[PATCH 5.11 385/601] kthread: Fix PF_KTHREAD vs to_kthread() race

Greg Kroah-Hartman
2021-05-12T17:41:42Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 3a7956e25e1d7b3c148569e78895e1f3178122a9 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/kthread.c | 33 +++++++++++++++++++++++++++------
kernel/sched/core.c | 2 +-
kernel/sched/fair.c | 2 +-
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 f0056507a373..c5fcb5ce2194 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7290,7 +7290,7 @@ 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 d9182af98988..f217e5251fb2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7588,7 +7588,7 @@ 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)) {
--
2.30.2