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

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

On Fri, May 05, 2023 at 03:09:01PM +0300, Ville Syrjälä wrote:
> 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.

Well but for example when we do check like if(new_bw_state) in intel_bw.c,
we are also might be having potentially some silent bugs.
Would you guarantee that if we remove all if(crtc_state) and if(new_bw_state) checks
in our code, that there won't be NULL pointer dereferences? I bet you don't.

But IF you do, then lets remove it everywhere then, why keeping it there, if we are sure! :))

> 
> > 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.

Well it is kinda weird, that we don't have clear rules here.
As I understand this is Bigjoiner, so most likely that was the reason why intel_get_crtc_state
wasn't called.
I mean I was anyway planning to continue investigating that Bigjoiner logic here in fact,
however that fix could help at least CI team to continue testing further.

> 
> > 
> > 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.

Answered above...

> 
> > 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.

Well, I'm glad, we are at least discussing now, why you NAKed it, initially without
having discussion first.

> 
> > 
> > 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.

Sometimes I wait for weeks.

> 
> -- 
> Ville Syrjälä
> Intel