linux-kernel - Re: [PATCH V32 01/27] Add the ability to lock down access to the running kernel image

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Message-ID: <87tvehxvh0.fsf@dja-thinkpad.axtens.net>
Date:   Mon, 29 Apr 2019 14:54:03 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     Matthew Garrett <mjg59@...gle.com>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>
Cc:     James Morris <jmorris@...ei.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Michael Ellerman <mpe@...erman.id.au>, cmr <cmr@...ormatik.wtf>
Subject: Re: [PATCH V32 01/27] Add the ability to lock down access to the running kernel image

Hi, 

>>> I'm thinking about whether we should lock down the powerpc xmon debug
>>> monitor - intuitively, I think the answer is yes if for no other reason
>>> than Least Astonishment, when lockdown is enabled you probably don't
>>> expect xmon to keep letting you access kernel memory.
>>
>> The original patchset contained a sysrq hotkey to allow physically
>> present users to disable lockdown, so I'm not super concerned about
>> this case - I could definitely be convinced otherwise, though.

So Mimi contacted me offlist and very helpfully provided me with a much
better and less confused justification for disabling xmon in lockdown:

On x86, physical presence (== console access) is a trigger to
disable/enable lockdown mode.

In lockdown mode, you're not supposed to be able to modify memory. xmon
allows you to modify memory, and therefore shouldn't be allowed in
lockdown.

So, if you can disable lockdown on the console that's probably OK, but
it should be specifically disabling lockdown, not randomly editing
memory with xmon.

Regards,
Daniel