Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030368AbcJQRJ1 (ORCPT ); Mon, 17 Oct 2016 13:09:27 -0400 Received: from mail-vk0-f48.google.com ([209.85.213.48]:36514 "EHLO mail-vk0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030327AbcJQRJQ (ORCPT ); Mon, 17 Oct 2016 13:09:16 -0400 MIME-Version: 1.0 In-Reply-To: References: <1476551776-8099-1-git-send-email-ard.biesheuvel@linaro.org> <1476689310.19992.1.camel@sipsolutions.net> From: Andy Lutomirski Date: Mon, 17 Oct 2016 10:08:54 -0700 Message-ID: Subject: Re: [PATCH] crypto: ccm - avoid scatterlist for MAC encryption To: Ard Biesheuvel Cc: Johannes Berg , Sergey Senozhatsky , "" , Herbert Xu , "David S. Miller" , "" , "linux-kernel@vger.kernel.org" , Jouni Malinen Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1963 Lines: 36 On Mon, Oct 17, 2016 at 12:37 AM, Ard Biesheuvel wrote: > On 17 October 2016 at 08:28, Johannes Berg wrote: >> On Sat, 2016-10-15 at 18:16 +0100, Ard Biesheuvel wrote: >>> The CCM code goes out of its way to perform the CTR encryption of the >>> MAC using the subordinate CTR driver. To this end, it tweaks the >>> input and output scatterlists so the aead_req 'odata' and/or >>> 'auth_tag' fields [which may live on the stack] are prepended to the >>> CTR payload. This involves calling sg_set_buf() on addresses which >>> are not direct mapped, which is not supported. >> >>> Since the calculation of the MAC keystream involves a single call >>> into the cipher, to which we have a handle already given that the >>> CBC-MAC calculation uses it as well, just calculate the MAC keystream >>> directly, and record it in the aead_req private context so we can >>> apply it to the MAC in cypto_ccm_auth_mac(). This greatly simplifies >>> the scatterlist manipulation, and no longer requires scatterlists to >>> refer to buffers that may live on the stack. >> >> No objection from me, Herbert? >> >> I'm getting a bit nervous though - I'd rather have any fix first so >> people get things working again - so maybe I'll apply your other patch >> and mine first, and then we can replace yours by this later. >> > > Could we get a statement first whether it is supported to allocate > aead_req (and other crypto req structures) on the stack? If not, then > we have our work cut out for us. But if it is, I'd rather we didn't > apply the kzalloc/kfree patch, since it is just a workaround for the > broken generic CCM driver, for which a fix is already available. I'm not a crypto person, but I don't see why not. There's even a helper called SKCIPHER_REQUEST_ON_STACK. :) The only problem I know of is pointing a scatterlist at the stack, which is bad for much the same reason as doing real DMA from the stack.