linux-kernel - Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for host and guest FPUs

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: <aA-QqaKo825uIuW7@google.com>
Date: Mon, 28 Apr 2025 07:28:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"ebiggers@...gle.com" <ebiggers@...gle.com>, Dave Hansen <dave.hansen@...el.com>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, Stanislav Spassov <stanspas@...zon.de>, 
	"levymitchell0@...il.com" <levymitchell0@...il.com>, 
	"samuel.holland@...ive.com" <samuel.holland@...ive.com>, Xin3 Li <xin3.li@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Weijiang Yang <weijiang.yang@...el.com>, 
	"mingo@...hat.com" <mingo@...hat.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"tglx@...utronix.de" <tglx@...utronix.de>, "john.allen@....com" <john.allen@....com>, 
	"mlevitsk@...hat.com" <mlevitsk@...hat.com>, Chang Seok Bae <chang.seok.bae@...el.com>, 
	"vigbalas@....com" <vigbalas@....com>, "peterz@...radead.org" <peterz@...radead.org>, "hpa@...or.com" <hpa@...or.com>, 
	"bp@...en8.de" <bp@...en8.de>, 
	"aruna.ramakrishna@...cle.com" <aruna.ramakrishna@...cle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v5 3/7] x86/fpu/xstate: Differentiate default features for
 host and guest FPUs

On Mon, Apr 28, 2025, Chao Gao wrote:
> >I assume the kernel has bigger problems if CET_S is somehow tied to a
> >userspace task.
> 
> To be clear, CET_S here refers to the CET supervisor state, which includes SSP
> pointers for privilege levels 0 through 2. The IA32_S_CET MSR is not part of
> that state.
> 
> >
> >For KVM, it's just the one MSR, and KVM needs to support save/restore of that MSR
> >no matter what, 

Oh, it's not just one MSR.  I was indeed thinking this was just IA32_S_CET.  But
lucky for me, the statement holds for SSP0-SS2.

> so supporting it via XSAVE would be more work, a bit sketchy, and
> >create yet another way for userspace to do weird things when saving/restoring vCPU
> >state.
> 
> Agreed. One more issue of including CET_S into KVM_GET/SET_XSAVE{2} is:
> 
> XSAVE UABI buffers adhere to the standard format defined by the SDM, which
> never includes supervisor states. Attempting to incorporate supervisor states
> into UABI buffers would lead to many issues, such as deviating from the
> standard format and the need to define offsets for each supervisor state.