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Message-ID: <79c369d4-ba96-97d4-7bda-14672f442855@huawei.com>
Date: Fri, 17 Sep 2021 14:44:09 +0800
From: Zhang Qiao <zhangqiao22@...wei.com>
To: <mingo@...hat.com>, <peterz@...radead.org>,
<juri.lelli@...hat.com>, <vincent.guittot@...aro.org>
CC: <linux-kernel@...r.kernel.org>, <zhangqiao22@...wei.com>
Subject: Re: [PATCH v3] kernel/sched: Fix sched_fork() access an invalid
sched_task_group
在 2021/9/15 14:40, Zhang Qiao 写道:
> There is a small race between copy_process() and sched_fork()
> where child->sched_task_group point to an already freed pointer.
>
> parent doing fork() | someone moving the parent
> to another cgroup
> -------------------------------+-------------------------------
> copy_process()
> + dup_task_struct()<1>
> parent move to another cgroup,
> and free the old cgroup. <2>
> + sched_fork()
> + __set_task_cpu()<3>
> + task_fork_fair()
> + sched_slice()<4>
>
> In the worst case, this bug can lead to "use-after-free" and
> cause panic as shown above,
> (1)parent copy its sched_task_group to child at <1>;
> (2)someone move the parent to another cgroup and free the old
> cgroup at <2>;
> (3)the sched_task_group and cfs_rq that belong to the old cgroup
> will be accessed at <3> and <4>, which cause a panic:
>
> [89249.732198] BUG: unable to handle kernel NULL pointer
> dereference at 0000000000000000
> [89249.732701] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD
> 2029955067 PMD 0
> [89249.733005] Oops: 0000 [#1] SMP PTI
> [89249.733288] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded
> Tainted: G OE --------- - - 4.18.0.x86_64+ #1
> [89249.734318] RIP: 0010:sched_slice+0x84/0xc0
> ....
> [89249.737910] Call Trace:
> [89249.738181] task_fork_fair+0x81/0x120
> [89249.738457] sched_fork+0x132/0x240
> [89249.738732] copy_process.part.5+0x675/0x20e0
> [89249.739010] ? __handle_mm_fault+0x63f/0x690
> [89249.739286] _do_fork+0xcd/0x3b0
> [89249.739558] do_syscall_64+0x5d/0x1d0
> [89249.739830] entry_SYSCALL_64_after_hwframe+0x65/0xca
> [89249.740107] RIP: 0033:0x7f04418cd7e1
>
> Between cgroup_can_fork() and cgroup_post_fork(), the cgroup
> membership and thus sched_task_group can't change. So update
> child's sched_task_group at sched_post_fork() and move task_fork()
> and __set_task_cpu() (where accees the sched_task_group) from
> sched_fork() to sched_post_fork().
>
> Fixes: 8323f26ce342 ("sched: Fix race in task_group")
> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
> ---
> include/linux/sched/task.h | 3 ++-
> kernel/fork.c | 2 +-
> kernel/sched/core.c | 43 +++++++++++++++++++-------------------
> 3 files changed, 25 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index ef02be869cf2..ba88a6987400 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -54,7 +54,8 @@ extern asmlinkage void schedule_tail(struct task_struct *prev);
> extern void init_idle(struct task_struct *idle, int cpu);
>
> extern int sched_fork(unsigned long clone_flags, struct task_struct *p);
> -extern void sched_post_fork(struct task_struct *p);
> +extern void sched_post_fork(struct task_struct *p,
> + struct kernel_clone_args *kargs);
> extern void sched_dead(struct task_struct *p);
>
> void __noreturn do_task_dead(void);
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 38681ad44c76..0e4251dc5436 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2405,7 +2405,7 @@ static __latent_entropy struct task_struct *copy_process(
> write_unlock_irq(&tasklist_lock);
>
> proc_fork_connector(p);
> - sched_post_fork(p);
> + sched_post_fork(p, args);
> cgroup_post_fork(p, args);
> perf_event_fork(p);
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c4462c454ab9..57544053be5a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4328,8 +4328,6 @@ int sysctl_schedstats(struct ctl_table *table, int write, void *buffer,
> */
> int sched_fork(unsigned long clone_flags, struct task_struct *p)
> {
> - unsigned long flags;
> -
> __sched_fork(clone_flags, p);
> /*
> * We mark the process as NEW here. This guarantees that
> @@ -4375,24 +4373,6 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
>
> init_entity_runnable_average(&p->se);
>
> - /*
> - * The child is not yet in the pid-hash so no cgroup attach races,
> - * and the cgroup is pinned to this child due to cgroup_fork()
> - * is ran before sched_fork().
> - *
> - * Silence PROVE_RCU.
> - */
> - raw_spin_lock_irqsave(&p->pi_lock, flags);
> - rseq_migrate(p);
> - /*
> - * We're setting the CPU for the first time, we don't migrate,
> - * so use __set_task_cpu().
> - */
> - __set_task_cpu(p, smp_processor_id());
> - if (p->sched_class->task_fork)
> - p->sched_class->task_fork(p);
> - raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> -
> #ifdef CONFIG_SCHED_INFO
> if (likely(sched_info_on()))
> memset(&p->sched_info, 0, sizeof(p->sched_info));
> @@ -4408,8 +4388,29 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> return 0;
> }
>
> -void sched_post_fork(struct task_struct *p)
> +void sched_post_fork(struct task_struct *p, struct kernel_clone_args *kargs)
> {
> + unsigned long flags;
> +#ifdef CONFIG_CGROUP_SCHED
> + struct task_group *tg;
> +#endif
> +
> + raw_spin_lock_irqsave(&p->pi_lock, flags);
> +#ifdef CONFIG_CGROUP_SCHED
> + tg = container_of(kargs->cset->subsys[cpu_cgrp_id],
> + struct task_group, css);
> + p->sched_task_group = autogroup_task_group(p, tg);
> +#endif
> + rseq_migrate(p);
> + /*
> + * We're setting the CPU for the first time, we don't migrate,
> + * so use __set_task_cpu().
> + */
> + __set_task_cpu(p, smp_processor_id());
> + if (p->sched_class->task_fork)
> + p->sched_class->task_fork(p);
> + raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +
> uclamp_post_fork(p);
> }
hello, scheduler folks,
could you please have a look?
thanks.
---
Zhang Qiao
>
>
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