linux-kernel - Re: [PATCH] Use memzero_explicit to clear local buffers

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Message-ID: <20150104233637.GA20757@gondor.apana.org.au>
Date:	Mon, 5 Jan 2015 10:36:37 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Giel van Schijndel <me@...tis.eu>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"maintainer:X86 ARCHITECTURE..." <x86@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steve French <sfrench@...ba.org>,
	Rahul Bedarkar <rahulbedarkar89@...il.com>,
	Thomas Pugliese <thomas.pugliese@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	"open list:CRYPTO API" <linux-crypto@...r.kernel.org>,
	"open list:CERTIFIED WIRELES..." <linux-usb@...r.kernel.org>,
	"open list:COMMON INTERNET F..." <linux-cifs@...r.kernel.org>,
	"moderated list:COMMON INTERNET F..." 
	<samba-technical@...ts.samba.org>,
	Daniel Borkmann <dborkman@...hat.com>
Subject: Re: [PATCH] Use memzero_explicit to clear local buffers

On Sun, Jan 04, 2015 at 11:49:09PM +0100, Giel van Schijndel wrote:
>
> > sctx does not point to stack memory so this is bogus.
> > 
> > Only stack memory cleared just before it goes out of scope needs
> > memzero_explicit.
> 
> Is that because the compiler can't safely optimize memset(0) away for a
> variable with greater-than-local scope?

Exactly.  memzero_explicit is not a marker for sensitive data.
Its only purpose is to prevent the compiler from optimising away
zeroing that occurs at the end of a scope.

Daniel, we should add a comment so that people stop sending bogus
patches with memzero_explicit.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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