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

[Peter Gonda]

On Tue, Oct 3, 2023 at 1:29 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> On 10/3/2023 11:37 AM, Peter Gonda wrote:
> > On Fri, Sep 29, 2023 at 11:26 AM Peter Gonda <pgonda@xxxxxxxxxx> wrote:
> >>
> >> On Thu, Sep 28, 2023 at 4:49 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >>>
> >>> Peter Gonda wrote:
> >>>> On Mon, Sep 25, 2023 at 10:17 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >>>>>
> >>>>> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> >>>>>
> >>>>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx/ [1]
> >>>>> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> >>>>> Reviewed-by: Erdem Aktas <erdemaktas@xxxxxxxxxx>
> >>>>> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> >>>>
> >>>> 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.
> >
>
> Dan, do you think it is related to not allocating direct mapped memory (using
> kvalloc)?

But I think the issue is the stack allocated variable 'ext_req' here:

sev_report_new()
+       void *buf __free(kvfree) = kvzalloc(size, GFP_KERNEL);
+       if (!buf)
+               return -ENOMEM;
+
+       guard(mutex)(&snp_cmd_mutex);
+       certs_address = buf + report_size;
+       struct snp_ext_report_req ext_req = {
+               .data = { .vmpl = desc->privlevel },
+               .certs_address = (__u64)certs_address,
+               .certs_len = ext_size,
+       };
+       memcpy(&ext_req.data.user_data, desc->inblob, desc->inblob_len);


>
> --
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer