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Message-ID: <20231230172351.574091-11-michael.roth@amd.com>
Date: Sat, 30 Dec 2023 11:23:26 -0600
From: Michael Roth <michael.roth@....com>
To: <kvm@...r.kernel.org>
CC: <linux-coco@...ts.linux.dev>, <linux-mm@...ck.org>,
	<linux-crypto@...r.kernel.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<jroedel@...e.de>, <thomas.lendacky@....com>, <hpa@...or.com>,
	<ardb@...nel.org>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<vkuznets@...hat.com>, <jmattson@...gle.com>, <luto@...nel.org>,
	<dave.hansen@...ux.intel.com>, <slp@...hat.com>, <pgonda@...gle.com>,
	<peterz@...radead.org>, <srinivas.pandruvada@...ux.intel.com>,
	<rientjes@...gle.com>, <dovmurik@...ux.ibm.com>, <tobin@....com>,
	<bp@...en8.de>, <vbabka@...e.cz>, <kirill@...temov.name>,
	<ak@...ux.intel.com>, <tony.luck@...el.com>,
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	<jarkko@...nel.org>, <ashish.kalra@....com>, <nikunj.dadhania@....com>,
	<pankaj.gupta@....com>, <liam.merwick@...cle.com>, <zhi.a.wang@...el.com>,
	Alexey Kardashevskiy <aik@....com>
Subject: [PATCH v11 10/35] KVM: SEV: Do not intercept accesses to MSR_IA32_XSS for SEV-ES guests

When intercepts are enabled for MSR_IA32_XSS, the host will swap in/out
the guest-defined values while context-switching to/from guest mode.
However, in the case of SEV-ES, vcpu->arch.guest_state_protected is set,
so the guest-defined value is effectively ignored when switching to
guest mode with the understanding that the VMSA will handle swapping
in/out this register state.

However, SVM is still configured to intercept these accesses for SEV-ES
guests, so the values in the initial MSR_IA32_XSS are effectively
read-only, and a guest will experience undefined behavior if it actually
tries to write to this MSR. Fortunately, only CET/shadowstack makes use
of this register on SEV-ES-capable systems currently, which isn't yet
widely used, but this may become more of an issue in the future.

Additionally, enabling intercepts of MSR_IA32_XSS results in #VC
exceptions in the guest in certain paths that can lead to unexpected #VC
nesting levels. One example is SEV-SNP guests when handling #VC
exceptions for CPUID instructions involving leaf 0xD, subleaf 0x1, since
they will access MSR_IA32_XSS as part of servicing the CPUID #VC, then
generate another #VC when accessing MSR_IA32_XSS, which can lead to
guest crashes if an NMI occurs at that point in time. Running perf on a
guest while it is issuing such a sequence is one example where these can
be problematic.

Address this by disabling intercepts of MSR_IA32_XSS for SEV-ES guests
if the host/guest configuration allows it. If the host/guest
configuration doesn't allow for MSR_IA32_XSS, leave it intercepted so
that it can be caught by the existing checks in
kvm_{set,get}_msr_common() if the guest still attempts to access it.

Fixes: 376c6d285017 ("KVM: SVM: Provide support for SEV-ES vCPU creation/loading")
Cc: Alexey Kardashevskiy <aik@....com>
Suggested-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Michael Roth <michael.roth@....com>
---
 arch/x86/kvm/svm/sev.c | 19 +++++++++++++++++++
 arch/x86/kvm/svm/svm.c |  1 +
 arch/x86/kvm/svm/svm.h |  2 +-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 2efe3ed89808..f99435b6648f 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2978,6 +2978,25 @@ static void sev_es_vcpu_after_set_cpuid(struct vcpu_svm *svm)
 
 		set_msr_interception(vcpu, svm->msrpm, MSR_TSC_AUX, v_tsc_aux, v_tsc_aux);
 	}
+
+	/*
+	 * For SEV-ES, accesses to MSR_IA32_XSS should not be intercepted if
+	 * the host/guest supports its use.
+	 *
+	 * guest_can_use() checks a number of requirements on the host/guest to
+	 * ensure that MSR_IA32_XSS is available, but it might report true even
+	 * if X86_FEATURE_XSAVES isn't configured in the guest to ensure host
+	 * MSR_IA32_XSS is always properly restored. For SEV-ES, it is better
+	 * to further check that the guest CPUID actually supports
+	 * X86_FEATURE_XSAVES so that accesses to MSR_IA32_XSS by misbehaved
+	 * guests will still get intercepted and caught in the normal
+	 * kvm_emulate_rdmsr()/kvm_emulated_wrmsr() paths.
+	 */
+	if (guest_can_use(vcpu, X86_FEATURE_XSAVES) &&
+	    guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
+		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_XSS, 1, 1);
+	else
+		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_XSS, 0, 0);
 }
 
 void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm)
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index b6179696861a..18d55df7fa5f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -103,6 +103,7 @@ static const struct svm_direct_access_msrs {
 	{ .index = MSR_IA32_LASTBRANCHTOIP,		.always = false },
 	{ .index = MSR_IA32_LASTINTFROMIP,		.always = false },
 	{ .index = MSR_IA32_LASTINTTOIP,		.always = false },
+	{ .index = MSR_IA32_XSS,			.always = false },
 	{ .index = MSR_EFER,				.always = false },
 	{ .index = MSR_IA32_CR_PAT,			.always = false },
 	{ .index = MSR_AMD64_SEV_ES_GHCB,		.always = true  },
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 9ed9d72546b3..7f1fbd874c45 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -30,7 +30,7 @@
 #define	IOPM_SIZE PAGE_SIZE * 3
 #define	MSRPM_SIZE PAGE_SIZE * 2
 
-#define MAX_DIRECT_ACCESS_MSRS	46
+#define MAX_DIRECT_ACCESS_MSRS	47
 #define MSRPM_OFFSETS	32
 extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
 extern bool npt_enabled;
-- 
2.25.1


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