linux-kernel - [PATCH v4 1/6] KVM: x86: Add a helper to handle filtering of unpermitted XCR0 features

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Date:   Tue,  4 Apr 2023 17:45:15 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Aaron Lewis <aaronlewis@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: [PATCH v4 1/6] KVM: x86: Add a helper to handle filtering of
 unpermitted XCR0 features

From: Aaron Lewis <aaronlewis@...gle.com>

Add a helper, kvm_get_filtered_xcr0(), to dedup code that needs to account
for XCR0 features that require explicit opt-in on a per-process basis.  In
addition to documenting when KVM should/shouldn't consult
xstate_get_guest_group_perm(), the helper will also allow sanitizing the
filtered XCR0 to avoid enumerating architecturally illegal XCR0 values,
e.g. XTILE_CFG without XTILE_DATA.

No functional changes intended.

Signed-off-by: Aaron Lewis <aaronlewis@...gle.com>
Reviewed-by: Mingwei Zhang <mizhang@...gle.com>
[sean: rename helper, move to x86.h, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c |  2 +-
 arch/x86/kvm/x86.c   |  4 +---
 arch/x86/kvm/x86.h   | 13 +++++++++++++
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6972e0be60fa..542bcaab3592 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -996,7 +996,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->eax = entry->ebx = entry->ecx = 0;
 		break;
 	case 0xd: {
-		u64 permitted_xcr0 = kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
+		u64 permitted_xcr0 = kvm_get_filtered_xcr0();
 		u64 permitted_xss = kvm_caps.supported_xss;
 
 		entry->eax &= permitted_xcr0;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2c0ff40e5345..7bac4162cfae 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4567,9 +4567,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 			r = 0;
 		break;
 	case KVM_CAP_XSAVE2: {
-		u64 guest_perm = xstate_get_guest_group_perm();
-
-		r = xstate_required_size(kvm_caps.supported_xcr0 & guest_perm, false);
+		r = xstate_required_size(kvm_get_filtered_xcr0(), false);
 		if (r < sizeof(struct kvm_xsave))
 			r = sizeof(struct kvm_xsave);
 		break;
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 203fb6640b5b..b6c6988d99b5 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -315,6 +315,19 @@ extern struct kvm_caps kvm_caps;
 
 extern bool enable_pmu;
 
+/*
+ * Get a filtered version of KVM's supported XCR0 that strips out dynamic
+ * features for which the current process doesn't (yet) have permission to use.
+ * This is intended to be used only when enumerating support to userspace,
+ * e.g. in KVM_GET_SUPPORTED_CPUID and KVM_CAP_XSAVE2, it does NOT need to be
+ * used to check/restrict guest behavior as KVM rejects KVM_SET_CPUID{2} if
+ * userspace attempts to enable unpermitted features.
+ */
+static inline u64 kvm_get_filtered_xcr0(void)
+{
+	return kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
+}
+
 static inline bool kvm_mpx_supported(void)
 {
 	return (kvm_caps.supported_xcr0 & (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR))
-- 
2.40.0.348.gf938b09366-goog