Re: [Intel-gfx] [PATCH] drm/i915: Fix NULL ptr deref by checking new_crtc_state

Ville Syrjälä
2023-05-05T12:09:07Z

On Fri, May 05, 2023 at 02:41:24PM +0300, Lisovskiy, Stanislav wrote:
> On Fri, May 05, 2023 at 02:25:46PM +0300, Ville Syrjälä wrote:
> > On Fri, May 05, 2023 at 02:20:17PM +0300, Lisovskiy, Stanislav wrote:
> > > On Fri, May 05, 2023 at 02:06:34PM +0300, Ville Syrjälä wrote:
> > > > On Fri, May 05, 2023 at 02:05:27PM +0300, Lisovskiy, Stanislav wrote:
> > > > > On Fri, May 05, 2023 at 02:02:43PM +0300, Ville Syrjälä wrote:
> > > > > > On Fri, May 05, 2023 at 01:58:03PM +0300, Lisovskiy, Stanislav wrote:
> > > > > > > On Fri, May 05, 2023 at 01:54:14PM +0300, Ville Syrjälä wrote:
> > > > > > > > On Fri, May 05, 2023 at 11:22:12AM +0300, Stanislav Lisovskiy wrote:
> > > > > > > > > intel_atomic_get_new_crtc_state can return NULL, unless crtc state wasn't
> > > > > > > > > obtained previously with intel_atomic_get_crtc_state, so we must check it
> > > > > > > > > for NULLness here, just as in many other places, where we can't guarantee
> > > > > > > > > that intel_atomic_get_crtc_state was called.
> > > > > > > > > We are currently getting NULL ptr deref because of that, so this fix was
> > > > > > > > > confirmed to help.
> > > > > > > > >
> > > > > > > > > Fixes: 74a75dc90869 ("drm/i915/display: move plane prepare/cleanup to intel_atomic_plane.c")
> > > > > > > > > Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> > > > > > > > > ---
> > > > > > > > > drivers/gpu/drm/i915/display/intel_atomic_plane.c | 4 ++--
> > > > > > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/drivers/gpu/drm/i915/display/intel_atomic_plane.c b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> > > > > > > > > index 9f670dcfe76e..4125ee07a271 100644
> > > > > > > > > --- a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> > > > > > > > > +++ b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
> > > > > > > > > @@ -1029,7 +1029,7 @@ intel_prepare_plane_fb(struct drm_plane *_plane,
> > > > > > > > > int ret;
> > > > > > > > >
> > > > > > > > > if (old_obj) {
> > > > > > > > > - const struct intel_crtc_state *crtc_state =
> > > > > > > > > + const struct intel_crtc_state *new_crtc_state =
> > > > > > > > > intel_atomic_get_new_crtc_state(state,
> > > > > > > > > to_intel_crtc(old_plane_state->hw.crtc));
> > > > > > > > >
> > > > > > > > > @@ -1044,7 +1044,7 @@ intel_prepare_plane_fb(struct drm_plane *_plane,
> > > > > > > > > * This should only fail upon a hung GPU, in which case we
> > > > > > > > > * can safely continue.
> > > > > > > > > */
> > > > > > > > > - if (intel_crtc_needs_modeset(crtc_state)) {
> > > > > > > > > + if (new_crtc_state && intel_crtc_needs_modeset(new_crtc_state)) {
> > > > > > > >
> > > > > > > > NAK. We need to fix the bug instead of paparing over it.
> > > > > > >
> > > > > > > I had pushed this already.
> > > > > >
> > > > > > It didn't even finish CI. Please revert.
> > > > >
> > > > > Swati did run CI and verified that fix helps. I'm _not_ going to revert.
> > > >
> > > > Fine. I'll do it.
> > >
> > > Problem is that you don't even care to explain, why this fix is wrong, but simply
> > > act in authoritarian way, instead of having constructive discussion.
> >
> > I've explanined this one about a hundred times. The NULL pointer should
> > not happen. Someone needs to actually analyze what is happening instead
> > of just adding randomg NULL checks all over the place.
>
> I do get this point. However why are we doing those check in other places then?

We do then when they are actually necessary.

> Moreover I can remember that you told me to do this check even, when were reviewing
> my other patches. Because we always have to check result of this function, as it
> can be NULL, in case if intel_atomic_get_crtc_state wasn't called before, which
> could happen even in normal case, as I understand.

You can't apply that kind of general rule. Whether the crtc should have
already been added to the state or not is case dependent. In this case
that should never be the case since the plane was already added to the
state, and thus its crtc should also have been added.

>
> If we want to understand why it happens in particular here, great lets investigate,
> however I don't get why we are having same checks everywhere all over the place then
> and I can even find your words, that we need to do those checks as well.
>
> Also if this doesn't break anything,

You can't know that. You're trading a clearly reproducible
bug with a silent bug that can cause who knows what other
issues. That one will be impossible to debug.

> improves our CI results, not violating our coding
> practices, because once again worth mentioning we do check new_crtc_state for NULLness
> in many places.. then why it can't be the fix?
> If we find better solution thats great, but there are plenty of other things as well,
> if you haven't noticed.
>
> Can we somehow _stop_ these childish kindergarden level review arguing warfare, at least
> for sake of professional efficiency?

Not sure what that kindergarten level stuff is. I just
NAKed the patch.

>
> For all my next patches I will always add you to CC and _personally_ will ask to review,
> even though quite often when I do this - I get nothing.

I can't review everything in detail. But in any case you should
at least wait a day or two for review feedback, and you definitely
need to wait for CI results as well.

--
Ville Syrjälä
Intel