linux-kernel - Re: [PATCH v4 6/6] virt: tdx-guest: Add Quote generation support using TSM

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Date:   Thu, 28 Sep 2023 15:49:00 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Peter Gonda <pgonda@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>
CC:     <linux-coco@...ts.linux.dev>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Erdem Aktas <erdemaktas@...gle.com>, <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v4 6/6] virt: tdx-guest: Add Quote generation support
 using TSM_REPORTS

Peter Gonda wrote:
> On Mon, Sep 25, 2023 at 10:17 PM Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> >
> > In TDX guest, the attestation process is used to verify the TDX guest
> > trustworthiness to other entities before provisioning secrets to the
> > guest. The first step in the attestation process is TDREPORT
> > generation, which involves getting the guest measurement data in the
> > format of TDREPORT, which is further used to validate the authenticity
> > of the TDX guest. TDREPORT by design is integrity-protected and can
> > only be verified on the local machine.
> >
> > To support remote verification of the TDREPORT in a SGX-based
> > attestation, the TDREPORT needs to be sent to the SGX Quoting Enclave
> > (QE) to convert it to a remotely verifiable Quote. SGX QE by design can
> > only run outside of the TDX guest (i.e. in a host process or in a
> > normal VM) and guest can use communication channels like vsock or
> > TCP/IP to send the TDREPORT to the QE. But for security concerns, the
> > TDX guest may not support these communication channels. To handle such
> > cases, TDX defines a GetQuote hypercall which can be used by the guest
> > to request the host VMM to communicate with the SGX QE. More details
> > about GetQuote hypercall can be found in TDX Guest-Host Communication
> > Interface (GHCI) for Intel TDX 1.0, section titled
> > "TDG.VP.VMCALL<GetQuote>".
> >
> > Trusted Security Module (TSM) [1] exposes a common ABI for Confidential
> > Computing Guest platforms to get the measurement data via ConfigFS.
> > Extend the TSM framework and add support to allow an attestation agent
> > to get the TDX Quote data (included usage example below).
> >
> >   report=/sys/kernel/config/tsm/report/report0
> >   mkdir $report
> >   dd if=/dev/urandom bs=64 count=1 > $report/inblob
> >   hexdump -C $report/outblob
> >   rmdir $report
> >
> > GetQuote TDVMCALL requires TD guest pass a 4K aligned shared buffer
> > with TDREPORT data as input, which is further used by the VMM to copy
> > the TD Quote result after successful Quote generation. To create the
> > shared buffer, allocate a large enough memory and mark it shared using
> > set_memory_decrypted() in tdx_guest_init(). This buffer will be re-used
> > for GetQuote requests in the TDX TSM handler.
> >
> > Although this method reserves a fixed chunk of memory for GetQuote
> > requests, such one time allocation can help avoid memory fragmentation
> > related allocation failures later in the uptime of the guest.
> >
> > Since the Quote generation process is not time-critical or frequently
> > used, the current version uses a polling model for Quote requests and
> > it also does not support parallel GetQuote requests.
> >
> > Link: https://lore.kernel.org/lkml/169342399185.3934343.3035845348326944519.stgit@dwillia2-xfh.jf.intel.com/ [1]
> > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> > Reviewed-by: Erdem Aktas <erdemaktas@...gle.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> 
> Hey Dan,
> 
> I tried running your test commands on an SNP enabled guest. To build
> the kernel I just checked out linus/master and applied your series. I
> haven't done any debugging yet, so I will update with what I find.
> 
> root@...ntu2004:~#   hexdump -C $report/outblob
> [  219.871875] ------------[ cut here ]------------
> [  219.876642] kernel BUG at include/linux/scatterlist.h:187!

Ok, it does not like virtual address of one of the buffers, but my
changes "should" not have affected that as get_ext_report() internally
uses snp_dev->certs_data and snp_dev->response for bounce buffering the
actual request / response memory. First test I want to try once I can
get on an SNP system is compare this to the ioctl path just make sure
that succeeds.