Re: [PATCH v8 14/15] x86: Secure Launch late initcall platform module

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On 2/15/24 03:40, Ard Biesheuvel wrote:
On Wed, 14 Feb 2024 at 23:32, Ross Philipson <ross.philipson@xxxxxxxxxx> wrote:

From: "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>

The Secure Launch platform module is a late init module. During the
init call, the TPM event log is read and measurements taken in the
early boot stub code are located. These measurements are extended
into the TPM PCRs using the mainline TPM kernel driver.

The platform module also registers the securityfs nodes to allow
access to TXT register fields on Intel along with the fetching of
and writing events to the late launch TPM log.

Signed-off-by: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: garnetgrimm <grimmg@xxxxxxxxxxxx>
Signed-off-by: Ross Philipson <ross.philipson@xxxxxxxxxx>

There is an awful amount of code that executes between the point where
the measurements are taken and the point where they are loaded into
the PCRs. All of this code could subvert the boot flow and hide this
fact, by replacing the actual taken measurement values with the known
'blessed' ones that will unseal the keys and/or phone home to do a
successful remote attestation.

To set context, in general the motivation to employ an RTM, Static or Dynamic, integrity solution is to enable external platform validation, aka attestation. These trust chains are constructed from the principle of measure and execute that rely on the presence of a RoT for Storage (RTS) and a RoT for Reporting (RTR). Under the TCG architecture adopted by x86 vendors and now recently by Arm, those roles are fulfilled by the TPM. With this context, lets layout the assumptive trusts being made here,
  1. The CPU GETSEC instruction functions correctly
  2. The IOMMU, and by extension the PMRs, functions correctly
  2. The ACM authentication process functions correctly
  3. The ACM functions correctly
  4. The TPM interactions function correctly
  5. The TPM functions correctly

With this basis, let's explore your assertion here. The assertion breaks down into two scenarios. The first is that the at-rest kernel binary is corrupt, unintentionally (bug) or maliciously, either of which does not matter for the situation. For the sake of simplicity, corruption of the Linux kernel during loading or before the DRTM Event is considered an equivalent to corruption of the kernel at-rest. The second is that the kernel binary was corrupted in memory at some point after the DRTM event occurs.

For both scenarios, the ACM will correctly configure the IOMMU PMRs to ensure the kernel can no longer be tampered with in memory. After which, the ACM will then accurately measure the kernel (bzImage) and safely store the measurement in the TPM.

In the first scenario, the TPM will accurately report the kernel measurement in the attestation. The attestation authority will be able to detect if an invalid kernel was started and can take whatever remediation actions it may employ.

In the second scenario, any attempt to corrupt the binary after the ACM has configured the IOMMU PMR will fail.


At the very least, this should be documented somewhere. And if at all
possible, it should also be documented why this is ok, and to what
extent it limits the provided guarantees compared to a true D-RTM boot
where the early boot code measures straight into the TPMs before
proceeding.

I can add a rendition of the above into the existing section of the documentation patch that already discusses separation of the measurement from the TPM recording code. As to the limits it incurs on the DRTM integrity, as explained above, I submit there are none.

v/r,
dps

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