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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: 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
This commit is contained in:
Bin Lai 2021-12-24 16:02:41 +08:00 committed by kaixuxiakx
parent 7f225c74b8
commit 6bbc9108ab
2 changed files with 24 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
kernel/kthread.c
View File

@ -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;
@ -176,10 +195,11 @@ 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;
probe_kernel_read(&data, &kthread->data, sizeof(data));
if (kthread)
probe_kernel_read(&data, &kthread->data, sizeof(data));
return data;
}
@ -492,7 +512,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
bool kthread_is_per_cpu(struct task_struct *k)
{
struct kthread *kthread = to_kthread(k);
struct kthread *kthread = __to_kthread(k);
if (!kthread)
return false;

2
kernel/sched/fair.c
View File

@ -7363,7 +7363,7 @@ 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)) {