Re: [PATCH] drm/exynos: fix race condition UAF in exynos_g2d_exec_ioctl

[lm0963]

Hi Andi,

On Wed, May 31, 2023 at 4:19 PM Andi Shyti <andi.shyti@xxxxxxxxxx> 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@xxxxxxxxx>
> > > > ---
> > > >  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;

>
> 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
    }

>
> 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