linux-kernel - Re: [PATCH 3/3] x86: Disable running 32bit processes if ia32

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Message-ID: <42a342ad-d019-c6df-6414-995bb32a3ef7@suse.com>
Date:   Fri, 9 Jun 2023 00:29:46 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, mhocko@...e.com, jslaby@...e.cz
Subject: Re: [PATCH 3/3] x86: Disable running 32bit processes if ia32_disabled
 is passed



On 8.06.23 г. 14:25 ч., Andrew Cooper wrote:
> On 08/06/2023 1:25 am, Thomas Gleixner wrote:
>> On Thu, Jun 08 2023 at 00:43, Andrew Cooper wrote:
>>> And yes, wiring this into SIGSEGV/etc would be a sensible move.
>> The only option is to wire it into die_hard_crash_and_burn(). There is
>> no sane way to deliver a signal to the process which managed to run into
>> this. Appropriate info to parent or ptracer will still be delivered.
> 
> Hmm yeah.  Something has already gone seriously wrong to end up here, so
> terminating it is probably best.
> 
>> I really wish that we could disable syscall32 reliably on AMD and make
>> it raise #UD as it does on Intal.
> 
> You know that's changing in FRED, and will follow the AMD model?
> 
> The SYSCALL instruction is lower latency if it doesn't need to check %cs
> to conditionally #UD.
> 
>>> Furthermore, despite CET-SS explicitly trying to account for and protect
>>> against accidental mismatches, there are errata in some parts which let
>>> userspace forge legal return addresses on the shadow stack by dropping
>>> into 32bit mode because, there's a #GP check missing in a microflow.
>> Didn't we assume that there are no CPU bugs? :)
> 
> Tell me, is such a CPU delivered with or without a complimentary unicorn? :)
> 
>>> For usecases where there ought not to be any 32bit code at all (and
>>> there absolutely are), it would be lovely if this could be enforced,
>>> rather than relying on blind hope that it doesn't happen.
>> I think it's rather clear what needs to be done here to achieve that,
>> but that's completely orthogonal to the intent of the patch series in
>> question which aims to make CONFIG_IA32_EMULATION a boot time decision.
> 
> Fully inhibiting 32bit mode is stronger, because it impacts code which
> would otherwise operate in CONFIG_IA32_EMULATION=n configuration.
> 
> Which is fine, but I agree that it doesn't want to be confused with
> being a "runtime CONFIG_IA32_EMULATION" knob.
> 
> If the runtime form could also come with an equivalent Kconfig form,
> that would be awesome too.


Actually that's something I'm working on. I.e be able to set the default 
state of IA32_EMULATION at compile time (i.e disabled or enabled) and 
provide a boottime switch to override this. That way upstream can retain 
the current behavior, while distros can set the "default disabled" 
config-time switch and finally users will have the ability to override 
the config-switch at boottime.


> 
> ~Andrew