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From: Ahmad Fatoum <>
To: Trent Piepho <>
Cc: "" <>
Subject: Re: [PATCH 1/5] common: machine_id: support /chosen/barebox, machine-id-path override
Date: Wed, 15 Sep 2021 12:55:25 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>


On 30.06.21 22:13, Trent Piepho wrote:
> On Wed, Jun 30, 2021 at 3:27 AM Ahmad Fatoum <> wrote:
>> On 28.06.21 21:50, Trent Piepho wrote:
>>> On Sun, Jun 27, 2021 at 11:41 PM Ahmad Fatoum <> wrote:
>>> On a board I did before Barebox had machine-id support, we used two
>>> sources of serial number to generate the machine id.  One was the imx7
>>> unique id and the other was a serial number in a i2c security chip.
>>> The imx id is predictable, so even hashed one can predict the
>>> machine-id exactly.
>> We happen to have stm32mp1 twins with consecutive MAC addresses.
>> I compared their serial numbers and while they clearly didn't start
>> at zero, all bytes were the same, except for two nibbles that were
>> one apart. So yes, if the i.MX UID follows a similar scheme, you
>> may be able to guess the machine-id of other devices in the same
>> batch if you already have the machine-id of one of them.
>>> We didn't really like that, so we combined two
>>  sources.
>> Is your issue one of privacy or security?
> Both.  We didn't like it being guessable for fishing attempts.
> Security was more on the cloud side, having the machine-ids guessable
> was not a good idea.  Yes, machine id is not to be used for access
> control, it is not a password.  But one does not want an attacker to
> have a list of every valid username even if the password is still
> secret.

Fair enough.

>> My understanding is that the machine id is not disclosed as matter
>> of privacy, so it's harder to track a device by manner of the unique
>> IDs embedded in its outgoing communication.
>>> It seems like there is no way to do that with this design?
>> You can write a driver that collects multiple sources and offers
>> a single NVMEM cell for consumption.
> I suppose this could be done, but it certainly seems complex.  I think
> one would need:
> Write code in board init function to get imx id, data from additional
> chip, put in global buffer.
> Add barebox specific device tree node, compatible =
> "mycompany,myboard-unique-data"
> Write small nvmem driver that exports the global buffer as nvmem device.
> I suppose the latter driver could be common if needing to create nvmem
> devices to hold data becomes commonly needed.

Yes. I think using a nvmem cell as an interface here is a good
trade off. It works for most and it's straight forward to extend
in future.
>> A user would expect a machine-id-path property to point at all info
>> used for determining the machine-id, not some of them.
> One could have a special name in the path, "internal" or something,
> that indicates data that code in Barebox will create.
>>> Or /chosen/barebox,machine-id could be a _list_ of paths, to be used
>>> in order.  But that requires a nvmem driver for each source.
>> That's fine by me and could be added in future. I can add a property
>> size check, so we leave open this avenue without breaking users.
>> The root device tree node already has a device in barebox, so board
>> could use that to offer custom info.
> If a property was created that simply contained the data directly,
> I.e. of_set_property(root, "extra-id-data", data, 8),
> would there be a way for barebox,machine-id to point to it?

Nope. But who would fix up such a property? If you get data
from a previous boot stage, you could put a nvmem-romem on top.

>>> The security chip wouldn't have worked for a nvmem driver.
>> Why not? Check out nvmem-rmem with just exports a memory region
>> as read-only nvmem device. You can do likewise. There are also
>> helpers to get a nvmem device out of a (i2c) regmap.
> To get the id, one needs to construct a command and then parse the
> response format to extract the data.  It is not as simple as some
> register contents.
>>> I'll also point out that just hashing the data is not a good idea to
>>> make a UUID.  Anyone who hashes the imx unique id will get the exact
>>> same UUID.  So it is not very universally unique!
>> The i.MX unique ID is unique across i.MX processors. Yes, it would
>> collide with an attacker that guess the ID, but is that really
>> a threat? Anyhow, there are boards using it like this already in the
> It comes up if some other software on the same board creates a UUID by
> hashing the imx id.  Anything that does this will get the same UUID.
> Suppose the board is not imx and has no unique built-in id other than
> a MAC address.  Anyone who hashes the mac address gets the same UUID
> and they collide.  It is not a security issue exactly, but a failure
> of the universally unique property of the UUID.

The alternative is not to have a default at all. So we provide
a default that works for most and if you need more than that,
you can extend it.

>> field. So this won't change, but for the new binding introduced here,
>> I can address issues that are raised.
>>> This is already a known issue when generating UUIDs that are not
>>> purely random.  See RFC4122 §4.3 for generating UUIDs in a namespace.
>> Users can do that, barebox will hashh it to get the format the OS expects.
>> It's probably a good idea to hint at RFC4122 for users interested in
>> generating their own material for use with the machine-id.
> I don't know what you mean by the format the OS expects.  The output
> of the §4.3 algorithm is a standard UUID.  It works perfectly well to
> pass it as a machine id and then systemd will use it as it is.  Since
> 2011, when systemd generates a machine id it follows RFC4422 §4.4 for
> random UUIDs.

systemd expects a 32 character lower case ID that's not all zeroes.
Newer systemd versions apparently generate machine IDs that qualify
as v4 UUIDs, but barebox doesn't and I just want to streamline the
existing implementation here. I don't intend to make the machine-id
UUID-compatible (although I don't mind extending the functionality
into that direction).


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barebox mailing list

      reply	other threads:[~2021-09-15 10:57 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-28  6:40 Ahmad Fatoum
2021-06-28  6:40 ` [PATCH 2/5] ARM: stm32mp: migrate to barebox,machine-id-path Ahmad Fatoum
2021-06-28  6:40 ` [PATCH 3/5] common: machine_id: deprecate machine_id_set_hashable Ahmad Fatoum
2021-06-28  9:50   ` Bastian Krause
2021-06-28 10:12     ` Ahmad Fatoum
2021-06-28  6:40 ` [PATCH 4/5] sandbox: dts: populate $global.machine_id Ahmad Fatoum
2021-06-28  6:40 ` [PATCH 5/5] ARM: dts: stm32mp: retire barebox, provide-mac-address in favor of NVMEM Ahmad Fatoum
2021-06-28  9:28 ` [PATCH 1/5] common: machine_id: support /chosen/barebox,machine-id-path override Ahmad Fatoum
2021-06-28  9:35 ` [PATCH 1/5] common: machine_id: support /chosen/barebox, machine-id-path override Bastian Krause
2021-06-28 10:11   ` Ahmad Fatoum
2021-06-28 20:20 ` Sascha Hauer
     [not found] ` <>
2021-06-30 10:27   ` Ahmad Fatoum
2021-06-30 20:13     ` Trent Piepho
2021-09-15 10:55       ` Ahmad Fatoum [this message]

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