linux-kernel - Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

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: <20250521194819.GGaC4uA6mwlMBoqDMt@fat_crate.local>
Date: Wed, 21 May 2025 21:48:19 +0200
From: Borislav Petkov <bp@...en8.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>,
	"Ahmed S. Darwish" <darwi@...utronix.de>,
	Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v4 1/6] x86/cpu: Use a new feature flag for 5 level paging

On Wed, May 21, 2025 at 09:41:00PM +0200, Thomas Gleixner wrote:
> > Right, if I had a time machine, I would fly back and define this thing in
> > a whole different way: X86_FEATURE would be what the kernel has enabled and if
> > the bits completely or partially overlap with the definition of a respective
> > CPUID bit, so be it. But they would not parrot CPUID.
> >
> > IOW, I would completely decouple X86_FEATURE_ bits from CPUID and have all
> > in-kernel users check X86_FEATURE flags and not touch CPUID at all.
> >
> > But ok, in another life...
> 
> We can do that now.

Yeah, we will have to because of alternatives...

> The fact that user space can check CPUID and make uninformed assumptions
> about the kernel's view was a design fail in the first place. 

Yeah.

> Of course everyone assumed that nothing needs the kernel to be involved and
> all of this will magically support itself, but that was delusional as we all
> know.

Yeah, look at glibc doing CPUID and making decisions about what to use. Now
imagine kernel disables something under its feet. Boom.

> In the long run we really want to disable user space CPUID and emulate
> it when the hardware supports CPUID faulting, which should be made an
> architectural feature IMO.

Both vendors do support it. I probably should do the AMD side. It is about
time...

> Let me restrict this further:
> 
>     arch/x86/kernel/cpu/cpuid_eval.c::cpuid_eval() (or whatever name the
>     bikeshed painting debate agrees on) should be the only place which
>     can use the actual CPUID instruction.

Right.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette