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

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Message-ID: <CAH6sp9PYkq_PTx9x86+xaw+azZOSQr7DkrDW_M6+_3yDf2KzWA@mail.gmail.com>
Date:   Thu, 23 Mar 2017 14:43:44 +0100
From:   Frans Klaver <fransklaver@...il.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org,
        DRI mailing list <dri-devel@...ts.freedesktop.org>,
        kernel-janitors@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/i915/kvmgt: avoid dereferencing a potentially null
 info pointer

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.

Cheers,
Frans