[PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation

[Mike Rapoport]

On Tue, Feb 02, 2021 at 03:34:29PM +0100, David Hildenbrand wrote:
> On 02.02.21 15:32, Michal Hocko wrote:
> > On Tue 02-02-21 15:26:20, David Hildenbrand wrote:
> > > On 02.02.21 15:22, Michal Hocko wrote:
> > > > On Tue 02-02-21 15:12:21, David Hildenbrand wrote:
> > > > [...]
> > > > > I think secretmem behaves much more like longterm GUP right now
> > > > > ("unmigratable", "lifetime controlled by user space", "cannot go on
> > > > > CMA/ZONE_MOVABLE"). I'd either want to reasonably well control/limit it or
> > > > > make it behave more like mlocked pages.
> > > > 
> > > > I thought I have already asked but I must have forgotten. Is there any
> > > > actual reason why the memory is not movable? Timing attacks?
> > > 
> > > I think the reason is simple: no direct map, no copying of memory.
> > 
> > This is an implementation detail though and not something terribly hard
> > to add on top later on. I was more worried there would be really
> > fundamental reason why this is not possible. E.g. security implications.
> 
> I don't remember all the details. Let's see what Mike thinks regarding
> migration (e.g., security concerns).

Thanks for considering me a security expert :-)

Yet, I cannot estimate how dangerous is the temporal exposure of
this data to the kernel via the direct map in the simple map/copy/unmap
sequence.

More secure way would be to map source and destination in a different page table
rather than in the direct map, similarly to the way text_poke() on x86
does.

I've left the migration callback empty for now because it can be added on
top and its implementation would depend on the way we do (or do not do)
pooling.

-- 
Sincerely yours,
Mike.