kthread: Fix PF_KTHREAD vs to_kthread() race

commit 2fe516a6cfb1822473a054aad48ddedd048b38a9 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: cf657aaf2619 ("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
Signed-off-by: Patrick Schaaf <bof@bof.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b2bac5d929d2bb79d6b0fedefce77767ad0d06c4..63f7eb3c2640d346f8d0b1b6e019b9574dc2f378 100644 (file)
--- 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 74cb20f32f72f1f7c6f72d027e46060d116c78f0..87d9fad9d01d6e25e75370d134bb6c082633d558 100644 (file)
--- 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)) {