +	INIT_LIST_HEAD(&em28xx_extension_devlist);
>>
>> This needs a mutex_lock(&em28xx_devlist_mutex);
>>
>> But actually, I don't think this is right: if there are multiple
>> em28xx devices, then I think if you disconnect one, then the other is
>> - with this code - also removed from the list.
>>
>> Can you give a link to the actual syzbot bug? I'm not at all sure you
>> are fixing the right thing here.
> 
> Hi, Hans!
> 
> I guess, I missed my coffee that morning, sorry :) This patch looks
> complety wrong, of course.
> 
> I've took a close look at this bug one more time, and I found the true
> root case of this bug.
> 
> If em28xx dev has dev_next pointer we need to close dev->next extension
> on disconnect to avoid UAF and corrupted list bug. So, something like
> this should work:
> 
> 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);
> +	}
>  	em28xx_release_resources(dev);
>  
>  	if (dev->dev_next) {
> 
> How to You feel about it? I am going to send this patch for syzbot
> testing

That looks a lot saner :-)

> 
> 
> Syzbot link:
> https://syzkaller.appspot.com/bug?id=3609bbf45bf63a8f6032f330eb3d34f51cc81493
> 
> 
> Again, sorry for v1 patch, I don't know how I came up with this fix :(

No problem, just post a v2 if syzbot passes.

Regards,

	Hans

>  
> 
> 
> With regards,
> Pavel Skripkin
>