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Date:   Mon, 7 Oct 2019 16:46:00 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        linux-crypto@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Stephan Mueller <smueller@chronox.de>
Subject: Re: [PATCH v2 5.4 regression fix] x86/boot: Provide memzero_explicit
Message-ID: <20191007144600.GB59713@gmail.com>
References: <20191007134724.4019-1-hdegoede@redhat.com>
 <20191007140022.GA29008@gmail.com>
 <1dc3c53d-785e-f9a4-1b4c-3374c94ae0a7@redhat.com>
 <20191007142230.GA117630@gmail.com>
 <2982b666-e310-afb7-40eb-e536ce95e23d@redhat.com>
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* Hans de Goede <hdegoede@redhat.com> wrote:

> Hi,
> 
> On 07-10-2019 16:22, Ingo Molnar wrote:
> > 
> > * Hans de Goede <hdegoede@redhat.com> wrote:
> > 
> > > Hi,
> > > 
> > > On 07-10-2019 16:00, Ingo Molnar wrote:
> > > > 
> > > > * Hans de Goede <hdegoede@redhat.com> wrote:
> > > > 
> > > > > 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>
> > > > > ---
> > > > > Changes in v2:
> > > > > - Add barrier_data() call after the memset, making the function really
> > > > >     explicit. Using barrier_data() works fine in the purgatory (build)
> > > > >     environment.
> > > > > ---
> > > > >    arch/x86/boot/compressed/string.c | 6 ++++++
> > > > >    1 file changed, 6 insertions(+)
> > > > > 
> > > > > diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c
> > > > > index 81fc1eaa3229..654a7164a702 100644
> > > > > --- a/arch/x86/boot/compressed/string.c
> > > > > +++ b/arch/x86/boot/compressed/string.c
> > > > > @@ -50,6 +50,12 @@ void *memset(void *s, int c, size_t n)
> > > > >    	return s;
> > > > >    }
> > > > > +void memzero_explicit(void *s, size_t count)
> > > > > +{
> > > > > +	memset(s, 0, count);
> > > > > +	barrier_data(s);
> > > > > +}
> > > > 
> > > > So the barrier_data() is only there to keep LTO from optimizing out the
> > > > seemingly unused function?
> > > 
> > > I believe that Stephan Mueller (who suggested adding the barrier)
> > > was also worried about people using this as an example for other
> > > "explicit" functions which actually might get inlined.
> > > 
> > > This is not so much about protecting against LTO as it is against
> > > protecting against inlining, which in this case boils down to the
> > > same thing. Also this change makes the arch/x86/boot/compressed/string.c
> > > and lib/string.c versions identical which seems like a good thing to me
> > > (except for the code duplication part of it).
> > > 
> > > But I agree a comment would be good, how about:
> > > 
> > > void memzero_explicit(void *s, size_t count)
> > > {
> > > 	memset(s, 0, count);
> > > 	/* Avoid the memset getting optimized away if we ever get inlined */
> > > 	barrier_data(s);
> > > }
> > 
> > Well, the standard construct for preventing inlining would be 'noinline',
> > right? Any reason that wouldn't work?
> 
> Good question. I guess the worry is that modern compilers are getting
> more aggressive with optimizing and then even if not inlined if the
> function gets compiled in the same scope, then the compiler might
> still notice it is only every writing to the memory passed in; and
> then optimize it away of the write happens to memory which lifetime
> ends immediately afterwards. I mean removing the call is not inlining,
> so compiler developers might decide that that is still fine to do.
> 
> IMHO with trickycode like this is is best to just use the proven
> version from lib/string.c
> 
> I guess I made the comment to specific though, so how about:
> 
> void memzero_explicit(void *s, size_t count)
> {
> 	memset(s, 0, count);
> 	/* Tell the compiler to never remove / optimize away the memset */
> 	barrier_data(s);
> }

Ok, I guess this will work.

Thanks,

	Ingo