linux-kernel - [PATCH 2/2] KVM: x86: Harden KVM against imbalanced load/put of guest FPU state

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Message-ID: <20251030185802.3375059-3-seanjc@google.com>
Date: Thu, 30 Oct 2025 11:58:02 -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, 
	Alexander Potapenko <glider@...gle.com>
Subject: [PATCH 2/2] KVM: x86: Harden KVM against imbalanced load/put of guest
 FPU state

Assert, via KVM_BUG_ON(), that guest FPU state isn't/is in use when
loading/putting the FPU to help detect KVM bugs without needing an assist
from KASAN.  If an imbalanced load/put is detected, skip the redundant
load/put to avoid clobbering guest state and/or crashing the host.

Note, kvm_access_xstate_msr() already provides a similar assertion.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/x86.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d1e048d14e88..67e5f735adf2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11807,6 +11807,9 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 /* Swap (qemu) user FPU context for the guest FPU context. */
 static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 {
+	if (KVM_BUG_ON(vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm))
+		return;
+
 	/* Exclude PKRU, it's restored separately immediately after VM-Exit. */
 	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, true);
 	trace_kvm_fpu(1);
@@ -11815,6 +11818,9 @@ static void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
 /* When vcpu_run ends, restore user space FPU context. */
 static void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
 {
+	if (KVM_BUG_ON(!vcpu->arch.guest_fpu.fpstate->in_use, vcpu->kvm))
+		return;
+
 	fpu_swap_kvm_fpstate(&vcpu->arch.guest_fpu, false);
 	++vcpu->stat.fpu_reload;
 	trace_kvm_fpu(0);
-- 
2.51.1.930.gacf6e81ea2-goog