Re: [PATCH 1/2] drm/core: Preserve the framebuffer after removing it.

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH 1/2] drm/core: Preserve the framebuffer after removing it.
Date: Wed, 9 Sep 2015 17:03:45 +0100	[thread overview]
Message-ID: <55F05861.9040302@linux.intel.com> (raw)
In-Reply-To: <20150909155615.GU2767@phenom.ffwll.local>

On 09/09/2015 04:56 PM, Daniel Vetter wrote:
> On Wed, Sep 09, 2015 at 04:47:08PM +0100, Tvrtko Ursulin wrote:
>>
>> On 09/09/2015 04:29 PM, Daniel Vetter wrote:
>>> On Wed, Sep 09, 2015 at 04:18:02PM +0100, Tvrtko Ursulin wrote:
>>>>
>>>> On 09/09/2015 04:04 PM, Daniel Vetter wrote:
>>>>> On Wed, Sep 09, 2015 at 03:51:50PM +0100, Tvrtko Ursulin wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> On 09/09/2015 03:40 PM, Maarten Lankhorst wrote:
>>>>>>> Previously RMFB and fd close chose to disable any plane that had
>>>>>>> an active framebuffer from this file. If it was a primary plane the
>>>>>>> crtc was disabled. However the fbdev code or any system compositor
>>>>>>> should restore the planes anyway so there's no need to do it twice.
>>>>>>>
>>>>>>> The old fb_id is zero'd, so there's no danger of being able to
>>>>>>> restore the fb from fb_id.
>>>>>>
>>>>>> What does this mean, say if the compositor dies last frame will remain on
>>>>>> the screen?
>>>>>
>>>>> Yes, and the commit message should mention that. It should also mention
>>>>> that other applications can't get at the data since we clear fb id still,
>>>>> so no information leak there.
>>>>
>>>> Perhaps I replied to the wrong patch from the series.
>>>>
>>>> Why is all this needed anyway? It sound pretty undesirable from the security
>>>> point of view to me. If it is exploitable to leave something sensitive on
>>>> screen that's not good.
>>>
>>> fd close is a super-painful context to do a full-blown modeset. It's
>>> userspace but we can't restart anything because no one ever checks the
>>> return value of close(). We could fix it by pushing this to a work item,
>>> but given that the rule itself seems dubious it's easier to adjust the abi
>>> imo. Framebuffers are somewhat global, so not deleting them makes imo
>>> sense.
>>>
>>> The big change is patch 2, which will make them survive for real.
>>
>> I don't follow this closely but it still sounds wrong. If modeset is a
>> concern then disable the planes and/or clear them?
>
> This is generic core code, you can't disable/clear planes in a generic way
> without doing a full modeset.
>
>> It really doesn't feel preservation of fb content is a good thing to do. If
>> the higher goal is to enable some smooth transitions clients should engineer
>> that themselves.
>>
>> In any case leaving content on screen sounds really bad to me.
>>
>> Reminds me of screen locker bugs which sometimes did not clear the screen
>> when displaying the unlock dialog. That was pretty common for a long period
>> in KDE. And this sounds like it could be attackable in a similar way.
>
> Afaik that's just userspace coordination fail - system deamons go into
> suspend without telling and waiting for the screenlocker to lock the
> screen. Hilarious, but not really something we can fix in the kernel.

It was just an example of a class of vulnerabilities which would be 
possible with these changes. If they, as you said, will preserve the 
last frame on screen when the compositor crashes.

For me this is serious enough not to go this route.

Tvrtko
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