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

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Date:	Mon, 5 Jan 2015 08:35:38 +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>
Subject: Re: [PATCH] Use memzero_explicit to clear local buffers

On Sun, Jan 04, 2015 at 07:05:40PM +0100, Giel van Schijndel wrote:
> When leaving a function use memzero_explicit instead of memset(0) to
> clear locally allocated/owned buffers. memset(0) may be optimized away.
> 
> All of the affected buffers contain sensitive data, key material or
> derivatives of one of those two.

Nack.
 
> diff --git a/arch/x86/crypto/sha256_ssse3_glue.c b/arch/x86/crypto/sha256_ssse3_glue.c
> index 8fad72f..b616e63 100644
> --- a/arch/x86/crypto/sha256_ssse3_glue.c
> +++ b/arch/x86/crypto/sha256_ssse3_glue.c
> @@ -164,7 +164,7 @@ static int sha256_ssse3_final(struct shash_desc *desc, u8 *out)
>  		dst[i] = cpu_to_be32(sctx->state[i]);
>  
>  	/* Wipe context */
> -	memset(sctx, 0, sizeof(*sctx));
> +	memzero_explicit(sctx, sizeof(*sctx));

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.

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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