linux-kernel - Re: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across arches

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <684115c9-4ca2-485e-94a4-706d075f3f6e@arm.com>
Date: Tue, 2 Dec 2025 11:04:09 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Mark Rutland <mark.rutland@....com>
Cc: Ard Biesheuvel <ardb@...nel.org>, Kees Cook <kees@...nel.org>,
 Ard Biesheuvel <ardb+git@...gle.com>, Will Deacon <will@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Jeremy Linton <jeremy.linton@....com>,
 Catalin Marinas <Catalin.Marinas@....com>,
 "Jason A . Donenfeld" <Jason@...c4.com>, linux-hardening@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 2/2] randomize_kstack: Unify random source across
 arches

On 02/12/2025 09:53, Mark Rutland wrote:
> 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 <ryan.roberts@....com> wrote:
>>>>> On 28/11/2025 11:01, Ard Biesheuvel wrote:
>>>>>> On Thu, 27 Nov 2025 at 12:00, Ryan Roberts <ryan.roberts@....com> 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 <linux/kernel.h>
>>>>>>>  #include <linux/jump_label.h>
>>>>>>>  #include <linux/percpu-defs.h>
>>>>>>> +#include <linux/prandom.h>
>>>>>>>
>>>>>>>  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(&current->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()

Yup. I'll have a play with inline vs outline. If I go with outline, I'll fix
these sites up.

> 
> Mark.