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

[Arvind Sankar <nivedita@alum.mit.edu>]

On Mon, Oct 07, 2019 at 11:34:09AM +0200, 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@alum.mit.edu>
> > >> Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get
> > >> input, memzero_explicit") Signed-off-by: Hans de Goede
> > >> <hdegoede@redhat.com>
> > >> ---
> > >> 
> > >>   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? 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?
> 

The common definition I think is the same as your attachment, i.e.
memset followed by barrier_data(). I don't think there is any reason not
to just copy that definition?

Alternatively, could the common definition not be made an inline or
macro? or is there a risk that could introduce unsafe optimizations to
eliminate it?