Re: [PATCH v2] mm/mremap: Fail map duplication attempts for private mappings

[Michal Hocko]

On Fri 21-07-17 14:18:31, Mike Kravetz wrote:
[...]
> >From 5c4a1602bd6a942544ed011dc0a72fd258e874b2 Mon Sep 17 00:00:00 2001
> From: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Date: Wed, 12 Jul 2017 13:52:47 -0700
> Subject: [PATCH] mm/mremap: Fail map duplication attempts for private mappings
> 
> mremap will attempt to create a 'duplicate' mapping if old_size
> == 0 is specified.  In the case of private mappings, mremap
> will actually create a fresh separate private mapping unrelated
> to the original.  This does not fit with the design semantics of
> mremap as the intention is to create a new mapping based on the
> original.
> 
> Therefore, return EINVAL in the case where an attempt is made
> to duplicate a private mapping.  Also, print a warning message
> (once) if such an attempt is made.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  mm/mremap.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index cd8a1b1..75b167d 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -383,6 +383,19 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>  	if (!vma || vma->vm_start > addr)
>  		return ERR_PTR(-EFAULT);
>  
> +	/*
> +	 * !old_len is a special case where an attempt is made to 'duplicate'
> +	 * a mapping.  This makes no sense for private mappings as it will
> +	 * instead create a fresh/new mapping unrelated to the original.  This
> +	 * is contrary to the basic idea of mremap which creates new mappings
> +	 * based on the original.  There are no known use cases for this
> +	 * behavior.  As a result, fail such attempts.
> +	 */
> +	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
> +		pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
>  	if (is_vm_hugetlb_page(vma))
>  		return ERR_PTR(-EINVAL);
>  
> -- 
> 2.7.5

-- 
Michal Hocko
SUSE Labs