Re: [PATCH 3/6] mremap: Don't allow MREMAP_DONTUNMAP on special_mappings and aio

[Dmitry Safonov]

On 12/28/20 7:33 PM, Dmitry Safonov wrote:
> [I moved your reply to avoid top-posting]
> 
> On 12/28/20 6:03 PM, Brian Geffon wrote:
>> On Mon, Oct 12, 2020 at 6:34 PM Dmitry Safonov <dima@xxxxxxxxxx> wrote:
>>>
>>> As kernel expect to see only one of such mappings, any further
>>> operations on the VMA-copy may be unexpected by the kernel.
>>> Maybe it's being on the safe side, but there doesn't seem to be any
>>> expected use-case for this, so restrict it now.
>>>
>>> Fixes: commit e346b3813067 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
>>> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
>>
>> I don't think this situation can ever happen MREMAP_DONTUNMAP is
>> already restricted to anonymous mappings (defined as not having
>> vm_ops) and vma_to_resize checks that the mapping is anonymous before
>> move_vma is called.
> 
> I've looked again now, I think it is possible. One can call
> MREMAP_DONTUNMAP without MREMAP_FIXED and without resizing. So that the
> old VMA is copied at some free address.
> 
> The calltrace would be: mremap()=>move_vma()
> [under if (flags & MREMAP_MAYMOVE)].
> 
> On the other side I agree with you that the fix could have been better
> if I realized the semantics that MREMAP_DONTUNMAP should only work with
> anonymous mappings.
> 
> Probably, a better fix would be to move
> :       if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
> :                       vma->vm_flags & VM_SHARED))
> :               return ERR_PTR(-EINVAL);
> 
> from vma_to_resize() into the mremap() syscall directly.
> What do you think?

Ok, I've misread the code now, it checks vma_to_resize() before.
I'll send a revert to this one.

Thanks for noticing,
          Dima