After 3 days of successfully running 5.4.143 with this patch attached and no issues, on a production workload (host + vms) of a busy webserver and mysql database, I request queueing this for a future 5.4 stable, like the 5.10 one requested by Borislav; copying his mail text in the hope that this is best form.

please queue for 5.4 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 Signed-off-by: Patrick Schaaf bof@bof.de

diff --git a/kernel/kthread.c b/kernel/kthread.c index b2bac5d929d2..22750a8af83e 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -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; }

@@ -490,9 +510,9 @@ 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;

@@ -1272,11 +1292,9 @@ EXPORT_SYMBOL(kthread_destroy_worker); */ void kthread_associate_blkcg(struct cgroup_subsys_state *css) { - struct kthread *kthread; + struct kthread *kthread = __to_kthread(current); +

- if (!(current->flags & PF_KTHREAD)) - return; - kthread = to_kthread(current); if (!kthread) return;

@@ -1298,13 +1316,10 @@ EXPORT_SYMBOL(kthread_associate_blkcg); */ struct cgroup_subsys_state *kthread_blkcg(void) { - struct kthread *kthread; + struct kthread *kthread = __to_kthread(current);

- if (current->flags & PF_KTHREAD) { - kthread = to_kthread(current); - if (kthread) - return kthread->blkcg_css; - } + if (kthread) + return kthread->blkcg_css; return NULL; } EXPORT_SYMBOL(kthread_blkcg); diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 74cb20f32f72..87d9fad9d01d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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)) {