linux-kernel - Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in rxkad

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Message-ID: <20160628075546.GA15690@gondor.apana.org.au>
Date:	Tue, 28 Jun 2016 15:55:46 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Howells <dhowells@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 02/29] rxrpc: Avoid using stack memory in SG lists in
 rxkad

On Tue, Jun 28, 2016 at 08:52:20AM +0100, David Howells wrote:
> Andy Lutomirski <luto@...nel.org> wrote:
> 
> > -	skcipher_request_set_crypt(req, &sg[1], &sg[0], sizeof(tmpbuf), iv.x);
> > +	skcipher_request_set_crypt(req, &sg, &sg, sizeof(tmpbuf), iv.x);
> 
> Don't the sg's have to be different?  Aren't they both altered by the process
> of reading/writing from them?

No they don't have to be different.

> > 	struct rxrpc_skb_priv *sp;
> > ...
> > +	swap(tmpbuf.xl, *(__be64 *)sp);
> > +
> > +	sg_init_one(&sg, sp, sizeof(tmpbuf));
> 
> ????  I assume you're assuming that the rxrpc_skb_priv struct contents can
> arbitrarily replaced temporarily...

Of course you can, it's per-skb state.

> And using an XCHG-equivalent instruction?  This won't work on a 32-bit arch
> (apart from one that sports CMPXCHG8 or similar).

No this is not using an atomic xchg, whatever gave you that idea?
-- 
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