linux-kernel - Re: [PATCH v3 08/10] x86/fpu/xstate: Add CET supervisor xfeature support

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: <74e49413-dd1a-4577-818f-b5b21b2a2b7e@intel.com>
Date: Fri, 7 Mar 2025 10:39:47 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Chao Gao <chao.gao@...el.com>, tglx@...utronix.de, x86@...nel.org,
 seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: peterz@...radead.org, rick.p.edgecombe@...el.com,
 weijiang.yang@...el.com, john.allen@....com, bp@...en8.de,
 Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH v3 08/10] x86/fpu/xstate: Add CET supervisor xfeature
 support

On 3/7/25 08:41, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@...el.com>
> 
> To support CET virtualization, KVM needs the kernel to save/restore CET
> supervisor xstate in guest FPUs when switching between guest and host
> FPUs.
> 
> Add CET supervisor xstate (i.e., XFEATURE_CET_KERNEL) support. Both the
> guest FPU and the kernel FPU will allocate memory for the new xstate.
> For the guest FPU, the xstate remains unused until the upcoming CET
> virtualization is added to KVM. For the kernel FPU, the xstate is unused
> until CET_S is enabled within the kernel.
> 
> Note CET_S may or may not be enabled within the kernel, so always
> allocating memory for XFEATURE_CET_KERNEL could potentially waste some
> XSAVE buffer space. If necessary, this issue can be addressed by making
> XFEATURE_CET_KERNEL a guest-only feature.

I feel like these changelogs are long but say very little.

This patch *WASTES* resources. Granted, it's only for a single patch,
but it's totally not obvious.

Could you work on tightening down the changelog, please?

> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -55,7 +55,7 @@ static const char *xfeature_names[] =
>  	"Protection Keys User registers",
>  	"PASID state",
>  	"Control-flow User registers",
> -	"Control-flow Kernel registers (unused)",
> +	"Control-flow Kernel registers",

This should probably be:

> +	"Control-flow Kernel registers (KVM only)",

or something similar for now. XFEATURE_CET_KERNEL is *VERY* different
from all of the other features and it's silly to pretend that it's the same.