Previously when we added a fence to a dma_resv object we always
assumed the the newer than all the existing fences.

With Jason's work to add an UAPI to explicit export/import that's not
necessary the case any more. So without this check we would allow
userspace to force the kernel into an use after free error.

Since the change is very small and defensive it's probably a good
idea to backport this to stable kernels as well just in case others
are using the dma_resv object in the same way.

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/dma-buf/dma-resv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 205acb2c744d..e3885c90a3ac 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -295,7 +295,8 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
                enum dma_resv_usage old_usage;

                dma_resv_list_entry(fobj, i, obj, &old, &old_usage);
-               if ((old->context == fence->context && old_usage >= usage) ||
+               if ((old->context == fence->context && old_usage >= usage &&
+                    dma_fence_is_later(fence, old)) ||
                    dma_fence_is_signaled(old)) {
                        dma_resv_list_set(fobj, i, fence, usage);
                        dma_fence_put(old);
--
2.25.1