livepatch: Don't block the removal of patches loaded after a forced transition

module_put() is currently never called in klp_complete_transition() when
klp_force is set. As a result, we might keep the reference count even when
klp_enable_patch() fails and klp_cancel_transition() is called.

This might give the impression that a module might get blocked in some
strange init state. Fortunately, it is not the case. The reference count
is ignored when mod->init fails and erroneous modules are always removed.

Anyway, this might be confusing. Instead, this patch moves
the global klp_forced flag into struct klp_patch. As a result,
we block only modules that might still be in use after a forced
transition. Newly loaded livepatches might be eventually completely
removed later.

It is not a big deal. But the code is at least consistent with
the reality.

Signed-off-by: Petr Mladek <[email protected]>
Acked-by: Joe Lawrence <[email protected]>
Acked-by: Miroslav Benes <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Jiri Kosina <[email protected]>

Author: pmladek

include/linux/livepatch.h

@@ -143,6 +143,7 @@ struct klp_object {
  * @kobj:	kobject for sysfs resources
  * @kobj_added: @kobj has been added and needs freeing
  * @enabled:	the patch is enabled (but operation may be incomplete)
+ * @forced:	was involved in a forced transition
  * @finish:	for waiting till it is safe to remove the patch module
  */
 struct klp_patch {
@@ -155,6 +156,7 @@ struct klp_patch {
 	struct kobject kobj;
 	bool kobj_added;
 	bool enabled;
+	bool forced;
 	struct completion finish;
 };
 

kernel/livepatch/core.c

@@ -45,7 +45,8 @@
  */
 DEFINE_MUTEX(klp_mutex);
 
-static LIST_HEAD(klp_patches);
+/* Registered patches */
+LIST_HEAD(klp_patches);
 
 static struct kobject *klp_root_kobj;
 
@@ -659,6 +660,7 @@ static int klp_init_patch_early(struct klp_patch *patch)
 	INIT_LIST_HEAD(&patch->list);
 	patch->kobj_added = false;
 	patch->enabled = false;
+	patch->forced = false;
 	init_completion(&patch->finish);
 
 	klp_for_each_object(patch, obj) {

kernel/livepatch/core.h

@@ -5,6 +5,7 @@
 #include <linux/livepatch.h>
 
 extern struct mutex klp_mutex;
+extern struct list_head klp_patches;
 
 static inline bool klp_is_object_loaded(struct klp_object *obj)
 {

kernel/livepatch/transition.c

@@ -33,8 +33,6 @@ struct klp_patch *klp_transition_patch;
 
 static int klp_target_state = KLP_UNDEFINED;
 
-static bool klp_forced = false;
-
 /*
  * This work can be performed periodically to finish patching or unpatching any
  * "straggler" tasks which failed to transition in the first attempt.
@@ -137,10 +135,10 @@ static void klp_complete_transition(void)
 		  klp_target_state == KLP_PATCHED ? "patching" : "unpatching");
 
 	/*
-	 * klp_forced set implies unbounded increase of module's ref count if
+	 * patch->forced set implies unbounded increase of module's ref count if
 	 * the module is disabled/enabled in a loop.
 	 */
-	if (!klp_forced && klp_target_state == KLP_UNPATCHED)
+	if (!klp_transition_patch->forced && klp_target_state == KLP_UNPATCHED)
 		module_put(klp_transition_patch->mod);
 
 	klp_target_state = KLP_UNDEFINED;
@@ -620,6 +618,7 @@ void klp_send_signals(void)
  */
 void klp_force_transition(void)
 {
+	struct klp_patch *patch;
 	struct task_struct *g, *task;
 	unsigned int cpu;
 
@@ -633,5 +632,6 @@ void klp_force_transition(void)
 	for_each_possible_cpu(cpu)
 		klp_update_patch_state(idle_task(cpu));
 
-	klp_forced = true;
+	list_for_each_entry(patch, &klp_patches, list)
+		patch->forced = true;
 }