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If that occurs in the wild, various folks believe that Microsoft willblacklist the bootloader that was used in the attack. If that's the samebootloader used to boot Linux, new systems, as well as old systems that getthe blacklist update, will no longer boot Linux. Matthew Garrett (atleast) is concerned about that scenario, so he has proposed kernel changes that would preventsuitably configured kernels from using kexec() among a handful ofother restrictions that could be used to circumvent secure boot.
That was back in early September, and those changes were relativelyuncontroversial except for the capability name Garrett chose(CAP_SECURE_FIRMWARE) and the kexec() restriction. Inmid-September, he followed up with a set ofpatches that were substantially the same, though the kexec()patch was removed and the capability was renamed toCAP_COMPROMISE_KERNEL. In the patch, he noted that "ifanyone wants to deploy these then they should disable kexec until support for signedkexec payloads has been merged".
Things went quiet for more than a month, but have since erupted into arather large thread. A query from JiriKosina about loading firmware into a secure boot kernel led to adiscussion of the threat model that is being covered by Garrett's patchset. While Garrett agreed that firmwareloading should eventually be dealt with via signatures, it is not as highon his priority list. An automated attack using crafted firmware would bevery hardware-specific, requiring reverse engineering, and "we'dprobably benefit from them doing that in the long run".
Garrett's focus on automated attacks makes it clear what threat models heis trying to thwart, so Kosina's next query, about resuming from hibernation, is an issuethat Garrett believes should be addressed. It turns out that Josh Boyerhas a patch to disable hibernation for secure boot systems, but that, like disablingkexec(), was not overwhelmingly popular.
For one thing, as James Bottomley pointedout, there will always be kernel bugs that allow circumvention of theserestrictions (e.g. by root reading the hibernation signing key or flippingthe capability bit): [...] a local privilege elevation attackusually exploits some bug (classically a buffer overflow) which executesarbitrary code in kernel context. In that case, the same attack vectorcan be used to compromise any in-kernel protection mechanism includingturning off the secure boot capability and reading the in-kernel privatesigning key.[...] The point I'm making is that given that the majority of exploits willalready be able to execute arbitrary code in-kernel, there's not muchpoint trying to consider features like this as attacker prevention. Weshould really be focusing on discussing why we'd want to prevent alegitimate local root from writing to the suspend partition in a secureboot environment.
But kernel exploits appear to be "off the table", at least in terms of thesecure boot circumvention that Garrett and others are concerned about.Kosina said:My understanding is that we are not trying to protect against root exploiting the kernel. We are trying to protect against root tampering with the kernel code and data through legitimate use of kernel-provided [facilities] (/dev/mem, ioperm, reprogramming devices to DMA to arbitrary memory locations, resuming from hibernation image that has been tampered with, etc).
It's not exactly clear why Microsoft would make a distinction between akernel exploit and using legitimate kernel services when making ablacklisting decision, though. But, for distributions that do ship signed kernels, theycan reduce the attack surface substantially: to only those kernels thatthey have signed, with whatever vulnerabilities are present in thoseparticular versions.
Eric Paris detailed one possible attack thatinstalls a crafted Linux boot environment (with a legitimately signedbootloader and kernel), whichsleeps after setting up a trojaned Windows hibernation image. Users wouldneed to wake the machine twice, but would end up running malware in asecure boot system.
Bottomley and others are, at the very least, uncomfortable with theidea of an "untrusted root". At the heart of the kernel changes for secureboot is removing the ability for root to make persistent changes to the bootenvironment. The patches that Garrett has proposed close many of the knownholes that would allow root to make those kinds of changes, but theargument is that there are likely to be others. As Alan Cox put it: With all the current posted RH patches I can still take overthe box as root trivially enough and you seem to have so far abolishedsuspend to disk, kexec and a pile of other useful stuff. To actually lockit down you'll have to do a ton more of this.
Another possible way to handle Linux being used as an attack vector againstWindows (which is how keys are likely to get blacklisted) is to change thebehavior of the Linux bootloaders. Bottomley suggested that a "present user" test on thefirst boot of the bootloader, which could be detected because the UEFI keydatabase and the "machine owner key" database do not contain the properkeys, would alleviate the problem. Garrett pointed out that the shim bootloader does not do this because it needs to be able to boot unattended,even on first boot. But, Bottomley sawthat as unfortunate:[...] what I'm telling you is that by deciding to allow automaticfirst boot, you're causing the windows attack vector problem. You couldeasily do a present user test only on first boot which would eliminateit. Instead, we get all of this.
Garrett, though, sees unattended first boot as an absolute requirement,especially for those who are trying to do automated installations for Linux systems.Others disagreed, not surprisingly, and the discussion still continues. Itshould be noted that the pre-bootloader that Bottomley released doesdo a present user test on first boot (and beyond, depending on whether theuser changes the secure boot configuration).
There does seem to be something of whack-a-mole problem here in terms offinding all of the ways that this "untrusted root" might be able to impactsecure boot. In addition, new kernel features will have to also bescrutinized to see whether they need to be disabled depending onCAP_COMPROMISE_KERNEL. Not trusting root is a very differentmodel than kernel developers (and users) are accustomed to. One canimagine that all of the different problem areas will be tracked downeventually, but it will be a fair amount of work. Whether that work istruly justified in support of a feature that is largely (though notcompletely) only useful for protecting Windows is a big question. On theother hand, not being able to [easily] boot Linux on x86 hardware because of keyblacklisting would be problematic too. This one will take some time toplay out.
If we want to work on other kernel security measures then I don't think it should be in the context of Secure Boot as that has been pushed as far as it will go and will take a few years of operational use to cool down. You can start a new project to help prevent unauthorized entry into the kernel, making kexec do signature checking maybe, but you can't _fundamentally_ prevent code from being loaded into the kernel after users pace is started, there are too many holes for that. The kernel team does their level best to plug holes as fast as they can and that's what we have to rely on for now.
UEFI secure boot kernel restrictions Posted Nov 8, 2012 15:22 UTC (Thu) by mjg59 (subscriber, #23239) [Link] (7 responses)
This is a problem with no easy answers and might not even be a solvable problem given the complexity needed for modern systems. Have you ever read Verner Vinge "Deepness in the Sky"?
UEFI secure boot kernel restrictions Posted Nov 8, 2012 16:33 UTC (Thu) by mjg59 (subscriber, #23239) [Link] (5 responses)