in line 1535, "dma_fence_put(fence);" drop the reference to fence and may
cause fence to be released. However, fence is used subsequently in line
1542 "fence->error". This may result in an use-after-free bug.

It can be fixed by recording fence->error in a variable before dropping
the reference to fence and referencing it after dropping.

The bug has been confirmed by Christian König on 2021-08-16. Now, I
resend this patch with my real name. I hope the patch can be updated
in a near future.

Signed-off-by: Wentao_Liang <Wentao_Liang_g@163.com>
---
 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 b28af04b0c3e..1d675a5838f2 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
@@ -1518,7 +1518,7 @@ 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;
 
 	for (i = 0; i < fence_count; i++) {
@@ -1533,14 +1533,15 @@ static int amdgpu_cs_wait_all_fences(struct amdgpu_device *adev,
 
 		r = dma_fence_wait_timeout(fence, true, timeout);
 		dma_fence_put(fence);
+		error = fence->error;
 		if (r < 0)
 			return r;
 
 		if (r == 0)
 			break;
 
-		if (fence->error)
-			return fence->error;
+		if (error)
+			return error;
 	}
 
 	memset(wait, 0, sizeof(*wait));
-- 
2.25.1

Am 28.07.22 um 14:12 schrieb Wentao_Liang:
> in line 1535, "dma_fence_put(fence);" drop the reference to fence and may
> cause fence to be released. However, fence is used subsequently in line
> 1542 "fence->error". This may result in an use-after-free bug.
>
> It can be fixed by recording fence->error in a variable before dropping
> the reference to fence and referencing it after dropping.
>
> The bug has been confirmed by Christian König on 2021-08-16. Now, I
> resend this patch with my real name. I hope the patch can be updated
> in a near future.

The subject line should be something like "drm/amdgpu: fix potential use 
after free".

>
> Signed-off-by: Wentao_Liang <Wentao_Liang_g@163.com>
> ---
>   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 b28af04b0c3e..1d675a5838f2 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
> @@ -1518,7 +1518,7 @@ 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;
>   
>   	for (i = 0; i < fence_count; i++) {
> @@ -1533,14 +1533,15 @@ static int amdgpu_cs_wait_all_fences(struct amdgpu_device *adev,
>   
>   		r = dma_fence_wait_timeout(fence, true, timeout);
>   		dma_fence_put(fence);
> +		error = fence->error;

That's still the wrong order, you need to get the fence error before 
dropping the reference.

Christian.

>   		if (r < 0)
>   			return r;
>   
>   		if (r == 0)
>   			break;
>   
> -		if (fence->error)
> -			return fence->error;
> +		if (error)
> +			return error;
>   	}
>   
>   	memset(wait, 0, sizeof(*wait));