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Re: [PATCH 0/3] SEV: fixes for -kernel launch with incompatible OVMF

From: Brijesh Singh
Subject: Re: [PATCH 0/3] SEV: fixes for -kernel launch with incompatible OVMF
Date: Tue, 2 Nov 2021 09:48:56 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 11/2/21 8:22 AM, Dov Murik wrote:

On 02/11/2021 12:52, Brijesh Singh wrote:
Hi Dov,

Overall the patch looks good, only question I have is that now we are
enforce qemu to hash the kernel, initrd and cmdline unconditionally for
any of the SEV guest launches. This requires anyone wanting to
calculating the expected measurement need to account for it. Should we
make the hash page build optional ?

The problem with adding a -enable-add-kernel-hashes QEMU option (or
suboption) is yet another complexity for the user.  I'd also argue that
adding these hashes can lead to a more secure VM boot process, so it
makes sense for it to be the default (and maybe introduce a
-allow-insecure-unmeasured-kernel-via-fw-cfg option to prevent the
measurement from changing due to addition of hashes?).

Maybe, on the other hand, OVMF should "report" whether it supports
hashes verification. If it does, it should have the GUID in the table
(near the reset vector), like the current OvmfPkg/AmdSev edk2 build. If
it doesn't support that, then the entry should not appear at all, and
then QEMU won't add the hashes (with patch 1 from this series).  This
means that in edk2 we need to remove the SEV Hash Table block from the
ResetVectorVtf0.asm for OvmfPkg, but include it in the AmdSev build.

By leaving it ON is conveying a wrong message to the user. The library used for verifying the hash is a NULL library for all the builds of Ovmf except the AmdSev package. In the NULL library case, OVMF does not perform any checks and hash table is useless. I will raise this on concern on your Ovmf patch series.

IMHO, if you want to turn it ON by default then make sure all the OVMF package builds supports validating the hash.

But the problem with this approach is that it prevents the future
unification of AmdSev and OvmfPkg, which is a possibility we discussed
(at least with Dave Gilbert), though not sure it's a good/feasible goal.

This is my exact concern, we are auto enabling the features in Qemu that is supported by AmdSev package only.

I am thinking this more for the SEV-SNP guest. As you may be aware that
with SEV-SNP the attestation is performed by the guest, and its possible
for the launch flow to pass 512-bits of host_data that gets included in
the report. If a user wants to do the hash'e checks for the SNP then
they can pass a hash of kernel, initrd and cmdline through a
launch_finish.ID_BLOCK.host_data and does not require a special hash
page. This it will simplify the expected hash calculation.

That is a new measured boot "protocol" that we can discuss, and see
whether it's better/easier than the existing one at hand that works on

What I don't understand in your suggestion is who performs a SHA256 of
the fw_cfg blobs (kernel/initrd/cmdline) so they can later be verified
(though ideally earlier is better).  Can you describe the details
(step-by-step) of an SNP VM boot with -kernel/-initrd/-append and how
the measurement/attestation is performed?

There are a multiple ways on how you can do a measured boot with the SNP.

1) VMPL0 (SVSM) can provide a complete vTPM (see the MSFT proposal on SNP mailing list).

2) Use your existing hashing approach with some changes to provide a bit more flexibility.

3) Use your existing hashing approach but zero out the hash page when -kernel is not used.

Let me expand #2.

While launching the SNP guest, a guest owner can provide a ID block that KVM will pass to the PSP during the guest launch flow. In the ID block there is a field called "host_data". A guest owner can do a hash of kernel/initrd/cmdline and include it in the "host_data" field. During the hash verification, the OVMF can call the SNP_GET_REPORT. The PSP will includes the "host_data" passed in the launch process in the report and OVMF can use it for the verification. Unlike the current implementation, this enables a guest owner to provides the hash without requiring any changes in the Qemu and thus affecting the measurement.

One thing to note that both #2 and #3 requires ovmf to connect to guest owner to validate the report before using the "host_data" or "hash page".


Adding a
special page requires a validation of that page. All the prevalidated
page need to be excluded by guest BIOS page validation flow to avoid the
double validation. The hash page is populated only when we pass -kernel
and it will be tricky to communicate this information to the guest BIOS
so that it can skip the validation.

So that again comes back to the earlier question of whether we should
always fill the hashes page or only sometimes, and how can OVMF tell.

How about: QEMU always prevalidates this page (either fills it with
zeros or with the hashes table), and the BIOS always excludes it?


Thoughts ?


On 11/1/21 5:21 AM, Dov Murik wrote:
Tom Lendacky and Brijesh Singh reported two issues with launching SEV
guests with the -kernel QEMU option when an old [1] or wrongly configured [2]
OVMF images are used.

The fixes in patches 1 and 2 allow such guests to boot by skipping the
kernel/initrd/cmdline hashes addition to the initial guest memory (and
warning the user).

Patch 3 is a refactoring of parts of the same function
sev_add_kernel_loader_hashes() to calculate all padding sizes at
compile-time.  This patch is not required to fix the issues above, but
is suggested as an improvement (no functional change intended).

Note that launch measurement security is not harmed by these fixes: a
Guest Owner that wants to use measured Linux boot with -kernel, must use
(and measure) an OVMF image that designates a proper hashes table area,
and that verifies those hashes when loading the binaries from QEMU via

The old OVMFs which don't publish the hashes table GUID or don't reserve
a valid area for it in MEMFD cannot support these hashes verification in
any case (for measured boot with -kernel).


Dov Murik (3):
   sev/i386: Allow launching with -kernel if no OVMF hashes table found
   sev/i386: Warn if using -kernel with invalid OVMF hashes table area
   sev/i386: Perform padding calculations at compile-time

  target/i386/sev.c | 34 +++++++++++++++++++++++-----------
  1 file changed, 23 insertions(+), 11 deletions(-)

base-commit: af531756d25541a1b3b3d9a14e72e7fedd941a2e

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