[personal profile] mjg59
A (well, now former) coworker let me know about a problem he was having with a Lenovo Thinkcentre M92p. It booted Fedora UEFI install media fine, but after an apparently successful installation refused to boot. UEFI installs on Windows worked perfectly. Secure Boot was quickly ruled out, but this could still have been a number of things. The most interesting observation was that the Fedora boot option didn't appear in the firmware boot menu at all, but Windows did. We spent a little while comparing the variable contents, gradually ruling out potential issues - Linux was writing an entry that had an extra 6 bytes in a structure, for instance[1], and a sufficiently paranoid firmware implementation may have been tripping up on that. Fixing that didn't help, though. Finally we tried just taking the Windows entry and changing the descriptive string. And it broke.

Every UEFI boot entry has a descriptive string. This is used by the firmware when it's presenting a menu to users - instead of "Hard drive 0" and "USB drive 3", the firmware can list "Windows Boot Manager" and "Fedora Linux". There's no reason at all for the firmware to be parsing these strings. But the evidence seemed pretty strong - given two identical boot entries, one saying "Windows Boot Manager" and one not, only the first would work. At this point I downloaded a copy of the firmware and started poking at it. Turns out that yes, actually, there is a function that compares the descriptive string against "Windows Boot Manager" and appears to return an error if it doesn't match. What's stranger is that it also checks for "Red Hat Enterprise Linux" and lets that one work as well.

This is, obviously, bizarre. A vendor appears to have actually written additional code to check whether an OS claims to be Windows before it'll let it boot. Someone then presumably tested booting RHEL on it and discovered that it didn't work. Rather than take out that check, they then addded another check to let RHEL boot as well. We haven't yet verified whether this is an absolute string match or whether a prefix of "Red Hat Enterprise Linux" is sufficient, and further examination of the code may reveal further workarounds. For now, if you want to run Fedora[2] on these systems you're probably best off changing the firmware to perform a legacy boot.

[1] src/include/efi.h: uint8_t padding[6]; /* Emperically needed */, says the efibootmgr source code. Unhelpful.
[2] Or Ubuntu, or Suse, or…

Re: Languages

Date: 2012-11-16 08:03 pm (UTC)
From: [identity profile] cxl [launchpad.net]
That's correct. My Chinese version of Windows 8 also names the UEFI entry "Windows Boot Manager".

Re: Languages

Date: 2013-08-29 10:43 am (UTC)
From: (Anonymous)
So, the poetic justice here is that, when somebody at Microsoft in the i18n department sends out a memo that 'Windows Boot Manager' is henceforth to be internationalized into the appropriate local language-glyphs, thereafter Windows 9 would refuse to install on the oh-sorry-purely-by-mistake 'buggy' firmware out there which only works when the bootstring is hardcoded in the usual English form.

Might even happen during the next win8 service pack, I suppose. Are those auto-download nowadays, installed behind the scenes at 3am by windows update? All the Lenovo and MSI and other boxen with such 'bugs' might wake up one Patch Tuesday unable to boot except in US/UK/Canada/Aussie/etc countries. Good thing we don't spell it manageur-vs-manager!


Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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