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

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Date:   Wed, 04 Apr 2018 16:22:06 +0000
From:   Matthew Garrett <mjg59@...gle.com>
To:     tytso@....edu, David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        luto@...nel.org, 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 6:52 AM Theodore Y. Ts'o <tytso@....edu> wrote:

> On Wed, Apr 04, 2018 at 02:33:37PM +0100, David Howells wrote:
> > Theodore Y. Ts'o <tytso@....edu> wrote:
> >
> > > 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?
> >
> > Lockdown mode restricts kexec to booting an authorised image (where the
> > authorisation may be by signature or by IMA).

> If that's true, then Matthew's assertion that lockdown w/o secure boot
> is insecure goes away, no?

If you don't have secure boot then an attacker with root can modify your
bootloader or kernel, and on next boot lockdown can be silently disabled.