kthread: Fix PF_KTHREAD vs to_kthread() race

The kthread_is_per_cpu() construct relies on only being called on
PF_KTHREAD tasks (per the WARN in to_kthread). This gives rise to the
following usage pattern:

	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))

However, as reported by syzcaller, this is broken. The scenario is:

	CPU0				CPU1 (running p)

	(p->flags & PF_KTHREAD) // true

					begin_new_exec()
					  me->flags &= ~(PF_KTHREAD|...);
	kthread_is_per_cpu(p)
	  to_kthread(p)
	    WARN(!(p->flags & PF_KTHREAD) <-- *SPLAT*

Introduce __to_kthread() that omits the WARN and is sure to check both
values.

Use this to remove the problematic pattern for kthread_is_per_cpu()
and fix a number of other kthread_*() functions that have similar
issues but are currently not used in ways that would expose the
problem.

Notably kthread_func() is only ever called on 'current', while
kthread_probe_data() is only used for PF_WQ_WORKER, which implies the
task is from kthread_create*().

Fixes: ac687e6e8c26 ("kthread: Extract KTHREAD_IS_PER_CPU")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>
Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973..6d3c488 100644 (file)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -84,6 +84,25 @@ static inline struct kthread *to_kthread(struct task_struct *k)
        return (__force void *)k->set_child_tid;
 }
 
+/*
+ * Variant of to_kthread() that doesn't assume @p is a kthread.
+ *
+ * Per construction; when:
+ *
+ *   (p->flags & PF_KTHREAD) && p->set_child_tid
+ *
+ * the task is both a kthread and struct kthread is persistent. However
+ * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
+ * begin_new_exec()).
+ */
+static inline struct kthread *__to_kthread(struct task_struct *p)
+{
+       void *kthread = (__force void *)p->set_child_tid;
+       if (kthread && !(p->flags & PF_KTHREAD))
+               kthread = NULL;
+       return kthread;
+}
+
 void free_kthread_struct(struct task_struct *k)
 {
        struct kthread *kthread;
@@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
  */
 void *kthread_func(struct task_struct *task)
 {
-       if (task->flags & PF_KTHREAD)
-               return to_kthread(task)->threadfn;
+       struct kthread *kthread = __to_kthread(task);
+       if (kthread)
+               return kthread->threadfn;
        return NULL;
 }
 EXPORT_SYMBOL_GPL(kthread_func);
@@ -199,10 +219,11 @@ EXPORT_SYMBOL_GPL(kthread_data);
  */
 void *kthread_probe_data(struct task_struct *task)
 {
-       struct kthread *kthread = to_kthread(task);
+       struct kthread *kthread = __to_kthread(task);
        void *data = NULL;
 
-       copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
+       if (kthread)
+               copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
        return data;
 }
 
@@ -514,9 +535,9 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
        set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
 }
 
-bool kthread_is_per_cpu(struct task_struct *k)
+bool kthread_is_per_cpu(struct task_struct *p)
 {
-       struct kthread *kthread = to_kthread(k);
+       struct kthread *kthread = __to_kthread(p);
        if (!kthread)
                return false;
 

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fcb35ae..4a0668a 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7667,7 +7667,7 @@ static void balance_push(struct rq *rq)
         * histerical raisins.
         */
        if (rq->idle == push_task ||
-           ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+           kthread_is_per_cpu(push_task) ||
            is_migration_disabled(push_task)) {
 
                /*

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7ea3b93..1d75af1 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7612,7 +7612,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
                return 0;
 
        /* Disregard pcpu kthreads; they are where they need to be. */
-       if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
+       if (kthread_is_per_cpu(p))
                return 0;
 
        if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {