This is a note to let you know that I've just added the patch titled kthread: Fix PF_KTHREAD vs to_kthread() race to the 5.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: kthread-fix-pf_kthread-vs-to_kthread-race.patch and it can be found in the queue-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 3a7956e25e1d7b3c148569e78895e1f3178122a9 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Date: Tue, 20 Apr 2021 10:18:17 +0200 Subject: kthread: Fix PF_KTHREAD vs to_kthread() race From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> Reviewed-by: Valentin Schneider <Valentin.Schneider@xxxxxxx> Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Patrick Schaaf <bof@xxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/kthread.c | 43 +++++++++++++++++++++++++++++-------------- kernel/sched/fair.c | 2 +- 2 files changed, 30 insertions(+), 15 deletions(-) --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -76,6 +76,25 @@ static inline struct kthread *to_kthread 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 *t */ 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_str 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); --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7301,7 +7301,7 @@ int can_migrate_task(struct task_struct 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)) { Patches currently in stable-queue which might be from peterz@xxxxxxxxxxxxx are queue-5.4/kthread-fix-pf_kthread-vs-to_kthread-race.patch