Re: [PATCH v15 22/23] KVM: selftests: Add support for TDX ucalls, via TDVMCALL_REPORT_FATAL_ERROR

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] kvm: selftests: ucall global state poisoning breaks mixed-VM testing
--

commit 742d058040571d70e8a67c18a52e7026f4abe945
Author: Sean Christopherson <seanjc@google.com>

KVM: selftests: Add support for TDX ucalls, via TDVMCALL_REPORT_FATAL_ERROR

This commit adds support for performing ucalls on TDX by utilizing the
TDVMCALL_REPORT_FATAL_ERROR hypercall. It replaces the default ucall
operations with TDX-specific handlers when a TDX VM is initialized.

> diff --git a/tools/testing/selftests/kvm/lib/x86/ucall.c b/tools/testing/selftests/kvm/lib/x86/ucall.c
> index c003df3c7b8a8..c1cd32d684646 100644
> --- a/tools/testing/selftests/kvm/lib/x86/ucall.c
> +++ b/tools/testing/selftests/kvm/lib/x86/ucall.c

[ ... ]

> @@ -37,6 +57,11 @@ static struct {
>  
>  void ucall_arch_init(struct kvm_vm *vm, gpa_t mmio_gpa)
>  {
> +	if (is_tdx_vm(vm)) {
> +		ucall_x86_ops.do_ucall = ucall_tdx_do_ucall;
> +		ucall_x86_ops.get_ucall = ucall_tdx_get_ucall;
> +	}
> +

[Severity: Medium]
Since ucall_x86_ops is a process-wide global struct, does modifying it here
without an else branch to restore the default handlers break mixed-VM testing?

If a process creates a TDX VM and later creates a non-TDX VM, the non-TDX VM
will inherit these TDX handlers. When sync_global_to_guest() is called, this
modified state syncs into the non-TDX VM's memory, which might cause the
guest to incorrectly issue TDCALL for ucalls, leading to an Invalid Opcode
exception in the non-TDX guest.

Additionally, host-side dispatch via ucall_arch_get_ucall() relies on this
globally assigned pointer:

void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu)
{
	return ucall_x86_ops.get_ucall(vcpu);
}

Will this cause the host to dispatch based on the last initialized VM rather
than dynamically checking is_tdx_vm(vcpu->vm)?

>  	sync_global_to_guest(vm, ucall_x86_ops);
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261001-tdx-selftests-v15-0-7c62a5d8a992@google.com?part=22