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

Michal Hocko
2021-02-03T12:09:51Z

On Tue 02-02-21 10:55:40, James Bottomley wrote:
> On Tue, 2021-02-02 at 20:15 +0200, Mike Rapoport wrote:
> > 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.
>
> Well the safest security statement is that we never expose the data to
> the kernel because it's a very clean security statement and easy to
> enforce. It's also the easiest threat model to analyse. Once we do
> start exposing the secret to the kernel it alters the threat profile
> and the analysis and obviously potentially provides the ROP gadget to
> an attacker to do the same. Instinct tells me that the loss of
> security doesn't really make up for the ability to swap or migrate but
> if there were a case for doing the latter, it would have to be a
> security policy of the user (i.e. a user should be able to decide their
> data is too sensitive to expose to the kernel).

The security/threat model should be documented in the changelog as
well. I am not a security expert but I would tend to agree that not
allowing even temporal mapping for data copying (in the kernel) is the
most robust approach. Whether that is generally necessary for users I do
not know.

From the API POV I think it makes sense to have two
modes. NEVER_MAP_IN_KERNEL which would imply no migrateability, no
copy_{from,to}_user, no gup or any other way for the kernel to access
content of the memory. Maybe even zero the content on the last unmap to
never allow any data leak. ALLOW_TEMPORARY would unmap the page from
the direct mapping but it would still allow temporary mappings for
data copying inside the kernel (thus allow CoW, copy*user, migration).
Which one should be default and which an opt-in I do not know. A less
restrictive mode to be default and the more restrictive an opt-in via
flags makes a lot of sense to me though.

--
Michal Hocko
SUSE Labs