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KVM: SEV: Do not intercept accesses to MSR_IA32_XSS for SEV-ES guests

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[ Upstream commit a26b7cd2254695f8258cc370f33280db0a9a3813 ]

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@amd.com>
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-Id: <20231016132819.1002933-4-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Stable-dep-of: b7e4be0a224f ("KVM: SEV-ES: Delegate LBR virtualization to the processor")
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Michael Roth 2023-10-16 08:27:32 -05:00 committed by Greg Kroah-Hartman
parent 2128bae4ec
commit b6e4076ca9
3 changed files with 21 additions and 1 deletions
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19
arch/x86/kvm/svm/sev.c
View File

@ -2994,6 +2994,25 @@ 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)

1
arch/x86/kvm/svm/svm.c
View File

@ -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 },

2
arch/x86/kvm/svm/svm.h
View File

@ -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;