linux-kernel - Re: [PATCH 5.4 regression fix] x86/boot: Provide memzero

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Message-ID: <12200313.ic8YZTgDOU@tauon.chronox.de>
Date:   Mon, 07 Oct 2019 11:34:09 +0200
From:   Stephan Mueller <smueller@...onox.de>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH 5.4 regression fix] x86/boot: Provide memzero_explicit

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@...m.mit.edu>
> >> Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get
> >> input, memzero_explicit") Signed-off-by: Hans de Goede
> >> <hdegoede@...hat.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?


> 
> Regards,
> 
> Hans



Ciao
Stephan
View attachment "memset_secure.c" of type "text/x-csrc" (4036 bytes)