[Devel] [PATCH v4 2/2] ve/kthread: fix race when work can be added to stopped kthread worker

[Kirill Tkhai]

On 22.03.2018 13:31, Pavel Tikhomirov wrote:
> Race can be reproduced with steps below:
> 
> 1) Add these test patch to increase the race probability:
> 
> kernel/kmod.c
> @@ -977,6 +977,8 @@ int call_usermodehelper_fns_ve(struct ve_struct *ve,
>         khelper = ve_is_super(ve) ? &khelper_worker : &ve->ve_umh_worker;
> 
>         if (ve_is_super(ve) || (get_exec_env() == ve)) {
> +               while(ve->is_running)
> +                       cond_resched();
>                 err = call_usermodehelper_by(khelper, path, argv, envp, wait, init,
>                                              cleanup, data);
>                 goto out_put;
> 
> 2) Set corepattern to type pipe in CT:
> 
> echo "|/bin/dd of=/vz/coredumps/core-%e-%t.%p" > /proc/sys/kernel/core_pattern
> 
> 3) Produce a segfault inside a container and next try to stop the
> container killing init.
> 
> Coredump creates "dd" work and ads it to ve_umh_worker, which is already
> stopped and will never handle these work, and our work will hang
> forever, and container will never stop:
> 
> [root at kuchy ~]# cat /proc/6064/stack
> [<ffffffff810a86f9>] __call_usermodehelper_exec+0x179/0x1a0
> [<ffffffff810a8eae>] call_usermodehelper_by+0x5e/0xa0
> [<ffffffff810a901c>] call_usermodehelper_fns_ve+0xec/0x170
> [<ffffffff8128600c>] do_coredump+0x4dc/0xaf0
> [<ffffffff810a1cc5>] get_signal_to_deliver+0x1c5/0x5e0
> [<ffffffff8102a387>] do_signal+0x57/0x6b0
> [<ffffffff8102aa3f>] do_notify_resume+0x5f/0xb0
> [<ffffffff816df6ec>] retint_signal+0x48/0x8c
> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> Fix is:
> 
> 1) When run __call_usermodehelper_exec for ve, check that umh for these
> ve is not PF_EXITING, before adding work. (These is done between memory
> barriers of helper_lock and helper_unlock (implict).)
> 
> 2) In do_exit first set PF_EXITING flag, and then in ve_stop_ns wait for
> running_helpers to become 0 before stopping umh. (Between setting and
> checking helpers there is a full memory barrier.)
> 
> Now we have 2 cases:
> 
> If __call_usermodehelper_exec thread comes to helper_lock after do_exit
> thread finished wait_khelpers, then the former already see that umh task
> is PF_EXITING (due to 2) and will not queue the work.
> 
> If __call_usermodehelper_exec comes to helper_lock before do_exit passes
> wait_khelpers, the former can see no PF_EXITING and can queue the work,
> and then do helper_unlock. After that if wait_khelpers see
> running_helpers == 0, it will also see the work queued (due to 1) and
> will run these work and complete it normaly.
> 
> So ither way work won't hang.
> 
> v3: using ve_struct::op_sem outside of ve code is bad so switch to a
> better solution and remove unneded lock.
> v4: actually remove lock in call_usermodehelper_fns_ve
> 
> https://jira.sw.ru/browse/PSBM-82490
> 
> Signed-of-by: Kirill Tkhai <ktkhai at virtuozzo.com>
> Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>

> ---
>  include/linux/kmod.h |  1 +
>  kernel/kmod.c        | 35 +++++++++++------------------------
>  kernel/ve/ve.c       |  2 ++
>  3 files changed, 14 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/kmod.h b/include/linux/kmod.h
> index 9bd5776b1302..cc9b9551a940 100644
> --- a/include/linux/kmod.h
> +++ b/include/linux/kmod.h
> @@ -98,6 +98,7 @@ enum umh_disable_depth {
>  extern void usermodehelper_init(void);
>  
>  extern int __usermodehelper_disable(enum umh_disable_depth depth);
> +extern void wait_khelpers(void);
>  extern void __usermodehelper_set_disable_depth(enum umh_disable_depth depth);
>  
>  static inline int usermodehelper_disable(void)
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 9c37e8b3ab01..1d2ea4fb17bb 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -803,6 +803,12 @@ int __usermodehelper_disable(enum umh_disable_depth depth)
>  	return -EAGAIN;
>  }
>  
> +void wait_khelpers(void)
> +{
> +	wait_event(running_helpers_waitq,
> +		atomic_read(&running_helpers) == 0);
> +}
> +
>  static void helper_lock(void)
>  {
>  	atomic_inc(&running_helpers);
> @@ -891,7 +897,8 @@ static int __call_usermodehelper_exec(struct subprocess_info *sub_info,
>  	if (sub_info->path[0] == '\0')
>  		goto out;
>  
> -	if (usermodehelper_disabled) {
> +	if (usermodehelper_disabled ||
> +	    (ve && (ve->init_task->flags & PF_EXITING))) {
>  		retval = -EBUSY;
>  		goto out;
>  	}
> @@ -975,30 +982,10 @@ int call_usermodehelper_fns_ve(struct ve_struct *ve,
>  	if (!ve)
>  		return -EFAULT;
>  
> -	if (ve_is_super(ve) || (get_exec_env() == ve)) {
> -		err = call_usermodehelper_by(path, argv, envp, wait, init, cleanup,
> -					     data, ve_is_super(ve) ? NULL : ve);
> -		goto out_put;
> -	}
> -
> -	if (wait > UMH_WAIT_EXEC) {
> -		printk(KERN_ERR "VE#%s: Sleeping call for containers UMH is "
> -				"not supported\n", ve->ve_name);
> -		err = -EINVAL;
> -		goto out_put;
> -	}
> -
> -	down_read(&ve->op_sem);
> -	err = -EPIPE;
> -	if (!ve->is_running)
> -		goto out;
> +	err = call_usermodehelper_by(path, argv, envp,
> +			wait, init, cleanup, data,
> +			ve_is_super(ve) ? NULL : ve);
>  
> -	err = call_usermodehelper_by(path, argv, envp, wait, init,
> -				     cleanup, data, ve);
> -
> -out:
> -	up_read(&ve->op_sem);
> -out_put:
>  	put_ve(ve);
>  	return err;
>  }
> diff --git a/kernel/ve/ve.c b/kernel/ve/ve.c
> index 68f84797a840..f562fb9e5cbf 100644
> --- a/kernel/ve/ve.c
> +++ b/kernel/ve/ve.c
> @@ -556,6 +556,8 @@ void ve_stop_ns(struct pid_namespace *pid_ns)
>  	if (!ve->ve_ns || ve->ve_ns->pid_ns != pid_ns)
>  		return;
>  
> +	wait_khelpers();
> +
>  	down_write(&ve->op_sem);
>  	/*
>  	 * Here the VE changes its state into "not running".
>