On Fri, 2025-08-29 at 15:39 -0700, Rick Edgecombe wrote:
> >
> > Anyways, I think we need to avoid the "synchronous" ioctl path anyways,
> > because taking kvm->slots_lock inside vcpu->mutex is gross. AFAICT it's not
> > actively problematic today, but it feels like a deadlock waiting to happen.
> >
> > The other oddity I see is the handling of kvm_tdx->state. I don't see how
> > this check in tdx_vcpu_create() is safe:
> >
> > if (kvm_tdx->state != TD_STATE_INITIALIZED)
> > return -EIO;
> >
> > kvm_arch_vcpu_create() runs without any locks held,
Oh, you're right. It's about those fields being set further down in the function
based on the results of KVM_TDX_INIT_VM, rather then TDX module locking. The
race would show if vCPU creation transitioned to TD_STATE_RUNNABLE in finalized
while another vCPU was getting created. Though I'm not sure exactly what would
go wrong, the code is wrong enough looking to be worth a fix.
> > and so TDX effectively has the same bug that SEV intra-host migration had,
> > where an in-flight vCPU creation could race with a VM-wide state transition
> > (see commit ecf371f8b02d ("KVM: SVM: Reject SEV{-ES} intra host migration if
> > vCPU creation is in-flight"). To fix that, kvm->lock needs to be taken and
> > KVM needs to verify there's no in-flight vCPU creation, e.g. so that a vCPU
> > doesn't pop up and contend a TDX-Module lock.
> >
> > We an even define a fancy new CLASS to handle the lock+check => unlock logic
> > with guard()-like syntax:
> >
> > CLASS(tdx_vm_state_guard, guard)(kvm);
> > if (IS_ERR(guard))
> > return PTR_ERR(guard);
> >
> > IIUC, with all of those locks, KVM can KVM_BUG_ON() both TDH_MEM_PAGE_ADD
> > and TDH_MR_EXTEND, with no exceptions given for -EBUSY. Attached patches
> > are very lightly tested as usual and need to be chunked up, but seem do to
> > what I want.
>
> Ok, the direction seem clear. The patch has an issue, need to debug.
Just this:
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index c595d9cb6dcd..e99d07611393 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2809,7 +2809,7 @@ static int tdx_td_finalize(struct kvm *kvm, struct
kvm_tdx_cmd *cmd)
static int tdx_get_cmd(void __user *argp, struct kvm_tdx_cmd *cmd)
{
- if (copy_from_user(cmd, argp, sizeof(cmd)))
+ if (copy_from_user(cmd, argp, sizeof(*cmd)))
return -EFAULT;
if (cmd->hw_error)
