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From:   Peter Gonda <pgonda@google.com>
Date:   Tue, 3 Oct 2023 12:37:00 -0600
Message-ID: <CAMkAt6rwSwES_hEac=_7Gg8wcGosHMPXA3J2g_BGC_c+NbVp1w@mail.gmail.com>
Subject: Re: [PATCH v4 6/6] virt: tdx-guest: Add Quote generation support
 using TSM_REPORTS
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     linux-coco@lists.linux.dev,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Erdem Aktas <erdemaktas@google.com>, peterz@infradead.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        dave.hansen@linux.intel.com
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On Fri, Sep 29, 2023 at 11:26=E2=80=AFAM Peter Gonda <pgonda@google.com> wr=
ote:
>
> On Thu, Sep 28, 2023 at 4:49=E2=80=AFPM Dan Williams <dan.j.williams@inte=
l.com> wrote:
> >
> > Peter Gonda wrote:
> > > On Mon, Sep 25, 2023 at 10:17=E2=80=AFPM Dan Williams <dan.j.williams=
@intel.com> wrote:
> > > >
> > > > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.=
intel.com>
> > > >
> > > > In TDX guest, the attestation process is used to verify the TDX gue=
st
> > > > trustworthiness to other entities before provisioning secrets to th=
e
> > > > guest. The first step in the attestation process is TDREPORT
> > > > generation, which involves getting the guest measurement data in th=
e
> > > > format of TDREPORT, which is further used to validate the authentic=
ity
> > > > 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 Encla=
ve
> > > > (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, t=
he
> > > > TDX guest may not support these communication channels. To handle s=
uch
> > > > cases, TDX defines a GetQuote hypercall which can be used by the gu=
est
> > > > to request the host VMM to communicate with the SGX QE. More detail=
s
> > > > about GetQuote hypercall can be found in TDX Guest-Host Communicati=
on
> > > > Interface (GHCI) for Intel TDX 1.0, section titled
> > > > "TDG.VP.VMCALL<GetQuote>".
> > > >
> > > > Trusted Security Module (TSM) [1] exposes a common ABI for Confiden=
tial
> > > > Computing Guest platforms to get the measurement data via ConfigFS.
> > > > Extend the TSM framework and add support to allow an attestation ag=
ent
> > > > to get the TDX Quote data (included usage example below).
> > > >
> > > >   report=3D/sys/kernel/config/tsm/report/report0
> > > >   mkdir $report
> > > >   dd if=3D/dev/urandom bs=3D64 count=3D1 > $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 co=
py
> > > > the TD Quote result after successful Quote generation. To create th=
e
> > > > shared buffer, allocate a large enough memory and mark it shared us=
ing
> > > > 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 fragmentat=
ion
> > > > related allocation failures later in the uptime of the guest.
> > > >
> > > > Since the Quote generation process is not time-critical or frequent=
ly
> > > > used, the current version uses a polling model for Quote requests a=
nd
> > > > it also does not support parallel GetQuote requests.
> > > >
> > > > Link: https://lore.kernel.org/lkml/169342399185.3934343.30358453483=
26944519.stgit@dwillia2-xfh.jf.intel.com/ [1]
> > > > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswa=
my@linux.intel.com>
> > > > Reviewed-by: Erdem Aktas <erdemaktas@google.com>
> > > > Signed-off-by: Dan Williams <dan.j.williams@intel.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@Ubuntu2004:~#   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.
>

I think there may be an issue with CONFIG_DEBUG_SG. That was the
warning we were getting in my above stack trace:

> [  219.876642] kernel BUG at include/linux/scatterlist.h:187!

This was for this line in enc_dec_message():

        sg_set_buf(&src[1], src_buf, hdr->msg_sz);

I am not sure why in sg_set_buf() virt_addr_valid() returns false for
the address given in the sev_report_new() which is from the variable
'ext_req' which is stack allocated?

static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
     unsigned int buflen)
{
#ifdef CONFIG_DEBUG_SG
    BUG_ON(!virt_addr_valid(buf));
#endif
    sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
}

When I disable CONFIG_DEBUG_SG in my config. Your patch seems to work,
well at least it doesn't crash the guest. I haven't checked if the
report is valid yet.