Re: [PATCH 5.4 regression fix] x86/boot: Provide memzero_explicit

[Hans de Goede]

Hi Stephan,

On 07-10-2019 11:34, Stephan Mueller wrote:
> Am Montag, 7. Oktober 2019, 11:06:04 CEST schrieb Hans de Goede:
>
> Hi Hans,
>
> > Hi Stephan,
> >
> > On 07-10-2019 10:59, Stephan Mueller wrote:
> > > Am Montag, 7. Oktober 2019, 10:55:01 CEST schrieb Hans de Goede:
> > >
> > > Hi Hans,
> > >
> > > > The purgatory code now uses the shared lib/crypto/sha256.c sha256
> > > > implementation. This needs memzero_explicit, implement this.
> > > >
> > > > Reported-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > > > Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get
> > > > input, memzero_explicit") Signed-off-by: Hans de Goede
> > > > <hdegoede@xxxxxxxxxx>
> > > > ---
> > > >
> > > >    arch/x86/boot/compressed/string.c | 5 +++++
> > > >    1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/arch/x86/boot/compressed/string.c
> > > > b/arch/x86/boot/compressed/string.c index 81fc1eaa3229..511332e279fe
> > > > 100644
> > > > --- a/arch/x86/boot/compressed/string.c
> > > > +++ b/arch/x86/boot/compressed/string.c
> > > > @@ -50,6 +50,11 @@ void *memset(void *s, int c, size_t n)
> > > >
> > > >    	return s;
> > > >    
> > > >    }
> > > >
> > > > +void memzero_explicit(void *s, size_t count)
> > > > +{
> > > > +	memset(s, 0, count);
> > >
> > > May I ask how it is guaranteed that this memset is not optimized out by
> > > the
> > > compiler, e.g. for stack variables?
> >
> > The function and the caller live in different compile units, so unless
> > LTO is used this cannot happen.
>
> Agreed in this case.
>
> I would just be worried that this memzero_explicit implementation is assumed
> to be protected against optimization when used elsewhere since other
> implementations of memzero_explicit are provided with the goal to be protected
> against optimizations.
> > Also note that the previous purgatory private (vs shared) sha256
> > implementation had:
> >
> >           /* Zeroize sensitive information. */
> >           memset(sctx, 0, sizeof(*sctx));
> >
> > In the place where the new shared 256 code uses memzero_explicit() and the
> > new shared sha256 code is the only user of the
> > arch/x86/boot/compressed/string.c memzero_explicit() implementation.
> >
> > With that all said I'm open to suggestions for improving this.
>
> What speaks against the common memzero_explicit implementation?

Nothing, but the purgatory is a standalone binary which runs between
2 kernels when doing kexec so it cannot use the function from lib/string.c
since it is not linked against the lib/string.o object.

> If you cannot
> use it, what about adding a barrier in the memzero_explicit implementation? Or
> what about adding some compiler magic as attached to this email?

Since the purgatory code is running in a somewhat limited environment,
with not all standard headers / macros available I was afraid that the
barrier_data() from the lib/string.c implementation would not work, so
I left it out. In hindsight I should have really given it a try first as
it seems to compile fine and there are no missing symbols in
arch/x86/purgatory/purgatory.ro when using it.

So I will send out a new version with the barrier_data() added making
the arch/x86/boot/compressed/string.c implementation identical to the
lib/string.c one.

Regards,

Hans