drm/xe/ufence: Prefetch ufence addr to catch bogus address

access_ok() only checks for addr overflow so also try to read the addr
to catch invalid addr sent from userspace.

Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/1630
Cc: Francois Dugast <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Matthew Brost <[email protected]>
Reviewed-by: Matthew Brost <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Nirmoy Das <[email protected]>
(cherry picked from commit 9408c45)
Signed-off-by: Lucas De Marchi <[email protected]>

Author: nirmoy

drivers/gpu/drm/xe/xe_sync.c

@@ -54,8 +54,9 @@ static struct xe_user_fence *user_fence_create(struct xe_device *xe, u64 addr,
 {
 	struct xe_user_fence *ufence;
 	u64 __user *ptr = u64_to_user_ptr(addr);
+	u64 __maybe_unused prefetch_val;
 
-	if (!access_ok(ptr, sizeof(*ptr)))
+	if (get_user(prefetch_val, ptr))
 		return ERR_PTR(-EFAULT);
 
 	ufence = kzalloc(sizeof(*ufence), GFP_KERNEL);