Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 56c77c1b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Greg Kroah-Hartman
Browse files

kthread: Fix PF_KTHREAD vs to_kthread() race 



commit 3a7956e25e1d7b3c148569e78895e1f3178122a9 upstream.

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: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarValentin Schneider <Valentin.Schneider@arm.com>
Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net


Signed-off-by: default avatarPatrick Schaaf <bof@bof.de>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent af3cf928

kernel/kthread.c

+29 −14
Original line number Diff line number Diff line
@@ -76,6 +76,25 @@ 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;
@@ -176,9 +195,10 @@ void *kthread_data(struct task_struct *task) 
 */

void *kthread_probe_data(struct task_struct *task)

{

	struct kthread *kthread = to_kthread(task);

	struct kthread *kthread = __to_kthread(task);

	void *data = NULL;



	if (kthread)

		probe_kernel_read(&data, &kthread->data, sizeof(data));

	return data;

}
@@ -490,9 +510,9 @@ 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;
@@ -1272,11 +1292,9 @@ EXPORT_SYMBOL(kthread_destroy_worker); 
 */

void kthread_associate_blkcg(struct cgroup_subsys_state *css)

{

	struct kthread *kthread;

	struct kthread *kthread = __to_kthread(current);





	if (!(current->flags & PF_KTHREAD))

		return;

	kthread = to_kthread(current);

	if (!kthread)

		return;
@@ -1298,13 +1316,10 @@ EXPORT_SYMBOL(kthread_associate_blkcg); 
 */

struct cgroup_subsys_state *kthread_blkcg(void)

{

	struct kthread *kthread;

	struct kthread *kthread = __to_kthread(current);



	if (current->flags & PF_KTHREAD) {

		kthread = to_kthread(current);

	if (kthread)

		return kthread->blkcg_css;

	}

	return NULL;

}

EXPORT_SYMBOL(kthread_blkcg);

kernel/sched/fair.c

+1 −1
Original line number Diff line number Diff line
@@ -7301,7 +7301,7 @@ 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)) {