linux-kernel - Re: [PATCH v2] KVM: VMX: Make CR4.CET a guest owned bit

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Message-ID: <aICAEVrblWxL9cv5@intel.com>
Date: Wed, 23 Jul 2025 14:24:17 +0800
From: Chao Gao <chao.gao@...el.com>
To: Mathias Krause <minipli@...ecurity.net>
CC: Sean Christopherson <seanjc@...gle.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <x86@...nel.org>, <pbonzini@...hat.com>,
	<dave.hansen@...el.com>, <rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>,
	<john.allen@....com>, <weijiang.yang@...el.com>, <xin@...or.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Borislav
 Petkov" <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter
 Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] KVM: VMX: Make CR4.CET a guest owned bit


It is recommended to first state what a patch does before providing
the background and motivation. See

https://docs.kernel.org/process/maintainer-kvm-x86.html#changelog

>There's no need to intercept changes to CR4.CET, as it's neither
>included in KVM's MMU role bits, nor does KVM specifically care about
>the actual value of a (nested) guest's CR4.CET value, beside for
>enforcing architectural constraints, i.e. make sure that CR0.WP=1 if
>CR4.CET=1.
>
>Intercepting writes to CR4.CET is particularly bad for grsecurity
>kernels with KERNEXEC or, even worse, KERNSEAL enabled. These features
>heavily make use of read-only kernel objects and use a cpu-local CR0.WP
>toggle to override it, when needed. Under a CET-enabled kernel, this
>also requires toggling CR4.CET, hence the motivation to make it
>guest-owned.
>
>Using the old test from [1] gives the following runtime numbers (perf
>stat -r 5 ssdd 10 50000):
>
>* grsec guest on linux-6.16-rc5 + cet patches:
>  2.4647 +- 0.0706 seconds time elapsed  ( +-  2.86% )
>
>* grsec guest on linux-6.16-rc5 + cet patches + CR4.CET guest-owned:
>  1.5648 +- 0.0240 seconds time elapsed  ( +-  1.53% )
>
>Not only makes not intercepting CR4.CET the test run ~35% faster, it's
>also more stable, less fluctuation due to less VMEXITs, I believe.
>
>Therefore, make CR4.CET a guest-owned bit where possible.
>
>This change is VMX-specific, as SVM has no such fine-grained control
>register intercept control.

Ah, that's why the shortlog is "KVM: VMX". I was wondering why the shortlog
specifically mentions VMX while the patch actually touches x86 common code.

>
>If KVM's assumptions regarding MMU role handling wrt. a guest's CR4.CET
>value ever change, the BUILD_BUG_ON()s related to KVM_MMU_CR4_ROLE_BITS
>and KVM_POSSIBLE_CR4_GUEST_BITS will catch that early.
>
>Link: https://lore.kernel.org/kvm/20230322013731.102955-1-minipli@grsecurity.net/ [1]
>Signed-off-by: Mathias Krause <minipli@...ecurity.net>

The patch looks good. So,

Reviewed-by: Chao Gao <chao.gao@...el.com>