Re: [PATCH v19 6/8] PM: hibernate: disable when there are active secretmem users

[James Bottomley]

On Tue, 2021-05-18 at 18:49 -0700, Dan Williams wrote:
> On Tue, May 18, 2021 at 6:33 PM James Bottomley <jejb@xxxxxxxxxxxxx>
> wrote:
> > On Tue, 2021-05-18 at 11:24 +0100, Mark Rutland wrote:
> > > On Thu, May 13, 2021 at 09:47:32PM +0300, Mike Rapoport wrote:
> > > > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > > > 
> > > > It is unsafe to allow saving of secretmem areas to the
> > > > hibernation snapshot as they would be visible after the resume
> > > > and this essentially will defeat the purpose of secret memory
> > > > mappings.
> > > > 
> > > > Prevent hibernation whenever there are active secret memory
> > > > users.
> > > 
> > > Have we thought about how this is going to work in practice, e.g.
> > > on mobile systems? It seems to me that there are a variety of
> > > common applications which might want to use this which people
> > > don't expect to inhibit hibernate (e.g. authentication agents,
> > > web browsers).
> > 
> > If mobile systems require hibernate, then the choice is to disable
> > this functionality or implement a secure hibernation store.   I
> > also thought most mobile hibernation was basically equivalent to
> > S3, in which case there's no actual writing of ram into storage, in
> > which case there's no security barrier and likely the inhibition
> > needs to be made a bit more specific to the suspend to disk case?
> > 
> > > Are we happy to say that any userspace application can
> > > incidentally inhibit hibernate?
> > 
> > Well, yes, for the laptop use case because we don't want suspend to
> > disk to be able to compromise the secret area.  You can disable
> > this for mobile if you like, or work out how to implement hibernate
> > securely if you're really suspending to disk.
> 
> Forgive me if this was already asked and answered. Why not document
> that secretmem is ephemeral in the case of hibernate and push the
> problem to userspace to disable hibernation? In other words
> hibernation causes applications to need to reload their secretmem, it
> will be destroyed on the way down and SIGBUS afterwards. That at
> least gives a system the flexibility to either sacrifice hibernate
> for secretmem (with a userspace controlled policy), or sacrifice
> secretmem using processes for hibernate.

Well, realistically, there are many possibilities for embedded if it
wants to use secret memory.  However, not really having much of an
interest in the use cases, it's not really for Mike or me to be acting
as armchair fly half.  I think the best we can do is demonstrate the
system for our use cases and let embedded kick the tyres for theirs if
they care, and if not they can disable the feature.

James