linux-kernel - Re: [PATCH AUTOSEL 5.10 2/6] x86/cpu: Don't clear X86_FEATURE_LAHF_LM flag in init_amd_k8() on AMD when running in a virtual machine

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: <20250418191224.GFaAKkGBnb01tGUVhW@fat_crate.local>
Date: Fri, 18 Apr 2025 21:12:24 +0200
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Pavel Machek <pavel@...x.de>, Sasha Levin <sashal@...nel.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Max Grobecker <max@...becker.info>, Ingo Molnar <mingo@...nel.org>,
	tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
	x86@...nel.org, thomas.lendacky@....com, perry.yuan@....com,
	mario.limonciello@....com, riel@...riel.com, mjguzik@...il.com,
	darwi@...utronix.de
Subject: Re: [PATCH AUTOSEL 5.10 2/6] x86/cpu: Don't clear
 X86_FEATURE_LAHF_LM flag in init_amd_k8() on AMD when running in a virtual
 machine

On Fri, Apr 18, 2025 at 11:31:27AM -0700, Sean Christopherson wrote:
> IMO, this is blatantly a QEMU bug (I verified the behavior when using "kvm64" on AMD).
> As per QEMU commit d1cd4bf419 ("introduce kvm64 CPU"), the vendor + FMS enumerates
> an Intel P4:
> 
>         .name = "kvm64",
>         .level = 0xd,
>         .vendor = CPUID_VENDOR_INTEL,
>         .family = 15,
>         .model = 6,
> 
> Per x86_cpu_load_model(), QEMU overrides the vendor when using KVM (at a glance,
> I can't find the code that actually overrides the vendor, gotta love QEMU's object
> model):

LOL, I thought I was the only one who thought this is madness. :-P

> 
>     /*
>      * vendor property is set here but then overloaded with the
>      * host cpu vendor for KVM and HVF.
>      */
>     object_property_set_str(OBJECT(cpu), "vendor", def->vendor, &error_abort);
> 
> Overriding the vendor but using Intel's P4 FMS is flat out wrong.  IMO, QEMU
> should use the same FMS as qemu64 for kvm64 when running on AMD.
> 
>         .name = "qemu64",
>         .level = 0xd,
>         .vendor = CPUID_VENDOR_AMD,
>         .family = 15,
>         .model = 107,
>         .stepping = 1,
> 
> Yeah, scraping FMS information is a bad idea, but what QEMU is doing is arguably
> far worse.

Ok, let's fix qemu. I don't have a clue, though, how to go about that so I'd
rely on your guidance here.

Because I really hate wagging the dog and "fixing" the kernel because something
else can't be bothered. I didn't object stronger to that fix because it is
meh, more of those "if I'm a guest" gunk which we sprinkle nowadays and that's
apparently not that awful-ish...

Thx.

-- 
Regards/Gruss,
    Boris.

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