Re: [PATCH v16 02/51] KVM: SEV: Read save fields from GHCB exactly once

[Tom Lendacky <thomas.lendacky@amd.com>]

On 9/19/25 17:32, Sean Christopherson wrote:
> Wrap all reads of GHCB save fields with READ_ONCE() via a KVM-specific
> GHCB get() utility to help guard against TOCTOU bugs.  Using READ_ONCE()
> doesn't completely prevent such bugs, e.g. doesn't prevent KVM from
> redoing get() after checking the initial value, but at least addresses
> all potential TOCTOU issues in the current KVM code base.
> 
> To prevent unintentional use of the generic helpers, take only @svm for
> the kvm_ghcb_get_xxx() helpers and retrieve the ghcb instead of explicitly
> passing it in.
> 
> Opportunistically reduce the indentation of the macro-defined helpers and
> clean up the alignment.
> 
> Fixes: 4e15a0ddc3ff ("KVM: SEV: snapshot the GHCB before accessing it")
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  arch/x86/kvm/svm/sev.c | 22 +++++++++++-----------
>  arch/x86/kvm/svm/svm.h | 25 +++++++++++++++----------
>  2 files changed, 26 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index f046a587ecaf..8d057dbd8a71 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3343,26 +3343,26 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
>  	BUILD_BUG_ON(sizeof(svm->sev_es.valid_bitmap) != sizeof(ghcb->save.valid_bitmap));
>  	memcpy(&svm->sev_es.valid_bitmap, &ghcb->save.valid_bitmap, sizeof(ghcb->save.valid_bitmap));
>  
> -	vcpu->arch.regs[VCPU_REGS_RAX] = kvm_ghcb_get_rax_if_valid(svm, ghcb);
> -	vcpu->arch.regs[VCPU_REGS_RBX] = kvm_ghcb_get_rbx_if_valid(svm, ghcb);
> -	vcpu->arch.regs[VCPU_REGS_RCX] = kvm_ghcb_get_rcx_if_valid(svm, ghcb);
> -	vcpu->arch.regs[VCPU_REGS_RDX] = kvm_ghcb_get_rdx_if_valid(svm, ghcb);
> -	vcpu->arch.regs[VCPU_REGS_RSI] = kvm_ghcb_get_rsi_if_valid(svm, ghcb);
> +	vcpu->arch.regs[VCPU_REGS_RAX] = kvm_ghcb_get_rax_if_valid(svm);
> +	vcpu->arch.regs[VCPU_REGS_RBX] = kvm_ghcb_get_rbx_if_valid(svm);
> +	vcpu->arch.regs[VCPU_REGS_RCX] = kvm_ghcb_get_rcx_if_valid(svm);
> +	vcpu->arch.regs[VCPU_REGS_RDX] = kvm_ghcb_get_rdx_if_valid(svm);
> +	vcpu->arch.regs[VCPU_REGS_RSI] = kvm_ghcb_get_rsi_if_valid(svm);
>  
> -	svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm, ghcb);
> +	svm->vmcb->save.cpl = kvm_ghcb_get_cpl_if_valid(svm);
>  
>  	if (kvm_ghcb_xcr0_is_valid(svm)) {
> -		vcpu->arch.xcr0 = ghcb_get_xcr0(ghcb);
> +		vcpu->arch.xcr0 = kvm_ghcb_get_xcr0(svm);
>  		vcpu->arch.cpuid_dynamic_bits_dirty = true;
>  	}
>  
>  	/* Copy the GHCB exit information into the VMCB fields */
> -	exit_code = ghcb_get_sw_exit_code(ghcb);
> +	exit_code = kvm_ghcb_get_sw_exit_code(svm);
>  	control->exit_code = lower_32_bits(exit_code);
>  	control->exit_code_hi = upper_32_bits(exit_code);
> -	control->exit_info_1 = ghcb_get_sw_exit_info_1(ghcb);
> -	control->exit_info_2 = ghcb_get_sw_exit_info_2(ghcb);
> -	svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm, ghcb);
> +	control->exit_info_1 = kvm_ghcb_get_sw_exit_info_1(svm);
> +	control->exit_info_2 = kvm_ghcb_get_sw_exit_info_2(svm);
> +	svm->sev_es.sw_scratch = kvm_ghcb_get_sw_scratch_if_valid(svm);
>  
>  	/* Clear the valid entries fields */
>  	memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 5d39c0b17988..5365984e82e5 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -913,16 +913,21 @@ void __svm_sev_es_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted,
>  void __svm_vcpu_run(struct vcpu_svm *svm, bool spec_ctrl_intercepted);
>  
>  #define DEFINE_KVM_GHCB_ACCESSORS(field)						\
> -	static __always_inline bool kvm_ghcb_##field##_is_valid(const struct vcpu_svm *svm) \
> -	{									\
> -		return test_bit(GHCB_BITMAP_IDX(field),				\
> -				(unsigned long *)&svm->sev_es.valid_bitmap);	\
> -	}									\
> -										\
> -	static __always_inline u64 kvm_ghcb_get_##field##_if_valid(struct vcpu_svm *svm, struct ghcb *ghcb) \
> -	{									\
> -		return kvm_ghcb_##field##_is_valid(svm) ? ghcb->save.field : 0;	\
> -	}									\
> +static __always_inline u64 kvm_ghcb_get_##field(struct vcpu_svm *svm)			\
> +{											\
> +	return READ_ONCE(svm->sev_es.ghcb->save.field);					\
> +}											\
> +											\
> +static __always_inline bool kvm_ghcb_##field##_is_valid(const struct vcpu_svm *svm)	\
> +{											\
> +	return test_bit(GHCB_BITMAP_IDX(field),						\
> +			(unsigned long *)&svm->sev_es.valid_bitmap);			\
> +}											\
> +											\
> +static __always_inline u64 kvm_ghcb_get_##field##_if_valid(struct vcpu_svm *svm)	\
> +{											\
> +	return kvm_ghcb_##field##_is_valid(svm) ? kvm_ghcb_get_##field(svm) : 0;	\
> +}
>  
>  DEFINE_KVM_GHCB_ACCESSORS(cpl)
>  DEFINE_KVM_GHCB_ACCESSORS(rax)