linux-kernel - Re: AMD SME encrpytion and PCI BAR pages to user space

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Date:   Wed, 21 Oct 2020 13:03:22 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Tom Lendacky <thomas.lendacky@....com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <linux-mm@...ck.org>,
        <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "Andy Lutomirski" <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Dave Young" <dyoung@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: AMD SME encrpytion and PCI BAR pages to user space

On Wed, Oct 21, 2020 at 10:30:23AM -0500, Tom Lendacky wrote:
> On 10/21/20 6:59 AM, Jason Gunthorpe wrote:
> > On Mon, Oct 19, 2020 at 11:36:16AM -0500, Tom Lendacky wrote:
> > 
> >>> io_remap_pfn_range()? Is there use cases where a caller actually wants
> >>> encrypted io memory?
> >>
> >> As long as you never have physical memory / ram being mapped in this path,
> >> it seems that applying pgprot_decrypted() would be ok.
> > 
> > I made a patch along these lines:
> > 
> > https://github.com/jgunthorpe/linux/commit/fc990842983f3530b72fcceafed84bd6075174a1
> > 
> > Just waiting for the 0-day bots to check it
> > 
> > I now have a report that SME works OK but when the same test is done
> > inside a VM with SEV it fails again - is there something else needed
> > for the SEV case?
> 
> Probably. I would assume that it is getting past the MMIO issue, since the
> above patch should cover SEV, too. But, with SEV, all DMA to and from the
> guest is unencrypted. I'm not familiar with how the DMA is setup and
> performed in this situation, but if the DMA is occurring to userspace
> buffers that are mapped as encrypted, then the resulting access will be
> ciphertext (either reading unencrypted data from the device as encrypted
> or writing encrypted data to the device that should be unencrypted). There
> isn't currently an API to allow userspace to change its mapping from
> encrypted to unencrypted.

Oh, interesting.. Yes the issue is no userspace DMA stuff uses the DMA
API correctly (because it is in userspace)

So SWIOTLB tricks don't work, I wish the dma_map could fail for these
situations

I would have guessed it used some vIOMMU and setup decrpytion just
like the host does..

Thanks,
Jason