[PATCH] drm/amdgpu: fix possible null-pointer dereference in amdgpu_ttm_tt_unpopulate()

[Tuo Li <islituo@gmail.com>]

The variable ttm is assigned to the variable gtt, and the variable gtt
is checked in:
  if (gtt && gtt->userptr)

This indicates that both ttm and gtt can be NULL.
If so, a null-pointer dereference will occur:
  if (ttm->page_flags & TTM_PAGE_FLAG_SG)

Also, some null-pointer dereferences will occur in the function
ttm_pool_free() which is called in:
  return ttm_pool_free(&adev->mman.bdev.pool, ttm);

To fix these possible null-pointer dereferences, the function returns
when ttm is NULL.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 3a55f08e00e1..0216ca085f11 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -1146,7 +1146,10 @@ static void amdgpu_ttm_tt_unpopulate(struct ttm_device *bdev,
 	struct amdgpu_ttm_tt *gtt = (void *)ttm;
 	struct amdgpu_device *adev;
 
-	if (gtt && gtt->userptr) {
+	if (ttm == NULL)
+		return;
+
+	if (gtt->userptr) {
 		amdgpu_ttm_tt_set_user_pages(ttm, NULL);
 		kfree(ttm->sg);
 		ttm->sg = NULL;
-- 
2.25.1