[PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation
rppt at kernel.org
Tue Feb 2 13:15:46 EST 2021
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
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
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)
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