Re: [PATCH v9 21/27] KVM: x86: Save and reload SSP to/from SMRAM

Chao Gao
2024-01-26T03:17:40Z

On Tue, Jan 23, 2024 at 06:41:54PM -0800, Yang Weijiang wrote:
>Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
>behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
>at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
>one of such registers on 64-bit Arch, and add the support for SSP. Note,
>on 32-bit Arch, SSP is not defined in SMRAM, so fail 32-bit CET guest
>launch.
>
>Suggested-by: Sean Christopherson <seanjc@google.com>
>Suggested-by: Chao Gao <chao.gao@intel.com>
>Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>---
> arch/x86/kvm/cpuid.c | 11 +++++++++++
> arch/x86/kvm/smm.c | 8 ++++++++
> arch/x86/kvm/smm.h | 2 +-
> 3 files changed, 20 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>index 3ab133530573..95233b0879a3 100644
>--- a/arch/x86/kvm/cpuid.c
>+++ b/arch/x86/kvm/cpuid.c
>@@ -149,6 +149,17 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
> if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
> return -EINVAL;
> }
>+ /*
>+ * Prevent 32-bit guest launch if shadow stack is exposed as SSP
>+ * state is not defined for 32-bit SMRAM.
>+ */
>+ best = cpuid_entry2_find(entries, nent, 0x80000001,
>+ KVM_CPUID_INDEX_NOT_SIGNIFICANT);
>+ if (best && !(best->edx & F(LM))) {
>+ best = cpuid_entry2_find(entries, nent, 0x7, 0);
>+ if (best && (best->ecx & F(SHSTK)))

F(LM) and F(SHSTK) are kernel-defined feature bits; they are not
bit masks defined by the CPU. You can use cpuid_entry_has() instead.

Shouldn't we do this only if SMM is supported, i.e., when

static_call(kvm_x86_has_emulated_msr)(kvm, MSR_IA32_SMBASE) is true.

>+ return -EINVAL;
>+ }

>
> /*
> * Exposing dynamic xfeatures to the guest requires additional
>diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
>index 45c855389ea7..7aac9c54c353 100644
>--- a/arch/x86/kvm/smm.c
>+++ b/arch/x86/kvm/smm.c
>@@ -275,6 +275,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
> enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
>
> smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
>+
>+ if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>+ KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
>+ vcpu->kvm);
> }
> #endif
>
>@@ -564,6 +568,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
> static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
> ctxt->interruptibility = (u8)smstate->int_shadow;
>
>+ if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>+ KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
>+ vcpu->kvm);
>+
> return X86EMUL_CONTINUE;
> }
> #endif
>diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
>index a1cf2ac5bd78..1e2a3e18207f 100644
>--- a/arch/x86/kvm/smm.h
>+++ b/arch/x86/kvm/smm.h
>@@ -116,8 +116,8 @@ struct kvm_smram_state_64 {
> u32 smbase;
> u32 reserved4[5];
>
>- /* ssp and svm_* fields below are not implemented by KVM */
> u64 ssp;
>+ /* svm_* fields below are not implemented by KVM */
> u64 svm_guest_pat;
> u64 svm_host_efer;
> u64 svm_host_cr4;
>--
>2.39.3
>