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@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;
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@xxxxxxxxxx>
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
