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

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Date:   Wed, 04 Apr 2018 16:20:43 +0000
From:   Matthew Garrett <mjg59@...gle.com>
To:     tytso@....edu, 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

On Wed, Apr 4, 2018 at 5:57 AM Theodore Y. Ts'o <tytso@....edu> wrote:

> 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?

It does - I was talking about the non-lockdown case. In the lockdown case
you can only kexec images you trust, so there's no problem. Red Hat have
been shipping a signed kdump image for years.