linux-kernel - Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when loading the KVM intel kernel module

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Date: Thu, 25 Apr 2024 15:43:50 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Tina Zhang <tina.zhang@...el.com>, Hang Yuan <hang.yuan@...el.com>, 
	Bo2 Chen <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Erdem Aktas <erdemaktas@...gle.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module

On Thu, Apr 25, 2024, Kai Huang wrote:
> On Thu, 2024-04-25 at 09:30 -0700, Sean Christopherson wrote:
> > On Tue, Apr 23, 2024, Kai Huang wrote:
> > And anecdotally, I know of at least one crash in our production environment where
> > a VMX instruction hit a seemingly spurious #UD, i.e. it's not impossible for a
> > ucode bug or hardware defect to cause problems.  That's obviously _extremely_
> > unlikely, but that's why I emphasized that sanity checking CR4.VMXE is cheap.
> 
> Yeah I agree it could happen although very unlikely.
> 
> But just to be sure:
> 
> I believe the #UD itself doesn't crash the kernel/machine, but should be
> the kernel unable to handle #UD in such case?

Correct, the #UD is likely not (immediately) fatal.
> 
> If so, I am not sure whether the CR4.VMX check can make the kernel any
> safer, because we can already handle the #UD for the SEAMCALL instruction.

It's not about making the kernel safer, it's about helping triage/debug issues.

> Yeah we can clearly dump message saying "CPU isn't in VMX operation" and
> return failure if we have the check, but if we don't, the worst situation
> is we might mistakenly report "CPU isn't in VMX operation" (currently code
> just treats #UD as CPU not in VMX operation) when CPU doesn't
> IA32_VMX_PROCBASED_CTLS3[5].
> 
> And for the IA32_VMX_PROCBASED_CTLS3[5] we can easily do some pre-check in
> KVM code during module loading to rule out this case.
>
> And in practice, I even believe the BIOS cannot turn on TDX if the
> IA32_VMX_PROCBASED_CTLS3[5] is not supported.  I can check on this.

Eh, I wouldn't worry about that too much.  The only reason I brought up that
check was to call out that we can't *know* with 100% certainty that SEAMCALL
failed due to the CPU not being post-VMXON.

> > Practically speaking it costs nothing, so IMO it's worth adding even if the odds
> > of it ever being helpful are one-in-and-million.
> 
> I think we will need to do below at somewhere for the common SEAMCALL
> function:
> 
> 	unsigned long flags;
> 	int ret = -EINVAL;
> 
> 	local_irq_save(flags);
> 
> 	if (WARN_ON_ONCE(!(__read_cr4() & X86_CR4_VMXE)))
> 		goto out;
> 
> 	ret = seamcall();
> out:
> 	local_irq_restore(flags);
> 	return ret;
> 
> to make it IRQ safe.
>
> And the odd is currently the common SEAMCALL functions, a.k.a,
> __seamcall() and seamcall() (the latter is a mocro actually), both return
> u64, so if we want to have such CR4.VMX check code in the common code, we
> need to invent a new error code for it.

Oh, I wasn't thinking that we'd check CR4.VMXE before *every* SEAMCALL, just
before the TDH.SYS.LP.INIT call, i.e. before the one that is most likely to fail
due to a software bug that results in the CPU not doing VMXON before enabling
TDX.

Again, my intent is to add a simple, cheap, and targeted sanity check to help
deal with potential failures in code that historically has been less than rock
solid, and in function that has a big fat assumption that the caller has done
VMXON on the CPU.

> That being said, although I agree it can make the code a little bit
> clearer, I am not sure whether it can make the code any safer -- even w/o
> it, the worst case is to incorrectly report "CPU is not in VMX operation",
> but shouldn't crash kernel etc.