linux-kernel - Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in relocate

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Date: Thu, 21 Mar 2024 16:02:11 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 "Huang, Kai" <kai.huang@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, dave.hansen@...el.com,
 bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
 luto@...nel.org, peterz@...radead.org, rick.p.edgecombe@...el.com,
 ashish.kalra@....com, chao.gao@...el.com, bhe@...hat.com,
 nik.borisov@...e.com, pbonzini@...hat.com, seanjc@...gle.com
Subject: Re: [PATCH v2 2/5] x86/kexec: do unconditional WBINVD in
 relocate_kernel()

On 3/20/24 18:10, Kirill A. Shutemov wrote:
> On Thu, Mar 21, 2024 at 09:48:28AM +1300, Huang, Kai wrote:
>>
>>>> Hi Tom,
>>>>
>>>> I am not aware of kexec() support status for SEV-ES/SEV-SNP guests.
>>>> Does patch 1 break them?
>>>
>>> SNP guests can kexec with some patches that are currently in process
>>> around shared to private memory conversions. ES guests can only kexec
>>> with a single vCPU. There was a recent patch series to add support for
>>> multiple vCPUs.
>>>
>>> Patch #1 doesn't break either ES or SNP because we still have an IDT and
>>> traditional kernel addressing in place, so the #VC can be handled.
>>
>> How about plain SEV guest?
>>
>>>
>>> Whereas patch #2 has switched to identity mapping and removed the IDT,
>>> so a #VC causes a triple fault.
>>
>> That makes sense.  Thanks.
>>
>> Hi Kirill,
>>
>> Does TDX guest have similar behaviour -- that WBINVD in stop_this_cpu() can
>> be handled although it causes #VE, while WBINVD in relocate_kernel() will
>> just triple fault the guest?
> 
> No. We never handle WBINVD #VE. Guest cannot handle WBINVD itself and the
> only option is to ask host to do this. We cannot guarantee host will do

Is the WBINVD performed or ignored in that case?

> anything useful with the request. I guess it can be potential attack
> vector if host strategically ignores WBINVD to induce bad guest behaviour.

With SNP, memory is coherent so there isn't a need for a WBINVD within a 
guest and so issuing it should not be an issue whether the hypervisor 
performs the operation or not. I don't know what can happen in the case 
where, say, you have a non-coherent TDISP device attached or such, but 
that would be very unusual/unlikely.

> 
> And it is not good from host PoV either. If it does WBINVD on every guest
> request we get guest->host DoS attack possibility.

Yeah, that can happen today, regardless of the type of VM running.

> 
> Tom, I am curious, how do you deal with these problems?

If the WBINVD is being intercepted, then it will generate a #VC and we use 
the GHCB protocol to communicate that back to the hypervisor to handle.

Thanks,
Tom

>