Re: [Patch v3 1/4] KVM: SVM: Re-queue events that were never injected

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] KVM: SVM: missing SVM_EVTINJ_VALID check in svm_complete_interrupts()
- [Medium] KVM: SVM: svm_complete_interrupts() clobbers VMCB event_inj and exit_int_info
--

commit d02578cb88ead63ae65c3cea00ca65df2ab7a2eb
Author: Pratik R. Sampat <prsampat@amd.com>

KVM: SVM: Re-queue events that were never injected

This commit updates svm_complete_interrupts() to detect undelivered injected
events in control->event_inj on #VMEXIT, migrate them to control->exit_int_info
so they are re-queued, and folds svm_cancel_injection() into this path.

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index fe0cba4731b2e..660dd1a3f497e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4330,6 +4330,7 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	u8 vector;
>  	int type;
> +	struct vmcb_control_area *control = &svm->vmcb->control;
>  	u32 exitintinfo = svm->vmcb->control.exit_int_info;
>  	bool nmi_l1_to_l2 = svm->nmi_l1_to_l2;
>  	bool soft_int_injected = svm->soft_int_injected;
> @@ -4337,6 +4338,27 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>  	svm->nmi_l1_to_l2 = false;
>  	svm->soft_int_injected = false;
>  
> +	/*
> +	 * Hardware clears EVENTINJ field when it injects an event.
> +	 * A non-empty EVENTINJ on #VMEXIT means the vCPU never entered guest
> +	 * mode, and thus that the event was never delivered.  Migrate the event
> +	 * to EXITINTINFO so that it's requeued instead of being dropped.
> +	 *
> +	 * An undelivered event doesn't imply a fatal VMEXIT_INVALID. With
> +	 * Enhanced SMT Protection, VMRUN may exit with an ordinary #VMEXIT
> +	 * without having injected that event into the guest.
> +	 *
> +	 * Clobbering EXITINTINFO is safe precisely because the vCPU never
> +	 * entered guest mode.
> +	 */
> +	if (control->event_inj) {
> +		control->exit_int_info = control->event_inj;
> +		control->exit_int_info_err = control->event_inj_err;
> +		control->event_inj = 0;
> +
> +		exitintinfo = control->exit_int_info;
> +	}

[Severity: High]
Should this check (control->event_inj & SVM_EVTINJ_VALID) rather than
control->event_inj?

In nested SVM, L1 can configure vmcb12->control.event_inj with
SVM_EVTINJ_VALID cleared while leaving non-zero vector, type, or
error-code-valid bits in bits 30:0.  nested_svm_check_event_inj() accepts
this, __nested_copy_vmcb_control_to_cache() preserves the non-reserved bits,
and nested_vmcb02_prepare_control() copies vmcb12_ctrl->event_inj into
vmcb02->control.event_inj:

arch/x86/kvm/svm/nested.c:nested_svm_check_event_inj() {
	if (!(event_inj & SVM_EVTINJ_VALID))
		return true;
    ...
}

arch/x86/kvm/svm/nested.c:__nested_copy_vmcb_control_to_cache() {
    ...
	to->event_inj           = from->event_inj & ~SVM_EVTINJ_RESERVED_BITS;
	to->event_inj_err       = from->event_inj_err;
}

arch/x86/kvm/svm/nested.c:nested_vmcb02_prepare_control() {
    ...
	vmcb02->control.event_inj           = vmcb12_ctrl->event_inj;
	vmcb02->control.event_inj_err       = vmcb12_ctrl->event_inj_err;
    ...
}

When SVM_EVTINJ_VALID is 0, VMRUN ignores EVENTINJ without clearing it and
enters L2 normally.  On the subsequent #VMEXIT from L2,
if (control->event_inj) evaluates to true and overwrites
control->exit_int_info and exitintinfo with the invalid event_inj value
(where SVM_EXITINTINFO_VALID is 0).

If L2 exited while vectoring an event, the valid exit_int_info populated by
hardware is clobbered and svm_complete_interrupts() returns early at
if (!(exitintinfo & SVM_EXITINTINFO_VALID)), dropping the L2 event.

In addition, if the invalid event_inj has non-zero SVM_EXITINTINFO_TYPE_MASK
bits and L2 exits with an #NPF that requires instruction emulation,
svm_check_emulate_instruction() inspects control->exit_int_info without
checking SVM_EXITINTINFO_VALID:

arch/x86/kvm/svm/svm.c:svm_check_emulate_instruction() {
    ...
	if ((svm->vmcb->control.exit_int_info & SVM_EXITINTINFO_TYPE_MASK) &&
	    !kvm_can_emulate_event_vectoring(emul_type))
		return X86EMUL_UNHANDLEABLE_VECTORING;
    ...
}

causing #NPF emulation to fail with X86EMUL_UNHANDLEABLE_VECTORING.

[Severity: Medium]
Also, by overwriting control->exit_int_info and zeroing control->event_inj in
the VMCB during svm_complete_interrupts(), won't subsequent exit handling in
svm_handle_exit() observe mutated VMCB fields?

In svm_vcpu_run(), trace_kvm_exit() runs before svm_complete_interrupts(),
while svm_handle_exit() runs afterward in vcpu_enter_guest():

arch/x86/kvm/svm/svm.c:svm_handle_exit() {
    ...
	if (is_guest_mode(vcpu)) {
		int vmexit;

		trace_kvm_nested_vmexit(vcpu, KVM_ISA_SVM);
    ...
	}

	if (svm_is_vmrun_failure(svm->vmcb->control.exit_code)) {
		kvm_run->exit_reason = KVM_EXIT_FAIL_ENTRY;
		kvm_run->fail_entry.hardware_entry_failure_reason
			= svm->vmcb->control.exit_code;
		kvm_run->fail_entry.cpu = vcpu->arch.last_vmentry_cpu;
		dump_vmcb(vcpu);
		return 0;
	}
    ...
}

When VMRUN fails with SVM_EXIT_ERR (for example, due to an invalid
control->event_inj configuration) or hits an unexpected exit while an event
is staged in control->event_inj, dump_vmcb() will log 0 for event_inj and
print the staged event_inj under exit_int_info, hiding the event_inj value
that was present at VMRUN.  Similarly, for nested exits,
trace_kvm_nested_vmexit() will record the migrated exit_int_info, disagreeing
with trace_kvm_exit() for the same exit.

Would it be better to read control->event_inj and control->event_inj_err into
local variables in svm_complete_interrupts() (while clearing
control->event_inj) instead of writing them into control->exit_int_info and
control->exit_int_info_err in the VMCB?

[ ... ]

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完美！ (Oops, no, wait, no extra text! Let's make sure the output ends with a blank line and only contains the report.)
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-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1792184487.git.prsampat@amd.com?part=1