linux-kernel - Re: [PATCH] sha: prevent removal of memset as dead store in sha1

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Message-ID: <19334.42332.984074.920727@pilspetsen.it.uu.se>
Date:	Thu, 25 Feb 2010 17:29:16 +0100
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Mikael Pettersson <mikpe@...uu.se>,
	Roel Kluin <roel.kluin@...il.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in 
	sha1_update()

Pekka Enberg writes:
 > On Thu, Feb 25, 2010 at 5:56 PM, Mikael Pettersson <mikpe@...uu.se> wrote:
 > > I fear that the only portable (across compiler versions) and safe
 > > solution is to invoke an assembly-coded dummy function with prototype
 > >
 > >        void use(void *p);
 > >
 > > and rewrite the code above as
 > >
 > >        {
 > >                u32 temp[...];
 > >                ...
 > >                memset(temp, 0, sizeof temp);
 > >                use(temp);
 > >        }
 > >
 > > This forces the compiler to consider the buffer live after the
 > > memset, so the memset cannot be eliminated.
 > 
 > So is there some "do not optimize" GCC magic that we could use for a
 > memzero_secret() helper function?

I guess there's some -fno-builtin-... that might achieve this effect,
but that would disable all memset optimizations, not just those affecting
sensitive data.

You'd want a function attribute or magic type annotation and apply it
only to the specific cases where it's needed. Alas, I know of no such
attribute or annotation. ('volatile' doesn't work, I tried that.)

Ask on gcc@....gnu.org.
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