pstore: Add proper unregister lock checking

The pstore backend lock wasn't being used during pstore_unregister().
Add sanity check and locking.

Link: https://lore.kernel.org/lkml/20200506152114.50375-7-keescook@chromium.org/Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/platform.c

@@ -69,8 +69,9 @@ static void pstore_dowork(struct work_struct *);
 static DECLARE_WORK(pstore_work, pstore_dowork);
 
 /*
- * psinfo_lock just protects "psinfo" during
- * calls to pstore_register()
+ * psinfo_lock protects "psinfo" during calls to
+ * pstore_register(), pstore_unregister(), and
+ * the filesystem mount/unmount routines.
  */
 static DEFINE_MUTEX(psinfo_lock);
 struct pstore_info *psinfo;
@@ -587,8 +588,6 @@ int pstore_register(struct pstore_info *psi)
 	psinfo = psi;
 	mutex_init(&psinfo->read_mutex);
 	sema_init(&psinfo->buf_lock, 1);
-	mutex_unlock(&psinfo_lock);
-
 
 	if (psi->flags & PSTORE_FLAGS_DMESG)
 		allocate_buf_for_compression();
@@ -620,12 +619,25 @@ int pstore_register(struct pstore_info *psi)
 
 	pr_info("Registered %s as persistent store backend\n", psi->name);
 
+	mutex_unlock(&psinfo_lock);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(pstore_register);
 
 void pstore_unregister(struct pstore_info *psi)
 {
+	/* It's okay to unregister nothing. */
+	if (!psi)
+		return;
+
+	mutex_lock(&psinfo_lock);
+
+	/* Only one backend can be registered at a time. */
+	if (WARN_ON(psi != psinfo)) {
+		mutex_unlock(&psinfo_lock);
+		return;
+	}
+
 	/* Stop timer and make sure all work has finished. */
 	pstore_update_ms = -1;
 	del_timer_sync(&pstore_timer);
@@ -644,6 +656,7 @@ void pstore_unregister(struct pstore_info *psi)
 
 	psinfo = NULL;
 	backend = NULL;
+	mutex_unlock(&psinfo_lock);
 }
 EXPORT_SYMBOL_GPL(pstore_unregister);