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 0C6A1ECAAD5 for ; Mon, 29 Aug 2022 16:40:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229724AbiH2QkG (ORCPT ); Mon, 29 Aug 2022 12:40:06 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55440 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229565AbiH2QkD (ORCPT ); Mon, 29 Aug 2022 12:40:03 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A873055AA; Mon, 29 Aug 2022 09:39:59 -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 ams.source.kernel.org (Postfix) with ESMTPS id 4F18BB81162; Mon, 29 Aug 2022 16:39:58 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3F9B0C433D6; Mon, 29 Aug 2022 16:39:56 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="LfnwDr2l" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1661791194; 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=oF1j60atJlDmJ/i/cn/SVD6+lw8BdA37ZDx7OhjUUlY=; b=LfnwDr2lKEuM/Iwq0wN8pMO7EITZzEXBK5Kj2tMINL0OnmV9YDrfbY2Q39x0MdvAPxPqU6 fNfDUe+vtWaZF0gRIt21dYVm25wBfkrMt3+G7os0IBL6FyocNXA5YinK8TyR1pNh4fvn6Y 6Am/7NMH2JQJRpwpE/jexjTKW7H4NVA= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 5635b464 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Mon, 29 Aug 2022 16:39:54 +0000 (UTC) Date: Mon, 29 Aug 2022 12:39:53 -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 Hi Eric, On Thu, Aug 25, 2022 at 11:15:58PM +0000, Eric Biggers wrote: > I'm wondering if people are aware of this issue, and whether anyone has any > thoughts on whether/where the kernel should be setting these new CPU flags. > There don't appear to have been any prior discussions about this. (Thanks to Maybe it should be set unconditionally now, until we figure out how to make it more granular. 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? Jason