linux-kernel - Re: [PATCH V6 10/10] vhost: use user

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Message-ID: <87bl1fcaxs.fsf@email.froward.int.ebiederm.org>
Date:   Fri, 17 Dec 2021 13:01:51 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Mike Christie <michael.christie@...cle.com>
Cc:     geert@...ux-m68k.org, vverma@...italocean.com, hdanton@...a.com,
        hch@...radead.org, stefanha@...hat.com, jasowang@...hat.com,
        mst@...hat.com, sgarzare@...hat.com,
        virtualization@...ts.linux-foundation.org,
        christian.brauner@...ntu.com, axboe@...nel.dk,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V6 10/10] vhost: use user_worker to check RLIMITs

Mike Christie <michael.christie@...cle.com> writes:

> For vhost workers we use the kthread API which inherit's its values from
> and checks against the kthreadd thread. This results in the wrong RLIMITs
> being checked. This patch has us use the user_worker helpers which will
> inherit its values/checks from the thread that owns the device similar to
> if we did a clone in userspace.
>
> Signed-off-by: Mike Christie <michael.christie@...cle.com>
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
>  drivers/vhost/vhost.c | 65 +++++++++++++++----------------------------
>  drivers/vhost/vhost.h |  7 ++++-
>  2 files changed, 28 insertions(+), 44 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index c9a1f706989c..8cf259d798c0 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -22,7 +22,6 @@
>  #include <linux/slab.h>
>  #include <linux/vmalloc.h>
>  #include <linux/kthread.h>
> -#include <linux/cgroup.h>
>  #include <linux/module.h>
>  #include <linux/sort.h>
>  #include <linux/sched/mm.h>
> @@ -344,17 +343,14 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>  static int vhost_worker(void *data)
>  {
>  	struct vhost_worker *worker = data;
> -	struct vhost_dev *dev = worker->dev;
>  	struct vhost_work *work, *work_next;
>  	struct llist_node *node;
>  
> -	kthread_use_mm(dev->mm);
> -
>  	for (;;) {
>  		/* mb paired w/ kthread_stop */
>  		set_current_state(TASK_INTERRUPTIBLE);
>  
> -		if (kthread_should_stop()) {
> +		if (test_bit(VHOST_WORKER_FLAG_STOP, &worker->flags)) {
>  			__set_current_state(TASK_RUNNING);
>  			break;
>  		}
> @@ -376,8 +372,9 @@ static int vhost_worker(void *data)
>  				schedule();
>  		}
>  	}
> -	kthread_unuse_mm(dev->mm);
> -	return 0;
> +
> +	complete(worker->exit_done);
> +	do_exit(0);

This code worries me.

It has the potential for a caller to do:

	vhost_worker_stop()
        module_put();

Then the exiting work thread tries to do:
	do_exit()

Except the code that calls do_exit has already been removed from the
kernel.  Maybe the vhost code can never be removed from the kernel
but otherwise I expect that is possible.

Eric