linux-kernel - Re: [PATCH] drm/i915/gvt: fix dropping obj reference twice

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Date:   Wed, 6 Nov 2019 16:43:49 +0800
From:   Zhenyu Wang <zhenyuw@...ux.intel.com>
To:     Pan Bian <bianpan2016@....com>
Cc:     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>,
        Daniel Vetter <daniel@...ll.ch>,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/i915/gvt: fix dropping obj reference twice

On 2019.11.06 15:31:07 +0800, Pan Bian wrote:
> The reference count of obj will be decremented twice if error occurs
> in dma_buf_fd(). Additionally, attempting to read the reference count of
> obj after dropping reference may lead to a use after free bug. Here, we
> drop obj's reference until it is not used.
> 
> Signed-off-by: Pan Bian <bianpan2016@....com>
> ---
>  drivers/gpu/drm/i915/gvt/dmabuf.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gvt/dmabuf.c b/drivers/gpu/drm/i915/gvt/dmabuf.c
> index 13044c027f27..4bfaefdf548d 100644
> --- a/drivers/gpu/drm/i915/gvt/dmabuf.c
> +++ b/drivers/gpu/drm/i915/gvt/dmabuf.c
> @@ -498,8 +498,6 @@ int intel_vgpu_get_dmabuf(struct intel_vgpu *vgpu, unsigned int dmabuf_id)
>  		goto out_free_gem;
>  	}
>  
> -	i915_gem_object_put(obj);
> -
>  	ret = dma_buf_fd(dmabuf, DRM_CLOEXEC | DRM_RDWR);
>  	if (ret < 0) {
>  		gvt_vgpu_err("create dma-buf fd failed ret:%d\n", ret);
> @@ -524,6 +522,8 @@ int intel_vgpu_get_dmabuf(struct intel_vgpu *vgpu, unsigned int dmabuf_id)
>  		    file_count(dmabuf->file),
>  		    kref_read(&obj->base.refcount));
>  
> +	i915_gem_object_put(obj);
> +
>  	return dmabuf_fd;
>  
>  out_free_dmabuf:

Looks fine to me. Thanks!

Reviewed-by: Zhenyu Wang <zhenyuw@...ux.intel.com>

-- 
Open Source Technology Center, Intel ltd.

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