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

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Message-ID: <20231031181340.30233c17@meshulam.tesarici.cz>
Date:   Tue, 31 Oct 2023 18:13:40 +0100
From:   Petr Tesařík <petr@...arici.cz>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Lutomirski, Andy" <luto@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.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>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "Cui, Dexuan" <decui@...rosoft.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "mikelley@...rosoft.com" <mikelley@...rosoft.com>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "hch@....de" <hch@....de>, "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 04/10] swiotlb: Use free_decrypted_pages()

On Tue, 31 Oct 2023 15:54:52 +0000
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Tue, 2023-10-31 at 11:43 +0100, Petr Tesařík wrote:
> > 
> > I admit I'm not familiar with the encryption/decryption API, but if a
> > __free_pages() is not sufficient here, then it is quite confusing.
> > The error label is reached only if set_memory_decrypted() returns
> > non-zero. My naive expectation is that the memory is *not* decrypted
> > in
> > that case and does not require special treatment. Is this assumption
> > wrong?  
> 
> Yea, the memory can still be decrypted, or partially decrypted. On x86,
> all the set_memory() calls can fail part way through the work, and they
> don't rollback the changes they had made up to that point.

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.

>[...]
> > OTOH I believe there is a bug in the logic. The subsequent
> > __free_pages() in swiotlb_alloc_tlb() would have to be changed to a
> > free_decrypted_pages(). However, I'm proposing a different approach
> > to
> > address the latter issue here:
> > 
> > https://lore.kernel.org/linux-iommu/20231026095123.222-1-petrtesarik@huaweicloud.com/T/  
> 
> Oh, yes, that makes sense. I was planning to send a patch to just leak
> the pages if set_memory_decrypted() fails, after my v2 linked above is
> accepted. It could have a different label than the phys_limit check
> error path added in your linked patch, so that case would still free
> the perfectly fine encrypted pages.

Hm, should I incorporate this knowledge into a v2 of my patch and
address both issues?

Petr T