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Date:   Wed, 31 May 2023 14:05:31 +0200
From:   Andi Shyti <andi.shyti@kernel.org>
To:     lm0963 <lm0963hack@gmail.com>
Cc:     inki.dae@samsung.com, sw0312.kim@samsung.com,
        kyungmin.park@samsung.com, airlied@gmail.com, daniel@ffwll.ch,
        krzysztof.kozlowski@linaro.org, alim.akhtar@samsung.com,
        dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/exynos: fix race condition UAF in
 exynos_g2d_exec_ioctl
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Hi Min,

On Wed, May 31, 2023 at 06:54:34PM +0800, lm0963 wrote:
> Hi Andi,
> 
> On Wed, May 31, 2023 at 4:19 PM Andi Shyti <andi.shyti@kernel.org> wrote:
> >
> > Hi Min,
> >
> > > > > If it is async, runqueue_node is freed in g2d_runqueue_worker on another
> > > > > worker thread. So in extreme cases, if g2d_runqueue_worker runs first, and
> > > > > then executes the following if statement, there will be use-after-free.
> > > > >
> > > > > Signed-off-by: Min Li <lm0963hack@gmail.com>
> > > > > ---
> > > > >  drivers/gpu/drm/exynos/exynos_drm_g2d.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/exynos/exynos_drm_g2d.c b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> > > > > index ec784e58da5c..414e585ec7dd 100644
> > > > > --- a/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> > > > > +++ b/drivers/gpu/drm/exynos/exynos_drm_g2d.c
> > > > > @@ -1335,7 +1335,7 @@ int exynos_g2d_exec_ioctl(struct drm_device *drm_dev, void *data,
> > > > >       /* Let the runqueue know that there is work to do. */
> > > > >       queue_work(g2d->g2d_workq, &g2d->runqueue_work);
> > > > >
> > > > > -     if (runqueue_node->async)
> > > > > +     if (req->async)
> > > >
> > > > did you actually hit this? If you did, then the fix is not OK.
> > >
> > > No, I didn't actually hit this. I found it through code review. This
> > > is only a theoretical issue that can only be triggered in extreme
> > > cases.
> >
> > first of all runqueue is used again two lines below this, which
> > means that if you don't hit the uaf here you will hit it
> > immediately after.
> 
> No, if async is true, then it will goto out, which will directly return.
> 
> if (runqueue_node->async)
>     goto out;   // here, go to out, will directly return
> 
> wait_for_completion(&runqueue_node->complete);      // not hit
> g2d_free_runqueue_node(g2d, runqueue_node);
> 
> out:
> return 0;

that's right, sorry, I misread it.

> > Second, if runqueue is freed, than we need to remove the part
> > where it's freed because it doesn't make sense to free runqueue
> > at this stage.
> 
> It is freed by g2d_free_runqueue_node in g2d_runqueue_worker
> 
> static void g2d_runqueue_worker(struct work_struct *work)
> {
>     ......
>     if (runqueue_node) {
>         pm_runtime_mark_last_busy(g2d->dev);
>         pm_runtime_put_autosuspend(g2d->dev);
> 
>         complete(&runqueue_node->complete);
>         if (runqueue_node->async)
>             g2d_free_runqueue_node(g2d, runqueue_node);        // freed here

this is what I'm wondering: is it correct to free a resource
here? The design looks to me a bit fragile and prone to mistakes.

The patch per se is OK. It doesn't make much difference to me
where you actually read async, although this patch looks a bit
safer:

Reviewed-by: Andi Shyti <andi.shyti@kernel.org> 

However some refactoring might be needed to make it a bit more
robust.

Thanks,
Andi

>     }
> 
> >
> > Finally, can you elaborate on the code review that you did so
> > that we all understand it?
> 
> queue_work(g2d->g2d_workq, &g2d->runqueue_work);
> msleep(100);        // add sleep here to let g2d_runqueue_worker run first
> if (runqueue_node->async)
>     goto out;
> 
> 
> >
> > Andi
> 
> 
> 
> --
> Min Li