Re: [PATCH v4 1/4] KVM: TDX: Synchronize user-return MSRs immediately after VP.ENTER

Sean Christopherson
2025-10-21T19:34:00Z

On Tue, Oct 21, 2025, Rick P Edgecombe wrote:
> On Tue, 2025-10-21 at 08:06 -0700, Sean Christopherson wrote:
> > I think we should be synchronizing only after a successful VP.ENTER with a real
> > > > TD exit, but today instead we synchronize after any attempt to VP.ENTER.
> >
> > Well this is all completely @#($*#.  Looking at the TDX-Module source, if the
> > TDX-Module synthesizes an exit, e.g. because it suspects a zero-step attack, it
> > will signal a "normal" exit but not "restore" VMM state.
>
> Oh yea, good point. So there is no way to tell from the return code if the
> clobbering happened.
>
> >
> > > If the MSR's do not get clobbered, does it matter whether or not they get
> > > restored.
> >
> > It matters because KVM needs to know the actual value in hardware.  If KVM thinks
> > an MSR is 'X', but it's actually 'Y', then KVM could fail to write the correct
> > value into hardware when returning to userspace and/or when running a different
> > vCPU.
> >
> > Taking a step back, the entire approach of updating the "cache" after the fact is
> > ridiculous.  TDX entry/exit is anything but fast; avoiding _at most_ 4x WRMSRs at
> > the start of the run loop is a very, very premature optimization.  Preemptively
> > load hardware with the value that the TDX-Module _might_ set and call it good.
> >
> > I'll replace patches 1 and 4 with this, tagged for stable@.
>
> Seems reasonable to me in concept, but there is a bug. It looks like some
> important MSR isn't getting restored right and the host gets into a bad state.
> The first signs start with triggering this:
>
> asmlinkage __visible noinstr struct pt_regs *fixup_bad_iret(struct pt_regs
> *bad_regs)
> {
> struct pt_regs tmp, *new_stack;
>
> /*
> * This is called from entry_64.S early in handling a fault
> * caused by a bad iret to user mode. To handle the fault
> * correctly, we want to move our stack frame to where it would
> * be had we entered directly on the entry stack (rather than
> * just below the IRET frame) and we want to pretend that the
> * exception came from the IRET target.
> */
> new_stack = (struct pt_regs *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) -
> 1;
>
> /* Copy the IRET target to the temporary storage. */
> __memcpy(&tmp.ip, (void *)bad_regs->sp, 5*8);
>
> /* Copy the remainder of the stack from the current stack. */
> __memcpy(&tmp, bad_regs, offsetof(struct pt_regs, ip));
>
> /* Update the entry stack */
> __memcpy(new_stack, &tmp, sizeof(tmp));
>
> BUG_ON(!user_mode(new_stack)); <---------------HERE
>
> Need to debug.

/facepalm

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 63abfa251243..cde91a995076 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -801,8 +801,8 @@ void tdx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
* state.
*/
for (i = 0; i < ARRAY_SIZE(tdx_uret_msrs); i++)
- kvm_set_user_return_msr(i, tdx_uret_msrs[i].slot,
- tdx_uret_msrs[i].defval);
+ kvm_set_user_return_msr(tdx_uret_msrs[i].slot,
+ tdx_uret_msrs[i].defval, -1ull);
}

static void tdx_prepare_switch_to_host(struct kvm_vcpu *vcpu)