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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: ac687e6e8c
("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
This commit is contained in:
parent
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@ -84,6 +84,25 @@ static inline struct kthread *to_kthread(struct task_struct *k)
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return (__force void *)k->set_child_tid;
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}
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/*
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* Variant of to_kthread() that doesn't assume @p is a kthread.
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*
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* Per construction; when:
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*
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* (p->flags & PF_KTHREAD) && p->set_child_tid
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*
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* the task is both a kthread and struct kthread is persistent. However
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* PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
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* begin_new_exec()).
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*/
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static inline struct kthread *__to_kthread(struct task_struct *p)
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{
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void *kthread = (__force void *)p->set_child_tid;
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if (kthread && !(p->flags & PF_KTHREAD))
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kthread = NULL;
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return kthread;
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}
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void free_kthread_struct(struct task_struct *k)
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{
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struct kthread *kthread;
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@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
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*/
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void *kthread_func(struct task_struct *task)
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{
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if (task->flags & PF_KTHREAD)
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return to_kthread(task)->threadfn;
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struct kthread *kthread = __to_kthread(task);
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if (kthread)
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return kthread->threadfn;
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return NULL;
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}
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EXPORT_SYMBOL_GPL(kthread_func);
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@ -199,10 +219,11 @@ EXPORT_SYMBOL_GPL(kthread_data);
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*/
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void *kthread_probe_data(struct task_struct *task)
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{
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struct kthread *kthread = to_kthread(task);
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struct kthread *kthread = __to_kthread(task);
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void *data = NULL;
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copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
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if (kthread)
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copy_from_kernel_nofault(&data, &kthread->data, sizeof(data));
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return data;
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}
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@ -514,9 +535,9 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
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set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
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}
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bool kthread_is_per_cpu(struct task_struct *k)
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bool kthread_is_per_cpu(struct task_struct *p)
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{
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struct kthread *kthread = to_kthread(k);
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struct kthread *kthread = __to_kthread(p);
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if (!kthread)
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return false;
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@ -7667,7 +7667,7 @@ static void balance_push(struct rq *rq)
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* histerical raisins.
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*/
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if (rq->idle == push_task ||
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((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
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kthread_is_per_cpu(push_task) ||
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is_migration_disabled(push_task)) {
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/*
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@ -7612,7 +7612,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
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return 0;
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/* Disregard pcpu kthreads; they are where they need to be. */
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if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
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if (kthread_is_per_cpu(p))
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return 0;
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if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {
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