Re: [PATCH V6 10/10] vhost: use user_worker to check RLIMITs

Eric W. Biederman
2021-12-17T19:02:03Z

Mike Christie <michael.christie@oracle.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@oracle.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.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