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From: Vlastimil Babka <vbabka@suse.cz>
Newsgroups: linux.kernel
Subject: Re: [PATCH] mm/mremap: Fail map duplication attempts for private  mappings
Date: Thu, 13 Jul 2017 21:20:02 +0200
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[+CC linux-api]

On 07/13/2017 05:58 PM, Mike Kravetz wrote:
> mremap will create a 'duplicate' mapping if old_size == 0 is
> specified.  Such duplicate mappings make no sense for private
> mappings.  If duplication is attempted for a private mapping,
> mremap creates a separate private mapping unrelated to the
> original mapping and makes no modifications to the original.
> This is contrary to the purpose of mremap which should return
> a mapping which is in some way related to the original.
> 
> Therefore, return EINVAL in the case where if an attempt is
> made to duplicate a private mapping.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/mremap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index cd8a1b1..076f506 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -383,6 +383,13 @@ 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 a mapping is 'duplicated'.
> +	 * Do not allow this for private mappings.
> +	 */
> +	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
> +		return ERR_PTR(-EINVAL);
> +
>  	if (is_vm_hugetlb_page(vma))
>  		return ERR_PTR(-EINVAL);
>  
>