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Jonathon Hall

Latest posts by Jonathon Hall (see all)

• Unexplained Potatoes - April 22, 2024
• PureBoot Not Vulnerable to UEFI Exploits (Again) - December 11, 2023
• 3D Print a Modular Librem 11 Back Cover - December 5, 2023

Through June and July, AMI MegaRAC BMC firmware has been in the news owing to high-severity vulnerabilities that have existed for years but only recently come to light.  Server hardware widely contains BMCs, and compromising one can give near-total control over the server.  Many vendors shipped this firmware with their own branding.

But what exactly is a BMC, and what can we do to stay secure?

Baseboard Management Controller

A BMC, or “Baseboard Management Controller”, manages a server remotely.  Physical access to servers is typically tightly controlled (or at least very inconvenient).  But like any computer, it could go offline, fail to boot, or otherwise misbehave.  If this was your PC at home, you’d go sit down at the keyboard and monitor to troubleshoot it, and maybe reinstall the OS from USB, but on a server we want to do this remotely.

That’s where the BMC comes in.  One of its key functions is to make the “physical” keyboard, monitor, and power button, available remotely over a web interface.  Often, you can attach a local ISO as a virtual USB disk, and the remote system thinks it is a real USB disk.  Remote serial ports and many other features are possible too.

All of this works if the OS’s networking is broken, or even if the entire OS is unbootable.  The BMC may even be able to flash the system BIOS, allowing recovery from a failed flash or broken firmware.  You may have done this before for virtual machines – the BMC brings remote management to physical systems.

The BMC is a second, smaller computer hanging off of the main computer.  Much like the embedded controller in a laptop, it is in charge of powering on or off the main system.  The BMC is always on when the server is plugged in, like the EC.  It is a complete computer, commonly containing an ARM CPU, DRAM, and its own firmware.

Bring Many Connections

To provide all these features, the BMC connects to many buses from the main system.  The BMC firmware fully controls some interfaces, while hardware functionality controls others.

• PCI Express: The BMC contains a PCIe video card.  It stores the framebuffer in BMC memory as well as scanning it out to a physical monitor, if there is one.  The BMC can observe the framebuffer and provide it over a network connection.
• LPC (“Low Pin Count”): The BMC can provide serial ports and snoop on POST codes from the main system.
• USB: The BMC acts as a USB keyboard, mouse, or storage device.
• SPI: The BMC may sit “between” the main system and its BIOS firmware chip, providing a way to flash the BIOS when the main system is offline.
• Ethernet: BMCs often can share the host’s physical network interface, eliminating an extra cable.  If they use separate networks, the BMC may share its network interface to the host over USB.

The main system is unlikely to consider any of these buses to be security boundaries.  The BMC has great power to lie about what is on the display, hide it from a physical monitor, or act as a malicious USB device.

Be Maximally Cautious

Purism disables BMC functionality on Librem Servers where possible.  PureBoot, when combined with a Librem Key, authenticate the system firmware to you, which complicates any tampering attack.

Purism Products and Availability Chart

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	Librem Server	Apr 2024 ($2,999+)	Shipping April, 2024	Learn More Buy Now