Re: [RFC PATCH 5/6] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

[Oleg Nesterov <oleg@redhat.com>]

On 07/01, Michal Hocko wrote:
>
> From: Michal Hocko <mhocko@suse.com>
>
> vhost driver relies on copy_from_user/get_user from a kernel thread.
> This makes it impossible to reap the memory of an oom victim which
> shares mm with the vhost kernel thread because it could see a zero
> page unexpectedly and theoretically make an incorrect decision visible
> outside of the killed task context.

And I still can't understand how, but let me repeat that I don't understand
this code at all.

> To quote Michael S. Tsirkin:
> : Getting an error from __get_user and friends is handled gracefully.
> : Getting zero instead of a real value will cause userspace
> : memory corruption.

Which userspace memory corruption? We are going to kill the dev->mm owner,
the task which did ioctl(VHOST_SET_OWNER) and (at first glance) the task
who communicates with the callbacks fired by vhost_worker().

Michael, could you please spell why should we care?

> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -492,6 +492,14 @@ static bool __oom_reap_task(struct task_struct *tsk)
>  		goto unlock_oom;
>  	}
>
> +	/*
> +	 * Tell all users of get_user_mm/copy_from_user_mm that the content
> +	 * is no longer stable. No barriers really needed because unmapping
> +	 * should imply barriers already and the reader would hit a page fault
> +	 * if it stumbled over a reaped memory.
> +	 */
> +	set_bit(MMF_UNSTABLE, &mm->flags);

And this is racy anyway.

Oleg.

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