firmware: qemu_fw_cfg: fix NULL-pointer deref on duplicate entries

Commit fe3c606 ("firmware: Fix a reference count leak.") "fixed"
a kobject leak in the file registration helper by properly calling
kobject_put() for the entry in case registration of the object fails
(e.g. due to a name collision).

This would however result in a NULL pointer dereference when the
release function tries to remove the never added entry from the
fw_cfg_entry_cache list.

Fix this by moving the list-removal out of the release function.

Note that the offending commit was one of the benign looking umn.edu
fixes which was reviewed but not reverted. [1][2]

[1] https://lore.kernel.org/r/202105051005.49BFABCE@keescook
[2] https://lore.kernel.org/all/[email protected]

Fixes: fe3c606 ("firmware: Fix a reference count leak.")
Cc: [email protected]      # 5.8
Cc: Qiushi Wu <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Johan Hovold <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>

drivers/firmware/qemu_fw_cfg.c

@@ -388,9 +388,7 @@ static void fw_cfg_sysfs_cache_cleanup(void)
 	struct fw_cfg_sysfs_entry *entry, *next;
 
 	list_for_each_entry_safe(entry, next, &fw_cfg_entry_cache, list) {
-		/* will end up invoking fw_cfg_sysfs_cache_delist()
-		 * via each object's release() method (i.e. destructor)
-		 */
+		fw_cfg_sysfs_cache_delist(entry);
 		kobject_put(&entry->kobj);
 	}
 }
@@ -448,7 +446,6 @@ static void fw_cfg_sysfs_release_entry(struct kobject *kobj)
 {
 	struct fw_cfg_sysfs_entry *entry = to_entry(kobj);
 
-	fw_cfg_sysfs_cache_delist(entry);
 	kfree(entry);
 }