In an effort to really drag this topic out (it’s now a trilogy), I’ve taken my previous Azure specific post and also the AWS specific post and decided to do some further research into whether the same is true in Google Cloud Platform (a.k.a GCP).
(If I was writing this like a true screenwriter, it would get shorter and faster each recap).
In July 2020, a GRUB2 bootloader vulnerability was discovered which could allow attackers to replace the bootloader on a machine which has Secure Boot turned on.
The vulnerability is designated CVE-2020-10713 and is rated 8.2 HIGH on the CVSS (see here).
Let’s recap what this is (honestly, please see my Azure post for details, it’s quite technical), and how it impacts a GCP virtual machine running SUSE Enterprise Linux 12, which is commonly used to run SAP systems such as SAP HANA or other SAP products.
What is the Vulnerability?
Essentially, some evil input data can be entered into some part of the GRUB2 program binaries, which is not checked/validated.
By carefully crafting the data that is the overflow, it is possible to cause a specifically targeted memory area to be overwritten.
As described by Eclypsium here (the security company that detected this) “Attackers exploiting this vulnerability can install persistent and stealthy bootkits or malicious bootloaders that could give them near-total control over the victim device“.
Essentially, the vulnerability allows an attacker with root privileges to replace the bootloader with a malicious one.
What is GRUB2?
GRUB2 is v2 of the GRand Unified Bootloader (see here for the manual).
It can be used to load the main operating system of a computer.
What is Secure Boot?
There are commonly two boot methods: “Legacy Boot” and “Secure Boot” (a.k.a UEFI boot).
Until Secure Boot was invented, the bootloader would sit in a designated location on the hard disk and would be executed by the computer BIOS to start the chain of processes for the computer start up.
With Secure Boot, certificates are used to secure the boot process chain.
This BootHole vulnerability means a new CA certificate needs to be implemented in every machine that uses Secure Boot!
But the attackers Need Root?
Yes, the vulnerability is in a GRUB2 configuration text file owned by the root user. Additional text added to the file can cause the buffer overflow.
Anti-virus can’t remove the bootloader if the bootloader boots first and “adjusts” the anti-virus.
NOTE: The flaw also exists if you also use the network boot capability (PXE boot).
What is the Patch?
Due to the complexity of the problem (did you read the prior Eclypsium link?), it needs more than one piece of software to be patched and in different layers of the boot chain.
The vulnerable GRUB2 software needs patching.
To be able to stop the vulnerable version of GRUB2 being re-installed and used, three things need to happen:
- The O/S vendor (SUSE) needs to adjust their code (known as the “shim”) so that it no longer trusts the vulnerable version of GRUB2. Again, this is a software patch from the O/S vendor (SUSE) which will need a reboot.
- Since someone with root could simply re-install O/S vendor code (the “shim”) that trusts the vulnerable version of GRUB2, the adjusted O/S vendor code will need signing and trusting by the certificates further up the chain.
- The revocation list of Secure Boot needs to be adjusted to prevent the vulnerable version of the O/S vendor code (“shim”) from being called during boot. (This is known as the “dbx” (exclusion database), which will need updating with a firmware update).
What is SUSE doing about it?
There needs to be a multi-pronged patching process because SUSE also found some additional bugs during their analysis.
You can see the SUSE page on CVE-2020-10713 here, which includes the mention of the additional bugs.
How does this impact GCP VMs?
In the previous paragraphs we found that a firmware update is needed to update the “dbx” exclusion database.
Since GCP virtual machines are hosted in a KVM based hypervisor, the “firmware” is actually software.
Whilst looking for details on “Secure Boot” in GCP virtual machines, we come across the Google Compute Engine’s “Shielded VM” option.
You can read about it in detail here.
In brief, in GCP a Shielded VM is deployed using a pre-defined set of Google specific guest operating systems:
As noted above, the documentation specifically mentions that the “firmware” underpinning the virtual machine contains Google’s Certificate Authority (CA) certificate, as the root of the trust chain.
This is important because the Eclypsium description of the vulnerability is specifically citing a problem with the Microsoft CA.
What this means is that Google actually decide on the trust chain themselves and can probably more rapidly adjust the firmware with a new CA certificate.
To reiterate, this is specific to Google specific VM images that you deploy as a Shielded VM.
Another point worth noting is that when creating a Shielded VM, you can enable the vTPM (virtual trusted platform module), which allows integrity monitoring of the boot process. Any change to the boot process and a validation alert is triggered. Whilst this would not prevent compromise, it would at least alert an administrator.
Reading the Google infrastructure security document, we find that just like AWS, Google have designed and are implementing their own security chip called Titan, on the physical hosts. This is used to ensure that physical hosts boot securely, but it is not clear if this chip is used in anyway for Shielded VMs booted on the physical host.
If we delve further into the GCP documentation we find that we also have the option to create a custom image for deployment into a Shielded VM.
See the documentation on how to create a custom Shielded VM image:
The above states that you can create your own Secure Boot capable VM image for deployment in GCP as a Shielded VM.
If we read further down that page under section “Default certificates“, we find a slight difference compared to the Google “curated” images:
The above is telling us, by default the standard Microsoft CA certificates are used for the Secure Boot setup of VMs created using a custom image (remember non-custom Secure Boot images use Google’s root CA) in GCP.
When it says “default values”, right now, they are the only values because of a small note further up the page:
OK, so you can only use the defaults for now. The same compromised defaults that will need fixing. 🤷♂️
What do we think needs to happen once Google create the ability to replace the certificates?
From reading those previously mentioned documents, I would guess that to rebuild the certificate database used during the creation of the custom Shielded VM image, you are going to need to re-create the VM image and then re-deploy a VM from that image!
The question remains, is SLES 12 supported as a Shielded VM guest-OS on GCP?
According to the Shielded VM page here, it is not by default. You will need to therefore create your own image:
The BootHole vulnerability is far reaching and will impact many, many devices (servers, laptops, IoT devices, TVs, fridges, cars?).
However, only those devices that actually *use* Secure Boot will truly be impacted, since the devices not using Secure Boot do not need to be patched (it’s fruitless).
If you run SLES 12 on GCP virtual machines, using public images, then by default you will not being using the Shielded VM instances, so there is no point patching to fix a vulnerability for which you are not affected.
You are only introducing more risk by patching.
If however, you do decide to patch (even if you don’t need to) then follow the advice from SUSE and patch to fix GRUB2, the “shim” and the other vulnerabilities that were found.
On a final closing point, you could be running a custom SLES image deployed in GCP as a Shielded VM. An image that your company has built and which uses Secure Boot. You would be wise to contact your cloud administrators to ensure that they are preparing for a VM rebuild and subsequent patching required to ensure that Secure Boot remains secure.
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