FAILED: patch "[PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8244a3bc27b3efd057da154b8d7e414670d5044f Mon Sep 17 00:00:00 2001
From: Anand K Mistry <[email protected]>
Date: Thu, 30 Sep 2021 09:00:07 +1000
Subject: [PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

drm_gem_ttm_mmap() drops a reference to the gem object on success. If the gem object's refcount == 1 on entry to drm_gem_prime_mmap(), that drop will free the gem object, and the subsequent drm_gem_object_get() will be a UAF. Fix by grabbing a reference before calling the mmap helper.

This issue was forseen when the reference dropping was adding in commit 9786b65bc61ac ("drm/ttm: fix mmap refcounting"):
"For that to work properly the drm_gem_object_get() call in drm_gem_ttm_mmap() must be moved so it happens before calling obj->funcs->mmap(), otherwise the gem refcount would go down to zero."

index deb23dbec8b5..d6c7f4f9a7a2 100644
@@ -719,11 +719,13 @@ int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
 	if (obj->funcs && obj->funcs->mmap) {
 		vma->vm_ops = obj->funcs->vm_ops;
 
+		drm_gem_object_get(obj);
 		ret = obj->funcs->mmap(obj, vma);
-		if (ret)
+		if (ret) {
+			drm_gem_object_put(obj);
 			return ret;
+		}
 		vma->vm_private_data = obj;
-		drm_gem_object_get(obj);
 		return 0;
 	}