linux-kernel - Re: [GIT PULL] Kernel lockdown for secure boot

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Message-ID: <20180408220030.GC4965@amd>
Date:   Mon, 9 Apr 2018 00:00:30 +0200
From:   Pavel Machek <pavel@....cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Matthew Garrett <mjg59@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        luto@...nel.org, David Howells <dhowells@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>, jmorris@...ei.org,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jforbes@...hat.com, linux-man@...r.kernel.org, jlee@...e.com,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-api@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot

Hi!

> > 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.)

I do have Motorola Droid 4 here (cellphone). It uses safestrap.. and
than it turn kexec's a lot (so that you can select Android vs. Jolla
vs. ... during boot).

So yes, kexec shows even in unexpected places.

And BTW.. the cellphone thingie is a situation where manufacturer
works against it users. Motorola does _not_ want me to run my own
kernels here.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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