linux-kernel - [PATCH v2 3/4] KVM: selftests: Disallow "get supported CPUID" before REQ_XCOMP_GUEST

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Message-ID: <20221128225735.3291648-4-seanjc@google.com>
Date:   Mon, 28 Nov 2022 22:57:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lei Wang <lei4.wang@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH v2 3/4] KVM: selftests: Disallow "get supported CPUID" before REQ_XCOMP_GUEST_PERM

Disallow using kvm_get_supported_cpuid() and thus caching KVM's supported
CPUID info before enabling XSAVE-managed features that are off-by-default
and must be enabled by ARCH_REQ_XCOMP_GUEST_PERM.  Caching the supported
CPUID before all XSAVE features are enabled can result in false negatives
due to testing features that were cached before they were enabled.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 .../selftests/kvm/lib/x86_64/processor.c       | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index 23067465c035..1d3829e652e6 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -601,21 +601,24 @@ void vcpu_arch_free(struct kvm_vcpu *vcpu)
 		free(vcpu->cpuid);
 }
 
+/* Do not use kvm_supported_cpuid directly except for validity checks. */
+static void *kvm_supported_cpuid;
+
 const struct kvm_cpuid2 *kvm_get_supported_cpuid(void)
 {
-	static struct kvm_cpuid2 *cpuid;
 	int kvm_fd;
 
-	if (cpuid)
-		return cpuid;
+	if (kvm_supported_cpuid)
+		return kvm_supported_cpuid;
 
-	cpuid = allocate_kvm_cpuid2(MAX_NR_CPUID_ENTRIES);
+	kvm_supported_cpuid = allocate_kvm_cpuid2(MAX_NR_CPUID_ENTRIES);
 	kvm_fd = open_kvm_dev_path_or_exit();
 
-	kvm_ioctl(kvm_fd, KVM_GET_SUPPORTED_CPUID, cpuid);
+	kvm_ioctl(kvm_fd, KVM_GET_SUPPORTED_CPUID,
+		  (struct kvm_cpuid2 *)kvm_supported_cpuid);
 
 	close(kvm_fd);
-	return cpuid;
+	return kvm_supported_cpuid;
 }
 
 static uint32_t __kvm_cpu_has(const struct kvm_cpuid2 *cpuid,
@@ -684,6 +687,9 @@ void __vm_xsave_require_permission(int bit, const char *name)
 		.addr = (unsigned long) &bitmask
 	};
 
+	TEST_ASSERT(!kvm_supported_cpuid,
+		    "kvm_get_supported_cpuid() cannot be used before ARCH_REQ_XCOMP_GUEST_PERM");
+
 	kvm_fd = open_kvm_dev_path_or_exit();
 	rc = __kvm_ioctl(kvm_fd, KVM_GET_DEVICE_ATTR, &attr);
 	close(kvm_fd);
-- 
2.38.1.584.g0f3c55d4c2-goog