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CHERI-flavored computer runs KDE for the first time • The Register


Wayland and the KDE Plasma desktop now run on CheriBSD, the special version of FreeBSD for Arm’s experimental Morello hardware.

The University of Cambridge’s Capability Hardware Enhanced RISC Instructions project, or CHERI for short, has been going on for several years, and usable results are starting to emerge. It aims to bake more hardware-level security protections into processors, and Arm’s Morello board includes CHERI’s research work.

And Ruslan Bukin, a researcher in Cambridge’s computer science department and also a FreeBSD contributor, is now porting the Wayland display server and KDE desktop to CheriBSD. As he said:

A few years ago in 2019, we reported when the project got government funding, and earlier this year, when the aforementioned Arm prototype hardware started shipping. Arriving six months later, this experimental port is an important step forward and a good sign.

The CHERI project originally focused on the MIPS processor architecture, but recently shifted its focus to include RISC-V and Arm as well. Not only is Arm an extremely important processor architecture these days, but since Arm Ltd started as a branch of Acorn Computers, it is also headquartered in Cambridge.

Digital security by hardware design

CHERI brings to modern processors two features of hardware-enforced safety and protection that were part of some computer designs in the early days: a tagged memory architecture and based on response capabilities.

Competencies a protection mechanism implemented in the hardware parts of some computers, such as Burroughs’ large systems – its descendants still exist today – and IBM’s early System/38 minicomputer. These systems evolved before the rise of Unix and Unix-like systems.

The S/38 evolved into the AS/400, now known as the IBM i, but the designers of those later systems dropped the security mechanism. Similarly, the Multics OS that inspired the creation of Unix has some similar features, but this is one of the things that Dennis Ritchie and Ken Thompson left behind in their smaller, simpler system.

The boffins at Cambridge’s Computer Laboratory worked on a way to add capabilities in a Unix-compatible way, and it’s called Capsicum, which has been part of FreeBSD since version 9.

The new desktop stack runs an experimental OS derived from FreeBSD called CheriBSD, which can use the hardware facilities of CHERI-enhanced Arm and RISC-V processors.

The project has an FAQ that explains a few more, as well as some less technical articles about the design and the OS, though it’s not exactly light reading. We highly recommend Chapter 13, Historical Context and Related Work, of this technical report [PDF]however.

Processor and compiler expert Mark Morgan Lloyd summed it up for us: “They’re trying not to be too rude, but they’re sure they think the industry is taking a wrong turn away from fine-grained hardware protection.”

In older systems, such as Multics, code running on a computer processor must run in one of several rings: the inner rings have more permissions and controls, and the outer rings have fewer. This rings-of-protection approach is also used by, for example, Intel chips, with a limited, simpler version, as we explained in our brief history of virtualization back in 2011. Most PC OSes never feature, chose instead to use the CPU’s memory management unit and page tables to primarily implement access protections.

CHERI brings a more granular level of protection. Programs may be limited to access only certain permitted areas of memory, in certain limited ways, and special hardware tags in memory areas to limit what they can use – whatever the OS’s security mechanisms can be tricked into believing.

CHERI doesn’t make computers cheaper or faster, breaking the pattern of many modern hardware developments. But if it succeeds in its goals, CHERI-flavored computers are more resistant to exploitation than ordinary ones. We suspect many organizations will be happy to pay for that. ®



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