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authorPeter Zijlstra <peterz@infradead.org>2021-04-20 10:18:17 +0200
committerPeter Zijlstra <peterz@infradead.org>2021-04-21 13:55:42 +0200
commit3a7956e25e1d7b3c148569e78895e1f3178122a9 (patch)
tree3fed8e39328534e9aa499002bb4d8890bb75eac9
parentad789f84c9a145f8a18744c0387cec22ec51651e (diff)
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) <peterz@infradead.org> Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com> Link: https://lkml.kernel.org/r/YH6WJc825C4P0FCK@hirez.programming.kicks-ass.net
-rw-r--r--kernel/kthread.c33
-rw-r--r--kernel/sched/core.c2
-rw-r--r--kernel/sched/fair.c2
3 files changed, 29 insertions, 8 deletions
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973c5740..6d3c488a0f82 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -84,6 +84,25 @@ 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;
@@ -168,8 +187,9 @@ EXPORT_SYMBOL_GPL(kthread_freezable_should_stop);
*/
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_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;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fcb35ae15619..4a0668acd876 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7667,7 +7667,7 @@ static void balance_push(struct rq *rq)
* histerical raisins.
*/
if (rq->idle == push_task ||
- ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+ kthread_is_per_cpu(push_task) ||
is_migration_disabled(push_task)) {
/*
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7ea3b93f5268..1d75af1ecfb4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7612,7 +7612,7 @@ 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)) {