This blog contains experience gained over the years of implementing (and de-implementing) large scale IT applications/software.

Is my GCP hosted SLES 12 Linux VM Affected by the BootHole Vulnerability

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).

Previously

(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:

  1. 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.
  2. 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.
  3. 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:

Summary:

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.

Useful Links:

Is my AWS hosted SLES 12 Linux VM Affected by the BootHole Vulnerability

In an effort to spin this story out a little further, I’ve taken my previous Azure specific post and decided to do some further research into whether the same is true in Amazon Web Services (a.k.a AWS).

Previously

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 other post for details, it’s quite technical), and how it impacts an AWS 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:

  1. 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.
  2. 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.
  3. 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 AWS VMs?

In the previous paragraphs we found that a firmware update is needed to update the “dbx” exclusion database.
Since AWS virtual machines are hosted in a KVM based hypervisor, the “firmware” is actually software.

Whilst looking for details on “Secure Boot” in AWS virtual machines, there is absolutely no mention of it being supported for Linux.
If we dig into the the VM import/export documents here on the AWS docs site, we find:

So the above states that for VMs imported/exported, “UEFI/EFI boot partitions are supported only for Windows boot volumes with VHDX as the image format. Otherwise, a VM’s boot volume must use Master Boot Record (MBR) partitions.“.
The words “…only for Windows…” are the key part of this.
Because if we scan just a little further down the page, it says that the UEFI boot partitions are actually “supported” for Windows, by being converted to MBR (not Secure Boot compatible):

I feel we can surmise that AWS does not support running Linux VMs with Secure Boot.
Apart from this little gem of information here.
This slide shows that the launch of the AWS Graviton2 chip enables ARM based Linux distributions to support Secure Boot.
We can read the Amazon EC2 User Guide here (updated August 28, 2020), to find that SLES 15 is the only SUSE Linux that supports ARM cpus on AWS:

So we know that Secure Boot is not available in AWS on any of the SLES x86 operating systems, and SLES 12 on ARM is not supported on Graviton based cpus.

Summary:

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 AWS virtual machines, you cannot possibly use Secure Boot, 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.

If you are running SLES 12 on AWS, then there is no specific order of patching, because you do not use Secure Boot, so there is no possibility of breaking the trust chain that doesn’t exist.

On a final closing point, you could be running a HANA system in AWS on what is known as “Bare Metal” (“High Memory Instances” or a.k.a “*.metal”). These are physical machines using the Nitro based hyper-visor. So whilst EC2 Virtual Machines can’t use Secure Boot, these “Bare Metal” machines may well do so through the use of the Nitro Security Chip (see a good deep dive here). You would be wise to contact your AWS account representative to establish if they will be patching the firmware.

Useful Links:

Is my Azure hosted SLES 12 Linux VM Affected by the BootHole Vulnerability

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 look at what this is and how it impacts a Microsoft Azure 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?

It is a “Classic Buffer Overflow” vulnerability in the GRUB2 bootloader for versions prior to 2.06.
Essentially, some evil input data can be entered into some part of the GRUB2 program binaries, which is not checked/validated.
The input data causes an overflow of the holding memory area into adjacent memory areas.
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, boot into it and then have further capability to effectively set up camp (a backdoor) on the server.
This backdoor would be hard to remove because the bootloader is one of the first things to be booted (anti-virus can’t remove the bootloader if the bootloader boots first and “adjusts” the anti-virus).

What is GRUB2?

GRUB2 is v2 of the GRand Unified Bootloader (see here for the manual).
It is used to load the main operating system of a computer.
Usually on Linux virtual machines, GRUB is used to load Linux. It is possible to install GRUB on machines that then boot into Windows.

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.
This is clearly quite insecure, since any program could put itself at the designated location and then be executed at boot up.

With Secure Boot, certificates are used to secure the boot process chain.
As with any certificate based process, at the top (root) level there needs to exist a certificate which is valid for many years and is ultimately trusted – the Certificate Authority (CA).
The next levels in the chain trust that CA certificate implicitly and if any point in the chain is compromised, then the trust is broken and will need re-establishing with new certificates.
Depending which level of the chain is compromised, will dictate the amount of effort needed to fix it.

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.
Once the attacker has used malware to instigate the overflow, and installed a malicious bootloader, they then have a backdoor to the server, which would be executed every time the server is rebooted.
This backdoor would be hard to remove because the bootloader is one of the first things to be booted (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.

First off, the vulnerable GRUB2 software needs patching; this is quite easy and will require a reboot of the Linux O/S.
The problem with patching just GRUB2, is that it is still possible for an attacker with root to re-install a vulnerable version of GRUB2 and then use that vulnerable version to compromise the system further.
Remember, the chain of trust is still trusting that vulnerable version of GRUB2.
Therefore, to be able to stop the vulnerable version of GRUB2 being re-installed and used, three things need to happen:

  1. 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.
  2. 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.
  3. 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.

They key point is that you *could* start patching, but if it were me, I would be tempted to wait until the SUSE “shim” has been updated with the new chain certificate, patch GRUB2 and then update the “dbx”.

How does this impact Azure VMs?

In the previous paragraphs we found that a firmware update is needed to update the “dbx” exclusion database.
Since Microsoft Azure is using the Hyper-V hypervisor, the “firmware” is actually software in Hyper-v.
See here, which says: “Secure Boot or UEFI firmware isn’t required on the physical Hyper-V host. Hyper-V provides virtual firmware to virtual machines that is independent of what’s on the Hyper-V host.

So the above would indicate that the Virtual Machine contains the necessary code from Hyper-V.
I would imagine that this is included at VM creation time.

If we dig into the VM details a little bit here on the Microsoft sites, we find:

So the above states that “…generation 2 VMs in Azure do not support Secure Boot…“.
The words “…in Azure…” are the key part of this.

OK, then how about Hyper-V in general (on-premise):

The above states “To Secure Boot generation 2 Linux virtual machines, you need to choose the UEFI CA Secure Boot template when you create the virtual machine.“.
BUT this is for Hyper-V in general, not for Azure virtual machines.

So we know that Secure Boot is not available in Azure on any of the generation 1 or generation 2 VMs (as of writing there are only 2).

Summary:

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 Azure virtual machines, you cannot possibly use Secure Boot, 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.

If you are running SLES on Azure, then there is no specific order of patching, because you do not use Secure Boot, so there is no possibility of breaking the trust chain that doesn’t exist.

On a final closing point, you could be running a HANA system in Azure on what is known as “HANA Large Instances” (HLI). These are physical machines. So whilst Virtual Machines can’t use Secure Boot, these physical machines may well do so. You would be wise to contact your Microsoft account representative to establish if they will be patching the firmware.

Useful Links:

Making saptune Actually Work & Patching to v2

Having recently spent some time analysing the performance of a HANA database system, I got down to the depths of Linux device I/O performance on an Azure hosted VM.

There was no reason to suspect any issue, because during the implementation of the VM image build process, we had followed all the relevant SAP notes.
In our case, on SUSE Enterprise Linux for SAP 12, we were explicitly following SAP Note 1275776 “Linux: Preparing SLES for SAP environments”.
Inside that SAP note, you go through the process of understanding the difference between sapconf and saptune, plus actually configure saptune (since it comes automatically with the “for SAP” versions of SLES 12).

Once configured, saptune should apply all the best practices that are encompassed in a number of SAP notes including SAP Note 2205917 “SAP HANA DB: Recommended OS settings for SLES 12 / SLES for SAP Applications 12”, which is itself needed during the HANA DB installation preparation work.
If you follow the note, there are a number of required O/S adjustments that are needed for HANA, which can be either applied manually, or (as recommended) automatically via saptune, provided the correct saptune profile is selected.

As part of our configuration, we had applied saptune solution profile S4HANA-DBSERVER (also noted in the SUSE documentation for SAP HANA).
This is applied using the standard:

saptune solution apply S4HANA-DBSERVER

You don’t get a lot of feedback from the saptune execution, but the fact there are no errors, indicates (normally) that it has done what has been requested.
You can check it has applied the profile by executing:

saptune solution list

The item that is starred in the returned list, is the profile that has been applied.
That’s it.

As part of my troubleshooting I even took the trouble of running the publicly available script sapconf_saptune_check (see here: https://github.com/scmschmidt/sapconf_saptune_check/blob/master/sapconf_saptune_check ), which just confirmed that saptune was indeed active/enabled and had a valid profile configured:

Back to the task of checking out the performance issue, and you can probably see where this is going now.
On investigation of the actual saptune profile contents, it was possible to see that a large majority of O/S changes had not been applied.
Specifically, we were not seeing the NOOP scheduler selected for the HANA disks devices.

By executing either of the following, you can check the currently selected scheduler:

grep -l ‘.*’ /sys/block/s??/queue/scheduler

or

cat /sys/block/s??/queue/scheduler

The selected scheduler will be in square brackets.
In my case, I was seeing “[cfq]” for all devices. Not good and not the recommendation from SAP and SUSE.
This setting should be automatically adjusted by the tuned daemon.

Looking at my version of saptune, I could see it was version 1.1.7 (from the output of the execution of the sapconf_saptune_check script).

Reading some of the recent blog posts from Soeren Schmidt here: https://blogs.sap.com/2019/05/03/a-new-saptune-is-knocking-on-your-door/
I could see that version 2 of saptune was now released.

Downloading the newer version (not installing directly!), reverting the old solution profile, installing the new saptune version and finally re-applying the same profile, confirmed that saptune was the culprit.

The new saptune2 fixed the issue, immediately activating a number of critical O/S changes, including the NOOP scheduler setting on each device.

The moral of the story, is therefore that as well as following the SAP processes, you still need to actually validate what it says it should have done.
The new saptune2 has been incorporated into our build process, plus the configuration check scripts will be specifically checking for it.
However, since the upgrade from saptune1 to saptune2 could cause issues if it just blindly re-applied the “new” profile settings, SAP have made saptune follow a backwards compatible upgrade process, whereby the O/S settings are retained as they were before the upgrade was executed.

Therefore, as per the SAP Note 2816790 “Differences between sapconf and saptune” links, the upgrade process for an already applied profile, is to revert it prior to the saptune upgrade, then applied the upgrade, then re-apply.
This could therefore not just be rolled out via our standard SLES patching routine. We had to develop an automated script that would specifically pre-patch saptune to saptune2 using the correct procedure, before we embarked on the next SLES patching round.

As a post-note, you should make yourself familiar with the coming changes to the SLES scheduler settings, with the introduction of the NONE scheduler (see below links for link to the blog).

Useful notes/links:
https://www.suse.com/c/sles-1112-os-tuning-optimisation-guide-part-1/
https://blogs.sap.com/2019/06/25/sapconf-versus-saptune-in-more-detail/
https://blogs.sap.com/2019/05/03/a-new-saptune-is-knocking-on-your-door/
https://www.suse.com/c/noop-now-named-none/

HowTo: Find the Datacentre Region and Physical Host of your Azure VM

With VMs hosted in Azure you need a fine balance between protection from hardware failure on the underlying Azure platform, plus performance from having the tiers of your SAP application being physically close together.

For this very purpose, Microsoft introduced Proximity Placement Groups (PPGs) to allow an administrator to ensure that specific tiers (e.g. application and database) are located close. Potentially even in the same server rack.
The PPGs also affect the location of the storage assigned to the VMs, although the storage infrastructure is actually transparent to administrators.

The PPGs still allow Azure to honor the Availability Sets, Fault Domains and Update Domains.

In this post, I show a method of finding the physical hostname of your Linux VM which could be part of a check before/after implementing a PPG.
NOTE: PPGs should be created at the time a VM is created, and assigned to the “lead” system of the rarest size. Example, an M-series VM is rare, so this should be the lead system when creating the PPG. This will anchor the other VMs to this M-series VMs location.

A separate post shows how to do this for a Windows VM.
On a Linux VM in Azure, as any Linux user, you can use the following to see the name of the physical host on which your VM is running:

awk -F 'H' '{ sub(/ostName/,"",$2); print $2 }' /var/lib/hyperv/.kvp_pool_3

Example output: DUB012345678910

In this case, we take the first 3 chars to be “Dublin”, which is in the EU North Azure region.
The remaining characters consist of the rack and physical hostname.

If you have 2 VMs in the same rack on the same physical host, then you will have minimal latency for networking between them.

Conversely, if you have 2 VMs on the same physical host, you are open to HA issues.

Therefore, you need a good balance for SAP.
You should expect to see SAP S/4HANA application servers and HANA DBs in the same Proximity Placement Groups, within the same rack, even potentially on the same host (providing you have availability sets across the tiers you will be safe).

Update: 23-Apr-2020
To get the above script output into a bash variable, the output contains hidden characters, we can use the following:

awk '{ gsub(/[^[:print:]]/,""); split ($0,a,"H"); sub(/ostName/,"",a[2]); print a[2]}' /var/lib/hyperv/.kvp_pool_3

Update: 05-Oct-2020
I have since found that there is another location where the above information can also be found.
Depending on your Linux O/S, you may also find the physical server name in the network scripts as follows:

grep BOOTSERVERNAME /var/run/netconfig/eth0/netconfig0

The aboe will return something like:
BOOTSERVERNAME=’AMS072nnnnnnnnn’