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

[Anshuman Khandual]

On 07/14/2017 04:03 AM, Mike Kravetz wrote:
> On 07/13/2017 12:11 PM, Vlastimil Babka wrote:
>> [+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@xxxxxxxxxx>
>> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>>
> In another e-mail thread, Andrea makes the case that mremap(old_size == 0)
> of private file backed mappings could possibly be used for something useful.
> For example to create a private COW mapping.  Of course, a better way to do
> this would be simply using the fd to create a private mapping.
> 
> If returning EINVAL for all private mappings is too general, the following
> patch adds a check to only return EINVAL for private anon mappings.
> 
> mm/mremap: Fail map duplication attempts for private anon mappings
> 
> mremap will create a 'duplicate' mapping if old_size == 0 is
> specified.  Such duplicate mappings make no sense for private
> anonymous mappings.  If duplication is attempted for a private
> anon 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 an attempt is made to
> duplicate a private anon mapping.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  mm/mremap.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index cd8a1b1..586ea3d 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -383,6 +383,14 @@ 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 anon mappings.
> +	 */
> +	if (!old_len && vma_is_anonymous(vma) &&
> +	    !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
> +		return ERR_PTR(-EINVAL);

Sounds better compared to rejecting everything private.