Re: [PATCH] kthread: Fix PF_KTHREAD vs to_kthread() race

[Patrick Schaaf <bof@bof.de>]

On Wed, Sep 1, 2021 at 11:44 AM Greg KH <greg@kroah.com> wrote:
>
> On Mon, Aug 30, 2021 at 01:05:56PM +0200, Borislav Petkov wrote:
> >
> > please queue for 5.10-stable.
> >
> > See https://bugzilla.kernel.org/show_bug.cgi?id=214159 for more info.
>
> Now queued up, thanks.

I just booted a further of my prod servers with that applied to
5.10.61, patch needed a little mangling, attached below. Once more (as
with the 5.4 one),  will report about stability in a few days,
initially looks fine.

best regards
  Patrick

Tested-By: Patrick Schaaf <bof@bof.de>

diff -purN linux-5.10.61-orig/kernel/kthread.c linux-5.10.61-p/kernel/kthread.c
--- linux-5.10.61-orig/kernel/kthread.c 2021-08-26 14:51:21.000000000 +0200
+++ linux-5.10.61-p/kernel/kthread.c    2021-09-01 09:52:31.056738951 +0200
@@ -84,6 +84,25 @@ 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;
@@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_shou
  */
 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_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;

diff -purN linux-5.10.61-orig/kernel/sched/fair.c
linux-5.10.61-p/kernel/sched/fair.c
--- linux-5.10.61-orig/kernel/sched/fair.c      2021-08-26
14:51:21.000000000 +0200
+++ linux-5.10.61-p/kernel/sched/fair.c 2021-09-01 09:48:06.860333848 +0200
@@ -7569,7 +7569,7 @@ 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)) {