From 6bbc9108ab256b12a2f1eaeed6ed638bdcdaef36 Mon Sep 17 00:00:00 2001 From: Bin Lai Date: Fri, 24 Dec 2021 16:02:41 +0800 Subject: [PATCH] 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) Reviewed-by: Valentin Schneider Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net --- kernel/kthread.c | 26 +++++++++++++++++++++++--- kernel/sched/fair.c | 2 +- 2 files changed, 24 insertions(+), 4 deletions(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 1d4c98a19..d6a429638 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; } @@ -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; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fb427ed11..8d3377d76 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -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)) {