[PATCH] drm/nouveau/mmu: fix Use after Free bug in nvkm_vmm_node_split

[Zheng Wang]

Here is a function call chain.
nvkm_vmm_pfn_map->nvkm_vmm_pfn_split_merge->nvkm_vmm_node_split
If nvkm_vma_tail return NULL in nvkm_vmm_node_split, it will
finally invoke nvkm_vmm_node_merge->nvkm_vmm_node_delete, which
will free the vma. However, nvkm_vmm_pfn_map didn't notice that.
It goes into next label and UAF happens.

Fix it by returning the return-value of nvkm_vmm_node_merge
instead of NULL.

Signed-off-by: Zheng Wang <zyytlz.wz@xxxxxxx>
---
 drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
index ae793f400ba1..84d6fc87b2e8 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/mmu/vmm.c
@@ -937,8 +937,8 @@ nvkm_vmm_node_split(struct nvkm_vmm *vmm,
 	if (vma->size != size) {
 		struct nvkm_vma *tmp;
 		if (!(tmp = nvkm_vma_tail(vma, vma->size - size))) {
-			nvkm_vmm_node_merge(vmm, prev, vma, NULL, vma->size);
-			return NULL;
+			tmp = nvkm_vmm_node_merge(vmm, prev, vma, NULL, vma->size);
+			return tmp;
 		}
 		tmp->part = true;
 		nvkm_vmm_node_insert(vmm, tmp);
-- 
2.25.1