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 854FB30DD18; Tue, 2 Dec 2025 09:39:44 +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=1764668386; cv=none; b=nsNnljWsaYkDXy70lh3AqXkoyb5wUkxxKvzGGj3Bj0VlF0VTWB2gez25gpiIYUM6QdSoyLpP109SZ42yYELid6VHQRwRYstY7sEGtboEI7hfUc6CMuImDQ1+hQSqvREyRsOUto7KeeCRohW4Y0BsAL10Ivujl7VE2xLBmgREJ9Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764668386; c=relaxed/simple; bh=4NOWD9mm3q4iy5ykB/OpQYkpUMPlP2yISDdr424kGfg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=C36f60wfStQWhYrtEaqpOTwMlPle3nqxr6egIt1sq8WUBZA7pvd7rWaR0P8Fh7Mx5ivaA3QbFmj/pLGtL7j1qrDxkCrIOXNnV77LYIc4K1dGKmSv3pHVExtq8caE+3CuYaebI7Spv6FR+7WG8QIguy7pNGCh6wz43Eq6XDTyCVo= 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 83C24153B; Tue, 2 Dec 2025 01:39:36 -0800 (PST) Received: from [10.57.87.167] (unknown [10.57.87.167]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 08C0A3F59E; Tue, 2 Dec 2025 01:39:41 -0800 (PST) Message-ID: <7da8d5cc-7b67-4f51-8f09-7ca401d06a3d@arm.com> Date: Tue, 2 Dec 2025 09:39:40 +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: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across arches Content-Language: en-GB To: Mark Rutland , Ard Biesheuvel Cc: 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 References: <20251127105958.2427758-1-ryan.roberts@arm.com> <20251127105958.2427758-3-ryan.roberts@arm.com> <9097505d-b18b-4f85-a02c-7f2865ad8bca@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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/ > > IIUC only x86 has noinstr validation, and from a quick scan, I expect > you see warnings from: > > * do_syscall_64() > * do_int80_syscall_32() > * __do_fast_syscall_32() > > For all of these, it is not safe to call instrumentable code before the > calls to {syscall_,}enter_from_user_mode{,_prepare}(). You'll need to > move the stack rnadomization after the existing instrumentation_begin() > calls. > > We'll need to go check the other architectures similarly. OK understood. I'll do it this way then. > > Mark.