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kthread: Fix PF_KTHREAD vs to_kthread() race

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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: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Valentin Schneider <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Patrick Schaaf <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Peter Zijlstra 2021-04-20 10:18:17 +02:00 • committed by Greg Kroah-Hartman
commit 56c77c1b52
2 changed files with 30 additions and 15 deletions
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2
kernel/sched/fair.c
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@ -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)) {