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

Fri May 5 13:57:54 UTC 2023

On Fri, May 05, 2023 at 04:42:33PM +0300, Lisovskiy, Stanislav wrote:
> On Fri, May 05, 2023 at 04:28:50PM +0300, Ville Syrjälä wrote:
> > On Fri, May 05, 2023 at 04:21:16PM +0300, Lisovskiy, Stanislav wrote:
> > > On Fri, May 05, 2023 at 04:11:52PM +0300, Ville Syrjälä wrote:
> > > > On Fri, May 05, 2023 at 03:54:58PM +0300, Lisovskiy, Stanislav wrote:
> > > > > On Fri, May 05, 2023 at 03:46:40PM +0300, Ville Syrjälä wrote:
> > > > > > On Fri, May 05, 2023 at 03:27:51PM +0300, Lisovskiy, Stanislav wrote:
> > > > > > > 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 at 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.
> > > > > > 
> > > > > > We have the checks where they are needed. The check in
> > > > > > intel_bw_atomic_check() (if that's the one you mean)
> > > > > > looks entirely correct to me.
> > > > > 
> > > > > Typo in my prev message, I meant intel_atomic_get_bw_state..but common idea is the same.
> > > > 
> > > > get_state() vs. get_{new,old}_state() are entirely different
> > > > things.
> > > > 
> > > > You use get_state() when you really want the state to be
> > > > included, and either
> > > > - know the state isn't included already, or
> > > > - you don't know wether the might have alerady been included
> > > > 
> > > > And one must of course remember that get_state() can
> > > > - fail so error handling is needed
> > > > - only be used during the check phase, and is illegal during the
> > > >   commit phase.
> > > 
> > > Sure I know this. I even remember we discussed this many times.
> > > 
> > > > 
> > > > The get_{new,old}_state() (or the various for loop variants)
> > > > you can use when you either:
> > > > - know that the state is included already
> > > > - are fine with the state potentially not being included
> > > 
> > > Don't you see that it is a bit of a contradiction in those 2 above??
> > > 
> > > You can't be "know that the state is included already" and 
> > > "are fine with the state potentially not being included" same time :)
> > > 
> > > Those 2 above actually mean that you CANNOT be sure, because you 
> > > are "fine with the state potentially not being included"! 
> > > Otherwise second one would have been redundant.
> > 
> > No. You are either fine with NULL, XOR you know that
> > the state is there already. There is no contradiction.
> 
> I do get that. But that way of calling the function is veeery counterintuitive.
> Means that you call it and check for NULLness..if you are fine with NULL and
> don't check for NULL..if you aren't fine with it and expect the state to be there.
> 
> That is really probabilistic design.
> I think we must enumerate all the cases where 

Not sure what you mean with enumerate. You can't just delcare
somewhere globally that in functions X and Y NULL is fine,
and in Z it is not. It depends on how X,Y,Z are implemented
and it may change any time the implementation is changed.

> 1) we expect new_state to be there and
>    then we don't need even any checks to be there, because we will then rely on get_state.
> 2) we don't expect it to be there and then call get_state always.
> 
> Because if you are "fine" with new_state being NULL, why even calling it?

Because
!NULL -> you have some work to do
 NULL -> you don't have work to do

> I don't get what means "fine" here - there should be a reason why it is not there right?
> crtc wasn't added to the state. get_crtc_state wasn't called - it means either we don't even 
> need to call get_new_*state at all or there _is_ a problem.
> 
> Stan
> 
> > 
> > > 
> > > So basically in theory you must also use that
> > > "you don't know wether the might have alerady been included" mentioned
> > > for *_get_state here as well.
> > 
> > No. This code _expects_ the state to be included. The fact
> > that it is not is a problem.
> > 
> > > 
> > > > 
> > > > That's just atomic 101.
> > > 
> > > Nope, it is not even that. It is 1st order logic here.
> > > 
> > > Stan
> > > 
> > > > 
> > > > -- 
> > > > Ville Syrjälä
> > > > Intel
> > 
> > -- 
> > Ville Syrjälä
> > Intel

-- 
Ville Syrjälä
Intel