[PATCH] drivers:gpu:drm:amd:amdgpu:amdgpu_cs.c:fix a potential use-after-free

[Wentao_Liang <Wentao_Liang_g@163.com>]

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