linux-kernel - Re: [RFC PATCH v2 12/18] KVM: TDX: Bug the VM if extended the initial measurement fails

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Message-ID: <b47e08bb6105a94bc88ee91aa7bdd055893eeda6.camel@intel.com>
Date: Fri, 29 Aug 2025 23:15:37 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Huang, Kai" <kai.huang@...el.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "Weiny, Ira" <ira.weiny@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "michael.roth@....com"
	<michael.roth@....com>, "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v2 12/18] KVM: TDX: Bug the VM if extended the initial
 measurement fails

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)