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 X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 42B72C54FCC for ; Mon, 20 Apr 2020 22:35:19 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 9180920857 for ; Mon, 20 Apr 2020 22:35:18 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="WaKFtBNj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9180920857 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=chromium.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18581-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 7898 invoked by uid 550); 20 Apr 2020 22:35:11 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 7862 invoked from network); 20 Apr 2020 22:35:11 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=gHx7qgEhMfCeRi4LwRcBINau80f3lsbADPFxDRDnQX8=; b=WaKFtBNjxAG6922c9M7PS6Sr9GtIoZi+Bab8wtI8WphnYmTcq8s4ZSzd4luipXOLU4 jf4QK74nyiAcI85rmEUYpbI/hBAhRHU50KzAVjDVdJlDL1gU+m4RHq55dCtYvfJ4lxlT nAFclQvexr0WpO9w144f4pIlUT9r5MqSnjeTI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=gHx7qgEhMfCeRi4LwRcBINau80f3lsbADPFxDRDnQX8=; b=Jj4VEbvKNbSlp1rxPDMhOGsU/thLs+DN5wpBCl7uBYVvaMfcQJUNgK5cqa/Shlolbe OOmOcYo3p66npR41GApCwZTy6TDcgWSUAlvItJXReKa/cZnQ6ZhtHLotphmh1Fx3OJps 97S+HLaBR3Yh5RBgS8yE/GyTz0IAvyTCbth/iqh3/zmXF1jov9FFPcFwG5o2+mxF0mDg dqa4NF1gr3yqnQNAS+6emmpsvDoythyk7KHB6K7USopjjSxWXYeIy1L3A6A3ul/rjOmf KP7xriyWrSVbgvaOJbNhO+/Q/h5edl1TZ2UN0Y/tPIxdIGpukfsJKCd/Osiiz0WZpnZO gG0g== X-Gm-Message-State: AGi0Pub2rV7fpLTnarRgFBXuNrS7iJ2GJ3/ydf09/PmIR/GE3HzF3RH/ 7U8a4FlzN3hvM49tJ2wO/mYEzA== X-Google-Smtp-Source: APiQypKAcbw6yPUzbM4nMMNMJKcG2W4526Isd/VpAs6Bq80MioCbLHvZ9ufOTrlBeRJfVRW8JpiX/g== X-Received: by 2002:a63:602:: with SMTP id 2mr18621933pgg.383.1587422099130; Mon, 20 Apr 2020 15:34:59 -0700 (PDT) Date: Mon, 20 Apr 2020 15:34:57 -0700 From: Kees Cook To: Will Deacon Cc: Thomas Gleixner , Elena Reshetova , x86@kernel.org, Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Mark Rutland , Alexander Potapenko , Ard Biesheuvel , Jann Horn , "Perla, Enrico" , kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support Message-ID: <202004201529.BB787BB@keescook> References: <20200324203231.64324-1-keescook@chromium.org> <20200324203231.64324-6-keescook@chromium.org> <20200420205458.GC29998@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420205458.GC29998@willie-the-truck> On Mon, Apr 20, 2020 at 09:54:58PM +0100, Will Deacon wrote: > On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote: > > + /* > > + * Since the compiler chooses a 4 bit alignment for the stack, > > + * let's save one additional bit (9 total), which gets us up > > + * near 5 bits of entropy. > > + */ > > + choose_random_kstack_offset(get_random_int() & 0x1FF); > > Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff > is 9 bits, and that is 4+5 but so what? Er, well, yes. I guess I was just trying to explain why there were 9 bits saved here and to document what I was seeing the compiler actually doing with the values. (And it serves as a comparison to the x86 comment which is explaining similar calculations in the face of x86_64 vs ia32.) Would something like this be better? /* * Since the compiler uses 4 bit alignment for the stack (1 more than * x86_64), let's try to match the 5ish-bit entropy seen in x86_64, * instead of having needlessly lower entropy. As a result, keep the * low 9 bits. */ -- Kees Cook