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KVM: x86: Deliver exception payload on KVM_GET_VCPU_EVENTS

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KVM allows the deferral of exception payloads when a vCPU is in guest
mode to allow the L1 hypervisor to intercept certain events (#PF, #DB)
before register state has been modified. However, this behavior is
incompatible with the KVM_{GET,SET}_VCPU_EVENTS ABI, as userspace
expects register state to have been immediately modified. Userspace may
opt-in for the payload deferral behavior with the
KVM_CAP_EXCEPTION_PAYLOAD per-VM capability. As such,
kvm_multiple_exception() will immediately manipulate guest registers if
the capability hasn't been requested.

Since the deferral is only necessary if a userspace ioctl were to be
serviced at the same as a payload bearing exception is recognized, this
behavior can be relaxed. Instead, opportunistically defer the payload
from kvm_multiple_exception() and deliver the payload before completing
a KVM_GET_VCPU_EVENTS ioctl.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Oliver Upton 2020-02-07 02:36:06 -08:00 committed by Paolo Bonzini
parent 684c0422da
commit a06230b62b
1 changed files with 16 additions and 13 deletions
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29
arch/x86/kvm/x86.c
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@ -498,19 +498,7 @@ static void kvm_multiple_exception(struct kvm_vcpu *vcpu,
vcpu->arch.exception.error_code = error_code;
vcpu->arch.exception.has_payload = has_payload;
vcpu->arch.exception.payload = payload;
/*
* In guest mode, payload delivery should be deferred,
* so that the L1 hypervisor can intercept #PF before
* CR2 is modified (or intercept #DB before DR6 is
* modified under nVMX). However, for ABI
* compatibility with KVM_GET_VCPU_EVENTS and
* KVM_SET_VCPU_EVENTS, we can't delay payload
* delivery unless userspace has enabled this
* functionality via the per-VM capability,
* KVM_CAP_EXCEPTION_PAYLOAD.
*/
if (!vcpu->kvm->arch.exception_payload_enabled ||
!is_guest_mode(vcpu))
if (!is_guest_mode(vcpu))
kvm_deliver_exception_payload(vcpu);
return;
}
@ -3803,6 +3791,21 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
{
process_nmi(vcpu);
/*
* In guest mode, payload delivery should be deferred,
* so that the L1 hypervisor can intercept #PF before
* CR2 is modified (or intercept #DB before DR6 is
* modified under nVMX). Unless the per-VM capability,
* KVM_CAP_EXCEPTION_PAYLOAD, is set, we may not defer the delivery of
* an exception payload and handle after a KVM_GET_VCPU_EVENTS. Since we
* opportunistically defer the exception payload, deliver it if the
* capability hasn't been requested before processing a
* KVM_GET_VCPU_EVENTS.
*/
if (!vcpu->kvm->arch.exception_payload_enabled &&
vcpu->arch.exception.pending && vcpu->arch.exception.has_payload)
kvm_deliver_exception_payload(vcpu);
/*
* The API doesn't provide the instruction length for software
* exceptions, so don't report them. As long as the guest RIP