linux-kernel - Re: [PATCH 04/10] swiotlb: Use free_decrypted

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Message-ID: <20231101072723.44d00721@meshulam.tesarici.cz>
Date:   Wed, 1 Nov 2023 07:27:23 +0100
From:   Petr Tesařík <petr@...arici.cz>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Cui, Dexuan" <decui@...rosoft.com>,
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        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 04/10] swiotlb: Use free_decrypted_pages()

Hi,

On Tue, 31 Oct 2023 17:29:25 +0000
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Tue, 2023-10-31 at 18:13 +0100, Petr Tesařík wrote:
> > Thank you for the explanation. So, after set_memory_decrypted()
> > fails,
> > the pages become Schroedinger-crypted, but since its true state
> > cannot
> > be observed by the guest kernel, it stays as such forever.
> > 
> > Sweet.
> >   
> Yes... The untrusted host (the part of the VMM TDX is defending
> against) gets to specify the return code of these operations (success
> or failure). But the coco(a general term for TDX and similar from other
> vendors) threat model doesn't include DOS. So the guest should trust
> the return code as far as trying to not crash, but not trust it in
> regards to the potential to leak data.
> 
> It's a bit to ask of the callers, but the other solution we discussed
> was to panic the guest if any weirdness is observed by the VMM, in
> which case the callers would never see the error. And of course
> panicing the kernel is Bad. So that is how we arrived at this request
> of the callers. Appreciate the effort to handle it on that side.
> 
> 
> > Hm, should I incorporate this knowledge into a v2 of my patch and
> > address both issues?  
> 
> That sounds good to me! Feel free to CC me if you would like, and I can
> scrutinize it for this particular issue.

I'm sorry I missed that free_decrypted_pages() is added by the very
same series, so I cannot use it just yet. I can open-code it and let
you convert the code to the new function. You may then also want to
convert another open-coded instance further down in swiotlb_free_tlb().

In any case, there is an interdependency between the two patches, so we
should agree in which order to apply them.

Petr T