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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
Date: Mon, 7 Oct 2019 16:22:30 +0200
Message-ID: <20191007142230.GA117630@gmail.com> (raw)
In-Reply-To: <1dc3c53d-785e-f9a4-1b4c-3374c94ae0a7@redhat.com>


* 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?

Thanks,

	Ingo

  reply index

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-07 13:47 Hans de Goede
2019-10-07 14:00 ` Ingo Molnar
2019-10-07 14:11   ` Hans de Goede
2019-10-07 14:22     ` Ingo Molnar [this message]
2019-10-07 14:29       ` Hans de Goede
2019-10-07 14:46         ` Ingo Molnar
2019-10-07 15:20           ` Arvind Sankar
2019-10-07 15:40             ` Ingo Molnar
2019-10-07 18:42               ` Arvind Sankar
2019-10-07 19:36                 ` Hans de Goede
2019-10-07 22:00                   ` [PATCH] lib/string: make memzero_explicit inline instead of external Arvind Sankar
2019-10-08 11:33                     ` [tip: x86/urgent] lib/string: Make memzero_explicit() " tip-bot2 for Arvind Sankar
2019-10-08 11:33                     ` tip-bot2 for Arvind Sankar
2019-10-10  2:52                     ` [PATCH] lib/string: make memzero_explicit " Dave Young
2019-10-10  6:56                       ` Dave Young
2019-10-07 14:49 ` [tip: x86/urgent] x86/boot: Provide memzero_explicit() tip-bot2 for Hans de Goede
2019-10-07 14:49 ` tip-bot2 for Hans de Goede

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