From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 07FA3ECAAD5 for ; Mon, 29 Aug 2022 19:06:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229577AbiH2TGK (ORCPT ); Mon, 29 Aug 2022 15:06:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56862 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229446AbiH2TGI (ORCPT ); Mon, 29 Aug 2022 15:06:08 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B1D5C7A774; Mon, 29 Aug 2022 12:06:06 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 4EE7761367; Mon, 29 Aug 2022 19:06:06 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B6CCBC433C1; Mon, 29 Aug 2022 19:06:04 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="QC1+uecv" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1661799963; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=twauLnP4jhuZFYkzgdwWPK5ISesn2VyXRMhrLPjnTC4=; b=QC1+uecvx+Bqwmsmmt3IoasaRmXX5drKwqo9ZxwPmfTys/vloq3aJIePJ7Z8NF3OqSmfFh VPtLCKcu6uSbc+5hkAFY7zQ7e7sxlDGQdUeMRGNiGIiVzOnFXDMqdfyNOZqFVLL2/xe3yU 1AAreUSHekO9UNKdbdYTiG54rc+Jnr8= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id b40d274c (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Mon, 29 Aug 2022 19:06:02 +0000 (UTC) Date: Mon, 29 Aug 2022 15:05:59 -0400 From: "Jason A. Donenfeld" To: Eric Biggers Cc: x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, Adam Langley , Ard Biesheuvel Subject: Re: Should Linux set the new constant-time mode CPU flags? Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 29, 2022 at 12:39:53PM -0400, Jason A. Donenfeld wrote: > In terms of granularity, I saw other folks suggesting making it per-task > (so, presumably, a prctl() knob), and others mentioning doing it just > for kernel crypto. For the latter, I guess the crypto API could set it > inside of its abstractions, and the various lib/crypto APIs could set it > at invocation time. I wonder, though, what's the cost of > enabling/disabling it? Would we in fact need a kind of lazy-deferred > disabling, like we have with kernel_fpu_end()? I also wonder what > crypto-adjacent code might wind up being missed if we're going function > by function. Like, obviously we'd set this for crypto_memneq, but what > about potential unprotected `==` of ID numbers that could leak some info > in various protocols? What other subtle nearby code should we be > thinking about, that relies on constant time logic but isn't neatly > folded inside a crypto_do_something() function? Another random note on this: I would hope that setting that MSR represents a speculation barrier or general instruction stream barrier, so that you can't do something naughty with the scheduler to toggle it rapidly and measure crypto timings somehow.