Re: [PATCH] drm/vc4: Fix NULL pointer dereference in vc4_save_hang_state()

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Wed, 17 Jan 2018 12:03:15 -0800
Eric Anholt <eric@anholt.net> wrote:

> Boris Brezillon <boris.brezillon@free-electrons.com> writes:
> 
> > When saving BOs in the hang state we skip one entry of the
> > kernel_state->bo[] array, thus leaving it to NULL. This leads to a NULL
> > pointer dereference when, later in this function, we iterate over all
> > BOs to check their ->madv state.
> >
> > Fixes: ca26d28bbaa3 ("drm/vc4: improve throughput by pipelining binning and rendering jobs")
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > ---
> >  drivers/gpu/drm/vc4/vc4_gem.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/vc4/vc4_gem.c b/drivers/gpu/drm/vc4/vc4_gem.c
> > index 6c32c89a83a9..19ac7fe0e5db 100644
> > --- a/drivers/gpu/drm/vc4/vc4_gem.c
> > +++ b/drivers/gpu/drm/vc4/vc4_gem.c
> > @@ -208,7 +208,7 @@ vc4_save_hang_state(struct drm_device *dev)
> >  			kernel_state->bo[j + prev_idx] = &bo->base.base;
> >  			j++;
> >  		}
> > -		prev_idx = j + 1;
> > +		prev_idx = j;  
> 
> Could we replace the whole "[j + prev_idx]" with a "[k++]" and maybe a
> WARN_ON_ONCE(k != state->bo_count) at the end?

Sure.

> 
> I really need to figure out if I can come up with a way to make IGT
> cases for GPU hangs on vc4, despite the validation.

I managed to trigger the NULL pointer dereference while debugging the
perfmon stuff, but it's fixed now, so I don't have a way to easily
force a reset.

> I found a bug in
> GPU reset due to BCL hangs when doing vc5, but I don't have a testcase.
> Maybe some submit flags that overwrite the BCL or RCL to do an infinite
> loop?