[PATCH v2 9/9] KVM: x86: never write to memory from kvm_vcpu_check_block

Paolo Bonzini
2022-08-11T21:06:35Z

kvm_vcpu_check_block() is called while not in TASK_RUNNING, and therefore
it cannot sleep. Writing to guest memory is therefore forbidden, but it
can happen on AMD processors if kvm_check_nested_events() causes a vmexit.

Fortunately, all events that are caught by kvm_check_nested_events() are
also recognized by kvm_vcpu_has_events() through vendor callbacks such as
kvm_x86_interrupt_allowed() or kvm_x86_ops.nested_ops->has_events(), so
remove the call and postpone the actual processing to vcpu_block().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/x86.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5e9358ea112b..9226fd536783 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10639,6 +10639,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
return 1;
}

+ if (is_guest_mode(vcpu)) {
+ /*
+ * Evaluate nested events before exiting the halted state.
+ * This allows the halt state to be recorded properly in
+ * the VMCS12's activity state field (AMD does not have
+ * a similar field and a vmexit always causes a spurious
+ * wakeup from HLT).
+ */
+ kvm_check_nested_events(vcpu);
+ }
+
if (kvm_apic_accept_events(vcpu) < 0)
return 0;
switch(vcpu->arch.mp_state) {
@@ -10662,9 +10673,6 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)

static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
{
- if (is_guest_mode(vcpu))
- kvm_check_nested_events(vcpu);
-
return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
!vcpu->arch.apf.halted);
}
--
2.31.1