linux-kernel - Re: [PATCH v16 23/51] KVM: x86: Allow setting CR4.CET if IBT or SHSTK is supported

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Message-ID: <ed332d81-9e22-41e3-95cb-9bebca1b00f6@linux.intel.com>
Date: Mon, 22 Sep 2025 15:25:04 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>,
 Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>,
 Xiaoyao Li <xiaoyao.li@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>,
 Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: Re: [PATCH v16 23/51] KVM: x86: Allow setting CR4.CET if IBT or SHSTK
 is supported



On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> From: Yang Weijiang <weijiang.yang@...el.com>
>
> Drop X86_CR4_CET from CR4_RESERVED_BITS and instead mark CET as reserved
> if and only if IBT *and* SHSTK are unsupported, i.e. allow CR4.CET to be
> set if IBT or SHSTK is supported.  This creates a virtualization hole if
> the CPU supports both IBT and SHSTK, but the kernel or vCPU model only
> supports one of the features.  However, it's entirely legal for a CPU to
> have only one of IBT or SHSTK, i.e. the hole is a flaw in the architecture,
> not in KVM.
>
> More importantly, so long as KVM is careful to initialize and context
> switch both IBT and SHSTK state (when supported in hardware) if either
> feature is exposed to the guest, a misbehaving guest can only harm itself.
> E.g. VMX initializes host CET VMCS fields based solely on hardware
> capabilities.
>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Signed-off-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>
> [sean: split to separate patch, write changelog]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>

> ---
>   arch/x86/include/asm/kvm_host.h | 2 +-
>   arch/x86/kvm/x86.h              | 3 +++
>   2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 554d83ff6135..39231da3a3ff 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -142,7 +142,7 @@
>   			  | X86_CR4_OSXSAVE | X86_CR4_SMEP | X86_CR4_FSGSBASE \
>   			  | X86_CR4_OSXMMEXCPT | X86_CR4_LA57 | X86_CR4_VMXE \
>   			  | X86_CR4_SMAP | X86_CR4_PKE | X86_CR4_UMIP \
> -			  | X86_CR4_LAM_SUP))
> +			  | X86_CR4_LAM_SUP | X86_CR4_CET))
>   
>   #define CR8_RESERVED_BITS (~(unsigned long)X86_CR8_TPR)
>   
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 65cbd454c4f1..f3dc77f006f9 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -680,6 +680,9 @@ static inline bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>   		__reserved_bits |= X86_CR4_PCIDE;       \
>   	if (!__cpu_has(__c, X86_FEATURE_LAM))           \
>   		__reserved_bits |= X86_CR4_LAM_SUP;     \
> +	if (!__cpu_has(__c, X86_FEATURE_SHSTK) &&       \
> +	    !__cpu_has(__c, X86_FEATURE_IBT))           \
> +		__reserved_bits |= X86_CR4_CET;         \
>   	__reserved_bits;                                \
>   })
>