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 8E9E230C343; Tue, 2 Dec 2025 09:53:14 +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=1764669196; cv=none; b=HP4bdiG2Qf0cSoGgwjQKgb+OgGlMVFpDD/E65nop1P3wG7Mm3wWIYN3QFXwUBZuymtfQUGseKsRFNDAPWKxP+AaccB5IJA5wsbxyO4ZaWLuCEH79jz3SzClATiv39sOfDxhIupEYeKrCHiEZndBHj1WmYbFK2N6/fsai2GZfuSg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764669196; c=relaxed/simple; bh=iP9xld7LkWPj8gLMi6glVawcS/VOgbtEdujgcuQDVK8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LmGwAEqKwi7ZD3LLwxni4PPkGB7q9OLBHIjkEDKE1HOAwDIum2mhNU3UE8TqNGz0fzjaf6hc9tB08gRrjD/YWP/SasIURQhFp5jPIseyadqYaLLVrXCnTvLEvcZfby/uCjHLveWRcY4vy/iU8al899U6ktIKoOgoUeCYD6AdY0A= 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 74072153B; Tue, 2 Dec 2025 01:53:06 -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 25B923F73B; Tue, 2 Dec 2025 01:53:12 -0800 (PST) Date: Tue, 2 Dec 2025 09:53:09 +0000 From: Mark Rutland To: Ryan Roberts Cc: Ard Biesheuvel , Kees Cook , Ard Biesheuvel , Will Deacon , Arnd Bergmann , Jeremy Linton , Catalin Marinas , "Jason A . Donenfeld" , linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across arches Message-ID: References: <20251127105958.2427758-1-ryan.roberts@arm.com> <20251127105958.2427758-3-ryan.roberts@arm.com> <9097505d-b18b-4f85-a02c-7f2865ad8bca@arm.com> <7da8d5cc-7b67-4f51-8f09-7ca401d06a3d@arm.com> Precedence: bulk X-Mailing-List: linux-hardening@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7da8d5cc-7b67-4f51-8f09-7ca401d06a3d@arm.com> On Tue, Dec 02, 2025 at 09:39:40AM +0000, Ryan Roberts wrote: > On 02/12/2025 09:35, Mark Rutland wrote: > > On Tue, Dec 02, 2025 at 10:15:22AM +0100, Ard Biesheuvel wrote: > >> On Mon, 1 Dec 2025 at 19:20, Ryan Roberts wrote: > >>> On 28/11/2025 11:01, Ard Biesheuvel wrote: > >>>> On Thu, 27 Nov 2025 at 12:00, Ryan Roberts wrote: > >>>>> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h > >>>>> index 089b1432f7e6..83c7e6710f6d 100644 > >>>>> --- a/include/linux/randomize_kstack.h > >>>>> +++ b/include/linux/randomize_kstack.h > >>>>> @@ -6,6 +6,7 @@ > >>>>> #include > >>>>> #include > >>>>> #include > >>>>> +#include > >>>>> > >>>>> DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, > >>>>> randomize_kstack_offset); > >>>>> @@ -45,9 +46,13 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, > >>>>> #define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100) > >>>>> #endif > >>>>> > >>>>> +static __always_inline u32 get_update_kstack_offset(void) > >>>>> +{ > >>>>> + return prandom_u32_state(¤t->kstack_rnd_state); > >>> > >>> I've got bot warnings because this is being called from noinstr code. I guess > >>> the best option is to just move add_random_kstack_offset() to after > >>> instrumentation is enabled for the affected arches. > >> > >> Just put instrumentation_begin()/instrumentation_end() around the call > >> to prandom_u32_state() - that seems to be the common approach for > >> punching holes into the 'noinstr' validation. > > > > That silences the warning, but isn't necessarily safe, so please DO NOT > > do that blindly. The instrumentation_{begin,end}() annotations are only > > supposed to be used when we know by construction that instrumentation is > > safe. > > > > Generally, if you can move this to after instrumentation is already > > enabled, that should be safe, and so that'd be the better approach. > > > > Ryan, can you share those warnings (e.g. link to those reports)? > > https://lore.kernel.org/oe-kbuild-all/202511302137.u4iIA2kf-lkp@intel.com/ > https://lore.kernel.org/oe-kbuild-all/202511302317.BGFIlAYX-lkp@intel.com/ Cool, so those are __do_fast_syscall_32() and do_syscall_64(), which I'd expect (and those are legitimate warnings). >From a quick scan of v6.18, it looks like arm64, loongarch, powerpc, and riscv perform all the necessary entry work before calling add_random_kstack_offset(), and we'll need to change: * s390's __do_syscall() * x86's do_syscall_64() * x86's do_int80_syscall_32() * x86's __do_fast_syscall_32() Mark.