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

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Message-ID: <a0309d42-a2ab-9270-b486-2325522256b6@canonical.com>
Date:   Wed, 21 Mar 2018 19:18:28 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Joe Perches <joe@...ches.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 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.

> 
>> 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];
>>