[GIT PULL] Kernel lockdown for secure boot

Theodore Y. Ts'o tytso at mit.edu
Wed Apr 4 12:57:43 UTC 2018

On Wed, Apr 04, 2018 at 04:30:18AM +0000, Matthew Garrett wrote:
> What I'm afraid of is this turning into a "security" feature that ends up
> being circumvented in most scenarios where it's currently deployed - eg,
> module signatures are mostly worthless in the non-lockdown case because you
> can just grab the sig_enforce symbol address and then kexec a preamble that
> flips it back to N regardless of the kernel config.

Whoa.  Why doesn't lockdown prevent kexec?  Put another away, why
isn't this a problem for people who are fearful that Linux could be
used as part of a Windows boot virus in a Secure UEFI context?

If lockdown simply included a requirement for a signed kernel for
kexec --- and if kernel signing aren't available, to simply not alow
kexec, wouldn't that take care of this case?

This wouldn't even be all that much of a burden for non-distro users
with lockdown enabled, since in my experience outside of enterprise
and data center use cases, kexec isn't used --- and in fact, very
often kexec doesn't even work outside of a very carefully selected and
bug-fixed set of device drivers.  (It often doesn't work in non-distro
kernels because very few upstream developers really care about kexec.)

					- Ted
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