linux-kernel - [RFC PATCH v3 5/8] KVM: SVM: Save shadow stack host state on VMRUN

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Message-ID: <20230817181820.15315-6-john.allen@amd.com>
Date:   Thu, 17 Aug 2023 18:18:17 +0000
From:   John Allen <john.allen@....com>
To:     <kvm@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <pbonzini@...hat.com>,
        <weijiang.yang@...el.com>, <rick.p.edgecombe@...el.com>,
        <seanjc@...gle.com>, <x86@...nel.org>, <thomas.lendacky@....com>,
        <bp@...en8.de>, John Allen <john.allen@....com>
Subject: [RFC PATCH v3 5/8] KVM: SVM: Save shadow stack host state on VMRUN

When running as an SEV-ES guest, the PL0_SSP, PL1_SSP, PL2_SSP, PL3_SSP,
and U_CET fields in the VMCB save area are type B, meaning the host
state is automatically loaded on a VMEXIT, but is not saved on a VMRUN.
The other shadow stack MSRs, S_CET, SSP, and ISST_ADDR are type A,
meaning they are loaded on VMEXIT and saved on VMRUN. PL0_SSP, PL1_SSP,
and PL2_SSP are currently unused. Manually save the other type B host
MSR values before VMRUN.

Signed-off-by: John Allen <john.allen@....com>
---
v3:
  - Don't save unused PL0_SSP, PL1_SSP, and PL2_SSP MSRs.
---
 arch/x86/kvm/svm/sev.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 2cd15783dfb9..021ead4dd201 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3097,6 +3097,15 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa)
 		hostsa->dr2_addr_mask = amd_get_dr_addr_mask(2);
 		hostsa->dr3_addr_mask = amd_get_dr_addr_mask(3);
 	}
+
+	if (boot_cpu_has(X86_FEATURE_SHSTK)) {
+		/*
+		 * MSR_IA32_U_CET and MSR_IA32_PL3_SSP are restored on VMEXIT,
+		 * save the current host values.
+		 */
+		rdmsrl(MSR_IA32_U_CET, hostsa->u_cet);
+		rdmsrl(MSR_IA32_PL3_SSP, hostsa->pl3_ssp);
+	}
 }
 
 void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
-- 
2.39.1