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

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Date:   Wed, 21 Mar 2018 12:09:21 -0700
From:   Joe Perches <joe@...ches.com>
To:     Colin 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 19:06 +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.

> 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];
>