linux-kernel - Re: [RFC PATCH] media: venus: Fix NULL pointer dereference in core selection

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Date:   Mon, 22 Jun 2020 14:51:44 +0300
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Doug Anderson <dianders@...omium.org>, amasule@...eaurora.org
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Jeffrey Kardatzke <jkardatzke@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        vgarodia@...eaurora.org,
        Mansur Alisha Shaik <mansur@...eaurora.org>
Subject: Re: [RFC PATCH] media: venus: Fix NULL pointer dereference in core
 selection

Hi Doug,

Thanks for the fix and sorry for the late reply.

On 6/2/20 6:39 AM, Doug Anderson wrote:
> Hi,
> 
> On Mon, Jun 1, 2020 at 3:03 PM Douglas Anderson <dianders@...omium.org> wrote:
>>
>> The newly-introduced function min_loaded_core() iterates over all of
>> the venus instances an tries to figure out how much load each instance
>> is putting on each core.  Not all instances, however, might be fully
>> initialized.  Specifically the "codec_freq_data" is initialized as
>> part of vdec_queue_setup(), but an instance may already be in the list
>> of all instances before that time.
>>
>> Let's band-aid this by checking to see if codec_freq_data is NULL
>> before dereferencing.
>>
>> NOTE: without this fix I was running into a crash.  Specifically there
>> were two venus instances.  One was doing start_streaming.  The other
>> was midway through queue_setup but hadn't yet gotten to initting
>> "codec_freq_data".
>>
>> Fixes: eff82f79c562 ("media: venus: introduce core selection")
>> Signed-off-by: Douglas Anderson <dianders@...omium.org>
>> ---
>> I'm not massively happy about this commit but it's the best I could
>> come up with without being much more of an expert in the venus codec.
>> If someone has a better patch then please just consider this one to be
>> a bug report and feel free to submit a better fix!  :-)
>>
>> In general I wonder a little bit about whether it's safe to be peeking
>> at all the instances without grabbing the "inst->lock" on each one.  I
>> guess it is since we do it both here and in load_scale_v4() but I
>> don't know why.
>>
>> One thought I had was that we could fully avoid accessing the other
>> instances, at least in min_loaded_core(), by just keeping track of
>> "core1_load" and "core2_load" in "struct venus_core".  Whenever we add
>> a new instance we could add to the relevant variables and whenever we
>> release an instance we could remove.  Such a change seems cleaner but
>> would require someone to test to make sure we didn't miss any case
>> (AKA we always properly added/removed our load from the globals).

Thanks for the suggestion (I also thought about something similar).  I
will try to cook something.

>>
>>  drivers/media/platform/qcom/venus/pm_helpers.c | 2 ++
>>  1 file changed, 2 insertions(+)
> 
> This fixes the same crash as the patch:
> 
> https://lore.kernel.org/r/1588314480-22409-1-git-send-email-mansur@codeaurora.org
> 

I'm going to take this approach because it takes into account the state
of the instance. The instance could be opened/created but the streaming
could not be started in near future, so it shouldn't be correct to take
its load when doing the calculations.

-- 
regards,
Stan