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KVM: x86: Harden KVM against imbalanced load/put of guest FPU state

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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 <yaoyuan@linux.alibaba.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251030185802.3375059-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-10-30 11:58:02 -07:00
parent 8819a49f9f
commit 9bc610b6a2
1 changed files with 6 additions and 0 deletions
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6
arch/x86/kvm/x86.c
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@@ -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);