From: Zhang Qiao zhangqiao22@huawei.com
[ Upstream commit 4ef0c5c6b5ba1f38f0ea1cedad0cad722f00c14a ]
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:
[] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 [] PGD 8000001fa0a86067 P4D 8000001fa0a86067 PUD 2029955067 PMD 0 [] Oops: 0000 [#1] SMP PTI [] CPU: 7 PID: 648398 Comm: ebizzy Kdump: loaded Tainted: G OE --------- - - 4.18.0.x86_64+ #1 [] RIP: 0010:sched_slice+0x84/0xc0
[] Call Trace: [] task_fork_fair+0x81/0x120 [] sched_fork+0x132/0x240 [] copy_process.part.5+0x675/0x20e0 [] ? __handle_mm_fault+0x63f/0x690 [] _do_fork+0xcd/0x3b0 [] do_syscall_64+0x5d/0x1d0 [] entry_SYSCALL_64_after_hwframe+0x65/0xca [] 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@huawei.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Acked-by: Tejun Heo tj@kernel.org Link: https://lkml.kernel.org/r/20210915064030.2231-1-zhangqiao22@huawei.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 ef02be869cf28..ba88a69874004 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 cbba21e3a58df..286c9d43163da 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2331,7 +2331,7 @@ 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 e165d28cf73be..9289da7f0ac4a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4096,8 +4096,6 @@ 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 @@ -4143,24 +4141,6 @@ 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)); @@ -4176,8 +4156,29 @@ 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); }