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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B20F7CFD340 for ; Mon, 24 Nov 2025 17:50:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=uMxwzkjUbHSlCREDuWZdbtMRE9C753ReQvDktnODA4A=; b=AKFzNMhRDOUql6oQWjvNuHZoKG nXRnAYanf2hwYi3p+Nog3c44WdJHeYUsSikFcx8s17RBMyfzoElroS8VN/Ep3JL7SU0tz/CQebRpK pV/knFe2xC06eI3SncwvOqur/7LIrP5EpsJTL+t7oJCjSVE3ClQzDFoyt2xjUWV7+k6I4JO97LF57 HqkJA6dofcrij1MEdWcFYG3fzMcvLlAWY5ZYbeaJg37FY4/8imRV0R5ui/uMlNQup8GMWunajwT1f evSccK4qFPNpd5fTv3bABkWugL40sgqoBLISrQrTHje8SUVXqQTaJnEQT7QtSO1YeD8JXRNnsfHWA fcW7DBog==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNahi-0000000C8I1-28nn; Mon, 24 Nov 2025 17:50:22 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNahf-0000000C8HX-0p2H for linux-arm-kernel@lists.infradead.org; Mon, 24 Nov 2025 17:50:20 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4374D1477; Mon, 24 Nov 2025 09:50:10 -0800 (PST) Received: from [10.57.88.238] (unknown [10.57.88.238]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4D8623F73B; Mon, 24 Nov 2025 09:50:16 -0800 (PST) Message-ID: Date: Mon, 24 Nov 2025 17:50:14 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [DISCUSSION] kstack offset randomization: bugs and performance Content-Language: en-GB To: Kees Cook , Will Deacon Cc: Arnd Bergmann , Ard Biesheuvel , Jeremy Linton , Catalin Marinas , Mark Rutland , "linux-arm-kernel@lists.infradead.org" , Linux Kernel Mailing List References: <66c4e2a0-c7fb-46c2-acce-8a040a71cd8e@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251124_095019_387023_CA625EC3 X-CRM114-Status: GOOD ( 23.13 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 24/11/2025 17:11, Kees Cook wrote: > > > On November 24, 2025 6:36:25 AM PST, Will Deacon wrote: >> On Mon, Nov 17, 2025 at 11:31:22AM +0000, Ryan Roberts wrote: >>> On 17/11/2025 11:30, Ryan Roberts wrote: >>>> Could this give us a middle ground between strong-crng and >>>> weak-timestamp-counter? Perhaps the main issue is that we need to store the >>>> secret key for a long period? >>>> >>>> >>>> Anyway, I plan to work up a series with the bugfixes and performance >>>> improvements. I'll add the siphash approach as an experimental addition and get >>>> some more detailed numbers for all the options. But wanted to raise it all here >>>> first to get any early feedback. >> >> FWIW, I share Mark's concerns about using a counter for this. Given that >> the feature currently appears to be both slow _and_ broken I'd vote for >> either removing it or switching over to per-thread offsets as a first >> step. > > That it has potential weaknesses doesn't mean it should be entirely removed. > >> We already have a per-task stack canary with >> CONFIG_STACKPROTECTOR_PER_TASK so I don't understand the reluctance to >> do something similar here. > > That's not a reasonable comparison: the stack canary cannot change arbitrarily for a task or it would immediately crash on a function return. :) > >> Speeding up the crypto feels like something that could happen separately. > > Sure. But let's see what Ryan's patches look like. The suggested changes sound good to me. Just to say I haven't forgotten about this; I ended up having to switch to something more urgent. Hoping to get back to it later this week. I don't think this is an urgent issue, so hopefully folks are ok waiting. I propose to post whatever I end up with then we can all disscuss from there. But the rough shape so far: Fixes: - Remove choose_random_kstack_offset() - arch passes random into add_random_kstack_offset() (fixes migration bypass) - Move add_random_kstack_offset() to el0_svc()/el0_svc_compat() (before enabling interrupts) to fix non-preemption requirement (arm64) Perf Improvements: - Based on Jeremy's prng, but buffer the 32 bits and use 6 bits per syscall (so cost of prng generation is amortized over 5 syscalls) - Reseed prng using get_random_u64() every 64K prng invocations (so cost of get_random_u64() is amortized over 64K*5 syscalls) - So while get_random_u64() still has a latency spike, it's so infrequent that it doesn't show up in p99.9 for my benchmarks. - If we want to change it to per-task, I think it's all amenable. - I'll leave the timer off limits for arm64. Although I'm seeing some inconsistencies in the performance measurements, so need to get that understood properly first. Thanks, Ryan > > -Kees > >