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

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Message-Id: <70C7A8C3-3DCC-4448-9FBD-534ADDE2D6E6@amacapital.net>
Date:   Mon, 2 Apr 2018 18:47:05 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        James Morris <jmorris@...ei.org>,
        David Howells <dhowells@...hat.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Justin Forbes <jforbes@...hat.com>,
        linux-man <linux-man@...r.kernel.org>, joeyli <jlee@...e.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [GIT PULL] Kernel lockdown for secure boot


> On Apr 2, 2018, at 5:59 PM, Kees Cook <keescook@...omium.org> wrote:
> 
>> On Mon, Apr 2, 2018 at 5:37 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> On 03/30/2018 05:46 PM, James Morris wrote:
>>> 
>>>> On Sat, 31 Mar 2018, David Howells wrote:
>>>> 
>>>> Date: Thu, 26 Oct 2017 17:37:38 +0100
>>>> 
>>>> Hi James,
>>>> 
>>>> Can you pull this patchset into security/next please?  It has been in
>>>> linux-next since the beginning of March.
>>>> 
>>>> It adds kernel lockdown support for EFI secure boot.
>>> 
>>> 
>>> Applied to
>>> git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git
>>> next-lockdown and next-testing
>>> 
>>> Are there any known coverage gaps now?
>>> 
>>> 
>>> 
>> 
>> This is an attempt at a review.  I'm replying here because I can't find the
>> actual relevant patch emails.
>> 
>> Cover letter:
>> 
>>> Here's a set of patches to institute a "locked-down mode" in the
>>> kernel and to trigger that mode if the kernel is booted in secure-boot >
>>> mode or through the command line.
>> 
>> I think this is seriously problematic in that it's not well defined.  It
>> sounds like "locked-down mode" means "make me feel good about something".
> 
> Naming of this feature has been multi-year bikeshedding, so if we
> could just leave the name, that'd be nice.

Fair enough. How about enum kernel_lockdown_level with three modes?

> 
> 

>> "Restrict /dev/{mem,kmem,port} when the kernel is locked down": this should
>> probably split into one restriction for read and one for write.
> 
> I think splitting read and write is only useful if there is a use-case
> for only blocking one of them. I struggle to imagine allowing write
> and blocking read, so really it's the case of wanting to allow read
> and disallow write. Is there actually a use-case for this? In all the
> "locked down" cases I've seen, both are desired.
> 

Let’s suppose for the sake of argument that UEFI really has a good reason to block writes. Blocking reads (kprobes, perf, etc) sounds extremely annoying, especially if running a stock distro, and I’d much rather not do it unless there’s a specific use case that needs it.