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

Ville Syrjälä
2023-05-05T13:11:58Z

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@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.

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

That's just atomic 101.

--
Ville Syrjälä
Intel