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 737D6CFD2F6 for ; Thu, 27 Nov 2025 16:59:05 +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=OYxHUrhvUcgT5oBRsyiThgUc5t2jF5qQX768Aozw98Y=; b=3aFDYE/EaOlDwCfKKTxrz/2ry4 tjkTgK2rTVlXKaT6XCsc9W4OuvOVeHGwYCro5ZbydXrZzPjtKb3CMMr36kK4292Zf1+kU/AnrcF6j gM6Uf5TWe1yjFYpeMca0rHc7i+MnJy10fpciZfvHeEpXMtWxib2Ndax0Htwyvks+LlGEIgBbo3hTj iNGMnfR/6QdE9M58i6kSYQSj6Cmy9hdgqhYHxGB64FVUfHuJdQWQQq5pQ3Wm2KZi5oKzlm0gzwsin iv8bWjtevhB4xiCi6yhUG6YhHDj/JeYlT94Zud4DfrfvbVbtQspfmm4t7PiJhmG8KufoplVwaoXyh CJw2AARg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vOfKe-0000000Gx53-0Mew; Thu, 27 Nov 2025 16:59:00 +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 1vOfKc-0000000Gx4T-0Bsy for linux-arm-kernel@lists.infradead.org; Thu, 27 Nov 2025 16:58:59 +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 55F47176A; Thu, 27 Nov 2025 08:58:48 -0800 (PST) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8B8CE3F66E; Thu, 27 Nov 2025 08:58:53 -0800 (PST) Date: Thu, 27 Nov 2025 16:58:50 +0000 From: Mark Rutland To: Ryan Roberts Cc: Ard Biesheuvel , Ard Biesheuvel , linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Kees Cook , Will Deacon , Arnd Bergmann , Jeremy Linton , Catalin Marinas , "Jason A. Donenfeld" Subject: Re: [RFC/RFT PATCH 0/6] Improve get_random_u8() for use in randomize kstack Message-ID: References: <20251127092226.1439196-8-ardb+git@google.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-20251127_085858_126863_88F04827 X-CRM114-Status: GOOD ( 20.27 ) 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 Thu, Nov 27, 2025 at 03:56:59PM +0000, Ryan Roberts wrote: > On 27/11/2025 15:03, Ard Biesheuvel wrote: > > So the question is really whether we want to dedicate 16 bytes per > > task for this. I wouldn't mind personally, but it is something our > > internal QA engineers tend to obsess over. > > Yeah that's a good point. I think it's a fair point that some people will obsesses over this, but I think the concern is misplaced. I know that people were very happy for the kernel FPSIMD context to disappear from task_struct, but 16 bytes is a fair amount smaller, and I'm pretty sure we can offset that with a small/moderate amount of work. AFAICT there are extant holes in task_struct that could easily account for 16 bytes. I can also see a few ways to rework arm64's thread_info and thread_struct (which are both embedded within task_struct) to save some space. > Is this something we could potentially keep at the start of the > kstack? Is there any precident for keeping state there at the moment? > For arm64, I know there is a general feeling that 16K for the stack > more than enough (but we are stuck with it because 8K isn't quite > enough). So it would be "for free". I guess it would be tricky to do > this in an arch-agnostic way though... We went out of our way to stop playing silly games like that when we moved thread_info into task_struct; please let's not bring that back. Mark.