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

Sean Christopherson
2022-09-20T00:56:07Z

On Thu, Aug 11, 2022, Paolo Bonzini wrote:
> 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);

Continuing the conversation with myself, this "needs" to check the return of
kvm_check_nested_events(). In quotes because KVM just signals "internal error"
if vmx_complete_nested_posted_interrupt() fails, i.e. the VM is likely dead anyways,
but again it's odd that the return of kvm_apic_accept_events() is checked but the
direct call to kvm_check_nested_events() is not.

> + }
> +
> 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
>