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

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Message-ID: <87a5xbjpk2.ffs@tglx>
Date:   Wed, 07 Jun 2023 16:49:01 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nikolay Borisov <nik.borisov@...e.com>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, mhocko@...e.com, jslaby@...e.cz,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 3/3] x86: Disable running 32bit processes if
 ia32_disabled is passed

On Wed, Jun 07 2023 at 16:38, Nikolay Borisov wrote:
> On 7.06.23 г. 15:53 ч., Thomas Gleixner wrote:
>> So why would boottime disabling of IA32_EMULATION affect X32_ABI in any
>> way?
>
> In this case it shouldn't affect it and the check should be
>
> ((elf_check_arch_ia32(x) && !ia32_disabled) || 
> (IS_ENABLED(CONFIG_X86_X32_ABI) && (x)->e_machine == EM_X86_64)).

Correct.

>>    1) What is the justification for setting the 'present' bit of
>>       GDT_ENTRY_DEFAULT_USER32_CS to 0?
>
> This was something which was suggested by Andrew Cooper on irc, to my 
> understanding the idea is that by not having a 32bit capable descriptor 
> it's impossible to run a 32bit code.

Right, but that's a completely separate change. If it is agreed on then
it needs to be consistent and not depend on this command line parameter.

> I guess the scenario where it might be relevant if someone starts a
> 64bit process and with inline assembly tries to run 32bit code
> somehow, though it might be a far fetched example and also the fact
> that the compat_elf_check_arch() forbids loading 32bit processes might
> be sufficient.

Guesswork is not helpful. Facts matter.

Fact is that today a 64bit application can do a far jump of far call
into a 32bit code segment based on the default descriptors or by setting
them up via LDT. That 32bit code obviously can't do compat syscalls if
IA32_EMULATION is disabled, but other than that it just works.

Whether that makes sense or not is a completely different question.

Ergo fact is that clearing the present bit is a user space visible
change, which can't be done nilly willy and burried into a patch
which is about making CONFIG_IA32_EMULATION a boot time switch.

Thanks,

        tglx