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

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Message-ID: <20150106194226.GM4806@salidar.dom.custoft.eu>
Date:	Tue, 6 Jan 2015 20:42:26 +0100
From:	Giel van Schijndel <me@...tis.eu>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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 Mon, Jan 05, 2015 at 10:36:37 +1100, Herbert Xu wrote:
> 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.

Question: are you sure the compiler won't optimize the call to memset(0)
way if it's immediately followed by kfree()?

Because one of my changes concerns that situation.

Another actually does change a stack-allocated buffer, I'll split that
one off right away.

-- 
Met vriendelijke groet,
With kind regards,
Giel van Schijndel
--
"When all you have is a hammer, everything starts to look like a nail."
  -- Abraham Maslow

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