[PATCH 5.12 433/677] kthread: Fix PF_KTHREAD vs to_kthread() race
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: 2021-05-12 18:57:35
Also in:
lkml
Subsystem:
scheduler, the rest · Maintainers:
Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Linus Torvalds
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 <redacted>
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 98191218d891..17ad829a114c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c@@ -7652,7 +7652,7 @@ 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 b5ffe808b07b..d078767f677f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c@@ -7608,7 +7608,7 @@ 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