Hi stable folks,

please queue for 5.10-stable.

See https://bugzilla.kernel.org/show_bug.cgi?id=214159 for more info.

--- 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) peterz@infradead.org Reviewed-by: Valentin Schneider Valentin.Schneider@arm.com Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net [ Drop the balance_push() hunk as it is not needed. ] Signed-off-by: Borislav Petkov bp@suse.de --- kernel/kthread.c | 33 +++++++++++++++++++++++++++------ kernel/sched/fair.c | 2 +- 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c index 9825cf89c614..508fe5278285 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -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;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 262b02d75007..bad97d35684d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7569,7 +7569,7 @@ 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)) {