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

[Mike Rapoport]

On Wed, Feb 03, 2021 at 01:09:30PM +0100, Michal Hocko wrote:
> 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:
> > 
> > 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.

The default is already NEVER_MAP_IN_KERNEL, so there is no explicit flag
for this. ALLOW_TEMPORARY should be opt-in, IMHO, and we can add it on top
later on.

-- 
Sincerely yours,
Mike.