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

From: Mike Rapoport
Date: Tue Oct 29 2019 - 05:29:03 EST

On Mon, Oct 28, 2019 at 11:08:08AM -0700, Matthew Wilcox wrote:
> On Mon, Oct 28, 2019 at 10:12:44AM -0700, Dave Hansen wrote:
> > Some other random thoughts:
> >
> > * The page flag is probably not a good idea. It would be probably
> > better to set _PAGE_SPECIAL on the PTE and force get_user_pages()
> > into the slow path.
> > * This really stops being "normal" memory. You can't do futexes on it,
> > cant splice it. Probably need a more fleshed-out list of
> > incompatible features.
> > * As Kirill noted, each 4k page ends up with a potential 1GB "blast
> > radius" of demoted pages in the direct map. Not cool. This is
> > probably a non-starter as it stands.
> > * The global TLB flushes are going to eat you alive. They probably
> > border on a DoS on larger systems.
> > * Do we really want this user interface to dictate the kernel
> > implementation? In other words, do we really want MAP_EXCLUSIVE,
> > or do we want MAP_SECRET? One tells the kernel what do *do*, the
> > other tells the kernel what the memory *IS*.
> > * There's a lot of other stuff going on in this area: XPFO, SEV, MKTME,
> > Persistent Memory, where the kernel direct map is a liability in some
> > way. We probably need some kind of overall, architected solution
> > rather than five or ten things all poking at the direct map.
> Another random set of thoughts:
> - Should devices be permitted to DMA to/from MAP_SECRET pages?

I can't say I have a clear cut yes or no here. One possible use case for
such pages is to read a secrets from storage directly into them. On the
other side, DMA to/from a device can be used to exploit those secrets...

> - How about GUP?

Do you mean GUP for "remote" memory? I'd say no.

> - Can I ptrace my way into another process's secret pages?


> - What if I splice() the page into a pipe?

I think it should fail.

Sincerely yours,