linux-kernel - Re: [PATCH] drm/i915/gvt: don't dereference 'workload' before null checking it

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Message-ID: <1521660250.7999.29.camel@perches.com>
Date:   Wed, 21 Mar 2018 12:24:10 -0700
From:   Joe Perches <joe@...ches.com>
To:     Colin Ian King <colin.king@...onical.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915/gvt: don't dereference 'workload' before null
 checking it

On Wed, 2018-03-21 at 19:18 +0000, Colin Ian King wrote:
> On 21/03/18 19:09, Joe Perches wrote:
> > On Wed, 2018-03-21 at 19:06 +0000, Colin King wrote:
> > > From: Colin Ian King <colin.king@...onical.com>
> > > 
> > > The pointer workload is dereferenced before it is null checked, hence
> > > there is a potential for a null pointer dereference on workload. Fix
> > > this by only dereferencing workload after it is null checked.
> > > 
> > > Detected by CoverityScan, CID#1466017 ("Dereference before null check")
> > 
> > Maybe true, but is it possible for workload to be null?
> > Maybe the null test should be removed instead.
> 
> From what I understand from the static analysis, there may be a
> potential for workload to be null, I couldn't rule it out so I went with
> the more paranoid stance of keeping the null check in.

workload cannot be null here.

Look at the uses of sr_oa_regs and see that
workload has already been dereferenced.

> > 
> > > Fixes: fa3dd623e559 ("drm/i915/gvt: keep oa config in shadow ctx")
> > > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > > ---
> > >  drivers/gpu/drm/i915/gvt/scheduler.c | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gvt/scheduler.c b/drivers/gpu/drm/i915/gvt/scheduler.c
> > > index 068126404151..f3010e365a48 100644
> > > --- a/drivers/gpu/drm/i915/gvt/scheduler.c
> > > +++ b/drivers/gpu/drm/i915/gvt/scheduler.c
> > > @@ -60,9 +60,9 @@ static void set_context_pdp_root_pointer(
> > >  static void sr_oa_regs(struct intel_vgpu_workload *workload,
> > >  		u32 *reg_state, bool save)
> > >  {
> > > -	struct drm_i915_private *dev_priv = workload->vgpu->gvt->dev_priv;
> > > -	u32 ctx_oactxctrl = dev_priv->perf.oa.ctx_oactxctrl_offset;
> > > -	u32 ctx_flexeu0 = dev_priv->perf.oa.ctx_flexeu0_offset;
> > > +	struct drm_i915_private *dev_priv;
> > > +	u32 ctx_oactxctrl;
> > > +	u32 ctx_flexeu0;
> > >  	int i = 0;
> > >  	u32 flex_mmio[] = {
> > >  		i915_mmio_reg_offset(EU_PERF_CNTL0),
> > > @@ -77,6 +77,10 @@ static void sr_oa_regs(struct intel_vgpu_workload *workload,
> > >  	if (!workload || !reg_state || workload->ring_id != RCS)
> > >  		return;
> > >  
> > > +	dev_priv = workload->vgpu->gvt->dev_priv;
> > > +	ctx_oactxctrl = dev_priv->perf.oa.ctx_oactxctrl_offset;
> > > +	ctx_flexeu0 = dev_priv->perf.oa.ctx_flexeu0_offset;
> > > +
> > >  	if (save) {
> > >  		workload->oactxctrl = reg_state[ctx_oactxctrl + 1];
> > >  
> 
>