v4: address changelog comments by Josh (thank you) ---8<--- When a KLP fails to apply, klp_reverse_transition will clear the TIF_PATCH_PENDING flag on all tasks, except for newly created tasks which are not on the task list yet. A similar race is possible for normal (forward) transitions, where TIF_PATCH_PENDING gets copied to the child, then later cleared in the parent. Meanwhile, fork will copy over the TIF_PATCH_PENDING flag from the parent to the child early on, in dup_task_struct -> setup_thread_stack. Much later, klp_copy_process will set child->patch_state to match that of the parent. However, the parent's patch_state may have been changed by KLP loading or unloading since it was initially copied over into the child. This results in the KLP code occasionally hitting this warning in klp_complete_transition: for_each_process_thread(g, task) { WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); task->patch_state = KLP_UNDEFINED; } Set, or clear, the TIF_PATCH_PENDING flag in the child task depending on whether or not it is needed at the time klp_copy_process is called, at a point in copy_process where the tasklist_lock is held exclusively, preventing races with the KLP code. The KLP code does have a few places where the state is changed without the tasklist_lock held, but those should not cause problems because klp_update_patch_state(current) cannot be called while the current task is in the middle of fork, klp_check_and_switch_task() which is called under the pi_lock, which prevents rescheduling, and manipulation of the patch state of idle tasks, which do not fork. This should prevent this warning from triggering again in the future, and close the race for both normal and reverse transitions. Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx> Reported-by: Breno Leitao <leitao@xxxxxxxxxx> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx> Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Fixes: d83a7cb375ee ("livepatch: change to a per-task consistency model") Cc: stable@xxxxxxxxxx --- kernel/livepatch/transition.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c index 5d03a2ad1066..30187b1d8275 100644 --- a/kernel/livepatch/transition.c +++ b/kernel/livepatch/transition.c @@ -610,9 +610,23 @@ void klp_reverse_transition(void) /* Called from copy_process() during fork */ void klp_copy_process(struct task_struct *child) { - child->patch_state = current->patch_state; - /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */ + /* + * The parent process may have gone through a KLP transition since + * the thread flag was copied in setup_thread_stack earlier. Bring + * the task flag up to date with the parent here. + * + * The operation is serialized against all klp_*_transition() + * operations by the tasklist_lock. The only exception is + * klp_update_patch_state(current), but we cannot race with + * that because we are current. + */ + if (test_tsk_thread_flag(current, TIF_PATCH_PENDING)) + set_tsk_thread_flag(child, TIF_PATCH_PENDING); + else + clear_tsk_thread_flag(child, TIF_PATCH_PENDING); + + child->patch_state = current->patch_state; } /* -- 2.35.1