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

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Message-ID: <38df6c8bfd384e5fefa8eb6fbc27c35b99c685ed.camel@intel.com>
Date: Tue, 21 Oct 2025 18:54:37 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Hunter, Adrian" <adrian.hunter@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>
CC: "x86@...nel.org" <x86@...nel.org>, "kas@...nel.org" <kas@...nel.org>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "hou,
 wenlong" <houwenlong.hwl@...group.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>
Subject: Re: [PATCH v4 1/4] KVM: TDX: Synchronize user-return MSRs immediately
 after VP.ENTER

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.