From: Sean Christopherson Date: Thu, 30 Oct 2025 18:58:02 +0000 (-0700) Subject: KVM: x86: Harden KVM against imbalanced load/put of guest FPU state X-Git-Tag: v6.18-rc6~42^2~4^2~7 X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=9bc610b6a2a71d1a6acac27e82a0bc8ca861c7ac;p=thirdparty%2Flinux.git 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. Reviewed-by: Yao Yuan Reviewed-by: Chao Gao Link: https://patch.msgid.link/20251030185802.3375059-3-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index d1e048d14e885..67e5f735adf2b 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);