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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:
Peter Zijlstra 2021-04-20 10:18:17 +02:00
parent ad789f84c9
commit 3a7956e25e
3 changed files with 29 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
kernel/kthread.c
View File

@ -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;

2
kernel/sched/core.c
View File

@ -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)) {
/*

2
kernel/sched/fair.c
View File

@ -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)) {