Re: [PATCH v5 0/3] init: add support to directly boot to a mapped device

[Kees Cook]

On Fri, Feb 26, 2016 at 8:53 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Mon, Feb 22 2016 at  1:55pm -0500,
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> On Sun, Feb 21, 2016 at 2:08 PM, Alasdair G Kergon <agk@xxxxxxxxxx> wrote:
>> > On Sat, Feb 20, 2016 at 10:13:49AM -0800, Kees Cook wrote:
>> >> This is a resurrection of a patch series from a few years back, first
>> >> brought to the dm maintainers in 2010. It creates a way to define dm
>> >> devices on the kernel command line for systems that do not use an
>> >> initramfs, or otherwise need a dm running before init starts.
>> >>
>> >> This has been used by Chrome OS for several years, and now by Brillo
>> >> (and likely Android soon).
>> >>
>> >> The last version was v4:
>> >> https://patchwork.kernel.org/patch/104860/
>> >> https://patchwork.kernel.org/patch/104861/
>> >
>> > Inconsistencies in the terminology here can be sorted out during review,
>> > and I see that you've taken on board some of my review comments from
>> > 2010, but what are your responses to the rest of them?
>>
>> Ah, sorry, the threads I could find were incomplete, so I wasn't able
>> to find those comments that were made to Will's 2010 submission. In
>> some of the cleanups I did I was very confused about "target" vs
>> "table", and tried to fix that. Regardless, I'm open to fixing
>> whatever is needed. :)
>>
>> Thanks for looking at this again!
>
> This work isn't going to fly as is.  I appreciate the effort and the
> goal (without understanding _why_) but: you're open-coding, duplicating
> and/or reinventing way too much in do_mounts_dm.c
>
> 1) You first need to answer: _why_ is using a proper initramfs not
> viable?  A very simple initramfs that issues dmsetup commands, etc,
> isn't so daunting is it?  Why is it so important for the kernel to
> natively provide a dmsetup interface?  Chrome, Android, etc cannot use
> initramfs?

That is correct: Chrome OS does not (and won't) use an initramfs. This
is mainly for reasons of boot speed, verified boot block size, and
maybe some other things I don't remember.

> 2) If you are able to adequately justify the need for dm=:
> I'd much rather the dm= kernel commandline be a simple series of
> comma-delimited dmsetup-like commands.
>
> You'd handle each command with extremely basic parsing:
>  <dm_ioctl_cmd> <args> [, <dm_ioctl_cmd> <args>]
> (inventing a special token to denote <newline>, to support tables with
> multiple entries, rather than relying on commas and counts, etc)

Sure, changing the syntax is fine by me. We'd need to plumb access to
the ioctl interface, though.

> and you'd then have do_mounts_dm.c open /dev/mapper/control directly and
> issue proper DM ioctls rather than adding all your shim code.  This last
> bit of opening /dev/mapper/control from init needs more research -- not
> sure if doing such a thing from kernel is viable/safe/acceptable.

Well, there's no /dev and no init since our dm is the root device
(dm-verity). We need everything up and running before we mount the
root filesystem, very similar to do_mount_md.c's purpose.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security