From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 13F572727EB for ; Tue, 25 Nov 2025 11:15:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764069306; cv=none; b=XFazd88zTQ7yupA4wgoIz/sgOtIhkrrqGffecMPhWCZU4R/BqkOq8b/WmUM7bzyUZbsTFu/svRbTitHBqhR5IYNjsYAQ3ajUpcsH8syB1lGNt868w0NVblVx8Ibp/kWAt6oFFqwqYmbfPmZsGJmz9ubQhatmmPb4MkCBeFrZK74= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764069306; c=relaxed/simple; bh=TcFM+KKhSy0dHBAWHwpEk3TR9vE3LN4wo8aOrgQ554M=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=He1NbXo4/lhkij5IENEEHgjEyeiJZOAggX1Zo4bbx1u+uabbImdAXgHOXN8a9C0PnpRjtVngdztWArdB+LB+/Lgwi8k3AEAMoAZtoNV3dsVtJ/n0wZcKtZ4eQamnt7wiA3Vlr7OleZ/6vjoYvayPDOxdESqxz5Tlm7tau/zLZMw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 8B2C21477; Tue, 25 Nov 2025 03:14:55 -0800 (PST) Received: from [10.1.30.209] (unknown [10.1.30.209]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8654D3F73B; Tue, 25 Nov 2025 03:15:01 -0800 (PST) Message-ID: Date: Tue, 25 Nov 2025 11:14:59 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [DISCUSSION] kstack offset randomization: bugs and performance Content-Language: en-GB To: Kees Cook Cc: Will Deacon , 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> <202511241250.EB2ADED@keescook> From: Ryan Roberts In-Reply-To: <202511241250.EB2ADED@keescook> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 24/11/2025 20:51, Kees Cook wrote: > 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) Err... I thought you were the one arguing against per-task state? I'm not really keen on having arm64 do a completely different thing to everyone else; It seems reasonable to me that we only need to (continue to) abstract the random source per-arch and the rest should remain common? Per my previous mails, I'm not really sure what choose_random_kstack_offset() is giving us in practice. Why not simplify?