linux-kernel - [PATCH] KVM: x86/cpuid: Stop exposing unknown AMX Tile Palettes and accelerator units

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Message-Id: <20220117065957.65335-1-likexu@tencent.com>
Date:   Mon, 17 Jan 2022 14:59:57 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jing Liu <jing2.liu@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86/cpuid: Stop exposing unknown AMX Tile Palettes and accelerator units

From: Like Xu <likexu@...cent.com>

Guest enablement of Intel AMX requires a good co-work from both host and
KVM, which means that KVM should take a more safer approach to avoid
the accidental inclusion of new unknown AMX features, even though it's
designed to be an extensible architecture.

Per current spec, Intel CPUID Leaf 1EH sub-leaf 1 and above are reserved,
other bits in leaves 0x1d and 0x1e marked as "Reserved=0" shall be strictly
limited by definition for reporeted KVM_GET_SUPPORTED_CPUID.

Fixes: 690a757d610e ("kvm: x86: Add CPUID support for Intel AMX")
Signed-off-by: Like Xu <likexu@...cent.com>
---
 arch/x86/kvm/cpuid.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index c55e57b30e81..3fde6610d314 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -661,7 +661,6 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 	case 0x17:
 	case 0x18:
 	case 0x1d:
-	case 0x1e:
 	case 0x1f:
 	case 0x8000001d:
 		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
@@ -936,21 +935,26 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		break;
 	/* Intel AMX TILE */
 	case 0x1d:
+		entry->ebx = entry->ecx = entry->edx = 0;
 		if (!kvm_cpu_cap_has(X86_FEATURE_AMX_TILE)) {
-			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+			entry->eax = 0;
 			break;
 		}
 
+		entry->eax = min(entry->eax, 1u);
 		for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
 			if (!do_host_cpuid(array, function, i))
 				goto out;
 		}
 		break;
-	case 0x1e: /* TMUL information */
+	/* TMUL Information */
+	case 0x1e:
+		entry->eax = entry->ecx = entry->edx = 0;
 		if (!kvm_cpu_cap_has(X86_FEATURE_AMX_TILE)) {
-			entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+			entry->ebx = 0;
 			break;
 		}
+		entry->ebx &= 0xffffffu;
 		break;
 	case KVM_CPUID_SIGNATURE: {
 		const u32 *sigptr = (const u32 *)KVM_SIGNATURE;
-- 
2.33.1