Re: [PATCH 09/10] vhost, mm: make sure that oom_reaper doesn't reap memory read by vhost

[Oleg Nesterov <oleg@redhat.com>]

Well. I promised to not argue, but I can't resist...

On 07/28, Michal Hocko wrote:
>
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -76,6 +76,28 @@ static inline unsigned long __copy_from_user_nocache(void *to,
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>
>  /*
> + * A safe variant of __get_user for for use_mm() users to have a
> + * gurantee that the address space wasn't reaped in the background
> + */
> +#define __get_user_mm(mm, x, ptr)				\
> +({								\
> +	int ___gu_err = __get_user(x, ptr);			\
> +	if (!___gu_err && test_bit(MMF_UNSTABLE, &mm->flags))	\
> +		___gu_err = -EFAULT;				\
> +	___gu_err;						\
> +})
> +
> +/* similar to __get_user_mm */
> +static inline __must_check long __copy_from_user_mm(struct mm_struct *mm,
> +		void *to, const void __user * from, unsigned long n)
> +{
> +	long ret = __copy_from_user(to, from, n);
> +	if ((ret >= 0) && test_bit(MMF_UNSTABLE, &mm->flags))
> +		return -EFAULT;
> +	return ret;
> +}

Still fail to understand why do we actually need this, but nevermind.

Can't we instead change handle_pte_fault() or do_anonymous_page() to
fail if MMF_UNSTABLE? We can realy pte_offset_map_lock(), MMF_UNSTABLE
must be visible under this lock.

We do not even need to actually disallow to re-populate the unmapped
pte afaics, so we can even change handle_mm_fault() to check
MMF_UNSTABLE after at the ens and return VM_FAULT_SIGBUS if it is set.

Oleg.

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