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 47D22CFD356 for ; Mon, 24 Nov 2025 20:52:10 +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:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=3DG8aNFGVcFB4CATalGn7NOhxZ0DeO8/oe9bHCpl8sw=; b=KWxnVoBT6ItuVGvc9Z/CNei/4I 5NlL+fKakUwgXFyus1UzM774ihqq+sPYnjQ4OoXAe+PSDpmDE243faFgFBhGYklPFIH5L5BZBp9Yd A6+x/6ucvsuWKTRj3WAGwBrvdEVKjsLbuwJISyfe1wJeGUENij9rd4NQiSknv9ceVG+MyNmkGT8Qu xuVSKsWBXdKV518eseJnJY2Xrf6TclNMOkKNx2pyhLA7hZogcEkZLlFAB8l3/JcKF2mHQWeSZIx6/ f+ZA4FQ+44woQTD4Zkbvqv4RgSy7zbKZXJmCQWWHIp8myLw9i95zIN+A79rtBC3rp0s01tpnZzuXi tRiCei1w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNdXV-0000000CJ9j-1h9A; Mon, 24 Nov 2025 20:52:01 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vNdXT-0000000CJ9E-1C8n for linux-arm-kernel@lists.infradead.org; Mon, 24 Nov 2025 20:52:00 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 9F18A44325; Mon, 24 Nov 2025 20:51:58 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7E166C16AAE; Mon, 24 Nov 2025 20:51:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764017518; bh=V+Uwt+ZdbJkBrqBbLMY3BQCOogr0mkRy+ckU06EArFU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=L02WjnJWfVbhAXiT/h0NFN+hEBPiubc2NS6O7J/ZHxdyXcS1jbjrfE+8/mkySFlWC +J1GyjhG4rnXHRJxtVcP+E+x5wH1wFw8JjpWj3CSwJnbww2zMaQrJth8PvqiFzyThv mte/qHteRbAC1XoXRdGk8DO/Vbx9zkUXEsOhNnqis4YOMsreRPRDuQLYZNomi0PS2d pvfgsmGDp43gT75M70iuE9R66srWy+7U2YGbiZFaY4pTZGvzflx+q2Tz+aOTxVHKJ3 13ZIb762wmHtP/QjIE/AY5p6Bq+j8B4q5pcIstUC6Szso82GawAH0AENekc5Ozk3wP axLDfP4QuBp+A== Date: Mon, 24 Nov 2025 12:51:58 -0800 From: Kees Cook To: Ryan Roberts Cc: Will Deacon , Arnd Bergmann , Ard Biesheuvel , Jeremy Linton , Catalin Marinas , Mark Rutland , "linux-arm-kernel@lists.infradead.org" , Linux Kernel Mailing List Subject: Re: [DISCUSSION] kstack offset randomization: bugs and performance Message-ID: <202511241250.EB2ADED@keescook> References: <66c4e2a0-c7fb-46c2-acce-8a040a71cd8e@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251124_125159_486320_493DFA9A X-CRM114-Status: GOOD ( 28.04 ) 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 Mon, Nov 24, 2025 at 05:50:14PM +0000, Ryan Roberts wrote: > 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) I thought we'd keep choose_random_kstack_offset() and just move everything into a per-task location? (And for arm64 only) -- Kees Cook