Linux has a shim problem. Which naturally leads to a reasonable question: What’s a shim, and why do we need it? The answer: Making Linux work wit Secure Boot, and an unintended quirk of the GPLv3.
Secure Boot is the verification scheme in modern machines that guarantees that only a trusted OS can boot. When Secure Boot was first introduced, many Linux fans suggested it was little more than an attempt to keep Linux distros off of consumer’s machines. That fear seems to have been unwarranted, as Microsoft has dutifully kept the Linux Shim signed, so we can all run Linux distros on our Secure Boot machines.
So the shim. It’s essentially a first-stage bootloader, that can boot a signed GRUB2 or other target. You might ask, why can’t we just ask Microsoft to sign GRUB2 directly? And that’s where the GPLv3 comes in. That license has an “anti-tivoization” section, which specifies “Installation Information” as part of what must be provided as part of GPLv3 compliance. And Microsoft’s legal team understands that requirement to apply to even this signing process. And it would totally defeat the point of Secure Boot to release the keys, so no GPLv3 code gets signed. Instead, we get the shim.
Now that we understand the shim, let’s cover how it’s broken. The most serious vulnerability is a buffer overflow in the HTTP file transfer code. The buffer is allocated based on the size in the HTTP header, but a malicious HTTP server can set that value incorrectly, and the shim code would happily write the real HTTP contents past the end of that buffer, leading to arbitrary code execution. You might ask, why in the world does the shim have HTTP code in it at all? The simple answer is to support UEFI HTTP Boot, a replacement for PXE boot.
The good news is that this vulnerability can only be triggered when using HTTP boot, and only by connecting to a malicious server or via a man-in-the-middle attack. With this in mind, it’s odd that this vulnerability is rated a 9.8. Specifically, it seems incorrect that this bug is rated low complexity, or a general network attack vector. In Red Hat’s own write-up of the vulnerability, they argue that the exploitation is high complexity, and is only possible from an adjacent network. There were a handful of lesser vulnerabilities found, and these were all fixed with shim 15.8. Continue reading “This Week In Security: Broken Shims, LassPass, And Toothbrushes?”
