linux-kernel - [PATCH v2 0/3] KVM: x86: CET vs. nVMX fix and hardening

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Message-ID: <20260128014310.3255561-1-seanjc@google.com>
Date: Tue, 27 Jan 2026 17:43:07 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Jim Mattson <jmattson@...gle.com>
Subject: [PATCH v2 0/3] KVM: x86: CET vs. nVMX fix and hardening

Fix a bug where KVM will clear IBT and SHSTK bits after nested VMX MSRs
have been configured, e.g. if the kernel is built with CONFIG_X86_CET=y
but CONFIG_X86_KERNEL_IBT=n.  The late clearing results in kvm-intel.ko
refusing to load as the CPU compatible checks generate their VMCS configs
with IBT=n and SHSTK=n, ultimately causing a mismatch on the CET entry
and exit controls.

Patch 2 hardens against similar bugs in the future by adding a flag and
WARNs to yell if KVM sets or clear feature flags outside of the dedicated
flow.

Patch 3 adds (very, very) long overdue printing of the mistmatching offsets
in the VMCS configs.

Chao, I didn't include any of your Reviewed-by's, as every patch changed
quite a bit from v1.

v2:
 - Isolate kvm_setup_xss_caps() from kvm_finalize_cpu_caps(). [Xiaoyao]
 - Fix the pr_cont() printing. [Chao]

v1: https://lore.kernel.org/all/20260123221542.2498217-1-seanjc@google.com

Sean Christopherson (3):
  KVM: x86: Explicitly configure supported XSS from
    {svm,vmx}_set_cpu_caps()
  KVM: x86: Harden against unexpected adjustments to kvm_cpu_caps
  KVM: VMX: Print out "bad" offsets+value on VMCS config mismatch

 arch/x86/kvm/cpuid.c   | 10 ++++++++--
 arch/x86/kvm/cpuid.h   | 12 +++++++++++-
 arch/x86/kvm/svm/svm.c |  6 +++++-
 arch/x86/kvm/vmx/vmx.c | 23 +++++++++++++++++++++--
 arch/x86/kvm/x86.c     | 30 +++++++++++++++++-------------
 arch/x86/kvm/x86.h     |  2 ++
 6 files changed, 64 insertions(+), 19 deletions(-)

base-commit: e81f7c908e1664233974b9f20beead78cde6343a
-- 
2.52.0.457.g6b5491de43-goog