linux-kernel - Re: [PATCH] lib/sha1: remove memsets and allocate workspace on the stack

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Date:	Mon, 8 Aug 2011 16:50:22 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Ramsay Jones <ramsay@...say1.demon.co.uk>,
	Nicolas Pitre <nico@...xnic.net>,
	Joachim Eastwood <manabian@...il.com>,
	Andreas Schwab <schwab@...ux-m68k.org>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH] lib/sha1: remove memsets and allocate workspace on the stack

On Mon, Aug 8, 2011 at 4:07 PM, Mandeep Singh Baines <msb@...omium.org> wrote:
>
> There is no loss of security due to removing the memset. It would be a
> bug for the stack to leak to userspace. However, a defence-in-depth
> argument could be made for keeping the clearing of the workspace.

So I'm nervous about this just because I can see the security crazies
rising up about this.

The fact is, in our current code in drivers/char/random.c, we do have
a memset() of the workspace buffer on the stack, and any competent
compiler should actually just remove it, because it's dead memory (and
the compiler can *see* that it's dead memory). Of course, I don't know
if gcc does notice that, but it's a prime example of code that "looks"
secure, but has absolutely zero actual real security. Getting rid of
the memset() is actually better for *real* security, in that at least
it's not some kind of pointless security theater. But I  can see some
people wanting to add a memory barrier or something to force the
memset() to actually take place.

So I dunno.

Arguably it's theoretically possible to find random data on the stack,
and maybe it can even be interesting (although I don't think the last
64 bytes of SHA1 state is all that exciting myself). Personally, I
consider it unlikely as hell to be relevant to anybody, and anybody
who has access to the kernel stack has *much* more direct security
holes than some random data that they can use. But the patch still
makes me worry about the brouhaha from some people.

                    Linus
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