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Subject: Re: [PATCH] drivers:gpu:drm:amd:amdgpu:fix a potential use-after-free
To:     lwt105 <3061522931@qq.com>, alexander.deucher@amd.com
Cc:     christian.koenig@amd.com, Xinhui.Pan@amd.com, airlied@linux.ie,
        daniel@ffwll.ch, sumit.semwal@linaro.org, nirmoy.das@amd.com,
        chenli@uniontech.com, JinhuiEric.Huang@amd.com,
        mh12gx2825@gmail.com, lee.jones@linaro.org, kevin1.wang@amd.com,
        luben.tuikov@amd.com, amd-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org
References: <tencent_D167B6522B17C0228524C70F9CA5EEBBEC09@qq.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
Message-ID: <fac65461-6ee2-e5dc-3af8-df9bfaecd6d4@gmail.com>
Date:   Mon, 16 Aug 2021 08:08:17 +0200
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Am 13.08.21 um 05:28 schrieb lwt105:
> in line 1503, "dma_fence_put(fence);" drop the reference to fence and may
> cause fence to be released. However, fence is used subsequently in line
> 1510 "fence->error". This may result in an use-after-free bug.
>
> It can be fixed by recording fence->error in an variable before dropping
> the reference to fence and referencing it after dropping.
>
> Signed-off-by: lwt105 <3061522931@qq.com>

Good catch.

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> index 30fa1f61e0e5..99d03180e113 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> @@ -1486,7 +1486,7 @@ static int amdgpu_cs_wait_all_fences(struct amdgpu_device *adev,
>   				     struct drm_amdgpu_fence *fences)
>   {
>   	uint32_t fence_count = wait->in.fence_count;
> -	unsigned int i;
> +	unsigned int i, error;
>   	long r = 1;

Would be nice to have if you could reuse the "r" variable here instead 
of a new one.

Regards,
Christian.

>   
>   	for (i = 0; i < fence_count; i++) {
> @@ -1500,6 +1500,7 @@ static int amdgpu_cs_wait_all_fences(struct amdgpu_device *adev,
>   			continue;
>   
>   		r = dma_fence_wait_timeout(fence, true, timeout);
> +		error = fence->error;
>   		dma_fence_put(fence);
>   		if (r < 0)
>   			return r;
> @@ -1507,8 +1508,8 @@ static int amdgpu_cs_wait_all_fences(struct amdgpu_device *adev,
>   		if (r == 0)
>   			break;
>   
> -		if (fence->error)
> -			return fence->error;
> +		if (error)
> +			return error;
>   	}
>   
>   	memset(wait, 0, sizeof(*wait));