linux-kernel - Re: [PATCH] drm/i915/kvmgt: avoid dereferencing a potentially null info pointer

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Date:   Fri, 24 Mar 2017 11:38:04 +0800
From:   Zhenyu Wang <zhenyuw@...ux.intel.com>
To:     Frans Klaver <fransklaver@...il.com>
Cc:     Colin King <colin.king@...onical.com>,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, kernel-janitors@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Zhi Wang <zhi.a.wang@...el.com>,
        "Zhang, Tina" <tina.zhang@...el.com>
Subject: Re: [PATCH] drm/i915/kvmgt: avoid dereferencing a potentially null
 info pointer

On 2017.03.23 14:43:44 +0100, Frans Klaver wrote:
> On Thu, Mar 23, 2017 at 1:22 PM, Colin King <colin.king@...onical.com> wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> >
> > info is being checked to see if it is a null pointer, however, vpgu is
> > dereferencing info before this check, leading to a potential null
> > pointer dereference.  If info is null, then the error message being
> > printed by macro gvt_vgpu_err and this requires vpgu to exist. We can
> > use a null vpgu as the macro has a sanity check to see if vpgu is null,
> > so this is OK.
> >
> > Detected with CoverityScan, CID#1420672 ("Dereference nefore null check")
> 
> s,nefore,before,
> 
> 
> >
> > Fixes: 695fbc08d80f ("drm/i915/gvt: replace the gvt_err with gvt_vgpu_err")
> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > ---
> >  drivers/gpu/drm/i915/gvt/kvmgt.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > index 1ea3eb270de8..f8619a772c44 100644
> > --- a/drivers/gpu/drm/i915/gvt/kvmgt.c
> > +++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > @@ -1339,9 +1339,9 @@ static int kvmgt_guest_init(struct mdev_device *mdev)
> >
> >  static bool kvmgt_guest_exit(struct kvmgt_guest_info *info)
> >  {
> > -       struct intel_vgpu *vgpu = info->vgpu;
> > -
> >         if (!info) {
> > +               struct intel_vgpu *vgpu = NULL;
> > +
> >                 gvt_vgpu_err("kvmgt_guest_info invalid\n");
> >                 return false;
> >         }
> 
> Does this mean the original gvt_err() macro is no longer there?
> 
> And apparently gvt_vgpu_err is a macro that depends on the specifics
> of its environment? Yikes.
> 

The null check is redundant there, we can just remove that block and extra
vgpu variable.

-- 
Open Source Technology Center, Intel ltd.

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