Re: [PATCH v2 3/3] KVM: TDX: Enable Bus Lock VM exit

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] The `vmx_handle_exit()` function silently drops negative error codes (fatal errors) when `bus_lock_detected` is true.
--

--- Patch [3]: [PATCH v2 3/3] KVM: TDX: Enable Bus Lock VM exit ---
commit 573aa824013208febeaa70c68e452e3344dd518c
Author: Xiaoyao Li <[email protected]>

KVM: TDX: Enable Bus Lock VM exit

This patch enables Bus Lock VM exit functionality for TDX guests by
programming the BUS_LOCK_DETECTION control in the TD VMCS and adding the
exit handler. It also handles clearing the bus_lock_detected bit to avoid
issues during the wait_for_sept_zap case.

> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 2c8844acbd43e..ee3c7a32361a4 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c

[ ... ]

> @@ -2150,6 +2154,22 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
>  	return 0;
>  }
>  
> +int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
> +{
> +	int ret = __tdx_handle_exit(vcpu, fastpath);
> +
> +	/* Exit to user space when bus lock was detected */
> +	if (vmx_get_exit_reason(vcpu).bus_lock_detected) {
> +		if (ret > 0) {
> +			vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;
> +			ret = 0;
> +		}

[Severity: High]
This is a pre-existing issue, but since you fixed the negative return value
handling here for tdx_handle_exit() in v2, should a similar fix be applied
to the pre-existing vmx_handle_exit() in arch/x86/kvm/vmx/vmx.c?

Looking at vmx_handle_exit(), if __vmx_handle_exit() returns a negative
error code (e.g. -ENOMEM) and bus_lock_detected is true, it unconditionally
overrides the negative return value to 0:

    if (vmx_get_exit_reason(vcpu).bus_lock_detected) {
        if (ret > 0)
            vcpu->run->exit_reason = KVM_EXIT_X86_BUS_LOCK;

        vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
        return 0;
    }

Because ret > 0 is false, it skips setting vcpu->run->exit_reason but still
returns 0 (success) to userspace.

Does this result in userspace receiving a successful return code but with a
stale or uninitialized exit_reason, likely leading to a VMM crash or
unexpected termination if a guest triggers a bus lock exit while KVM
hits an internal error?

> +
> +		vcpu->run->flags |= KVM_RUN_X86_BUS_LOCK;
> +	}
> +	return ret;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=3