Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

[Mike Rapoport]

On Mon, Oct 28, 2019 at 02:44:23PM -0600, Andy Lutomirski wrote:
> 
> > On Oct 27, 2019, at 4:17 AM, Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> > 
> > ïFrom: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > Hi,
> > 
> > The patch below aims to allow applications to create mappins that have
> > pages visible only to the owning process. Such mappings could be used to
> > store secrets so that these secrets are not visible neither to other
> > processes nor to the kernel.
> > 
> > I've only tested the basic functionality, the changes should be verified
> > against THP/migration/compaction. Yet, I'd appreciate early feedback.
> 
> Iâve contemplated the concept a fair amount, and I think you should
> consider a change to the API. In particular, rather than having it be a
> MAP_ flag, make it a chardev.  You can, at least at first, allow only
> MAP_SHARED, and admins can decide who gets to use it.  It might also play
> better with the VM overall, and you wonât need a VM_ flag for it â you
> can just wire up .fault to do the right thing.

I think mmap()/mprotect()/madvise() are the natural APIs for such
interface. Switching to a chardev doesn't solve the major problem of direct
map fragmentation and defeats the ability to use exclusive memory mappings
with the existing allocators, while mprotect() and madvise() do not.

-- 
Sincerely yours,
Mike.