Re: [Linux-ima-user] [RFC][PATCH] ima: add default rule for initramfsfiles

From: Roberto Sassu
Date: Wed Jul 14 2010 - 04:35:11 EST

On 07/14/2010 08:29 AM, Seiji Munetoh wrote:
On Wed, Jul 14, 2010 at 2:42 PM, Shaz<shazalive@xxxxxxxxx> wrote:

On Wed, Jul 14, 2010 at 3:08 AM, Seiji Munetoh<seiji.munetoh@xxxxxxxxx>
wrote:
On Thu, Jul 8, 2010 at 10:14 PM, Mimi Zohar<zohar@xxxxxxxxxxxxxxxxxx>
wrote:
On Tue, 2010-07-06 at 17:08 +0200, Roberto Sassu wrote:
This patch modifies the default policy shipped with IMA, in order to
avoid measurements
of files in the initial ramdisk. Those files can be measured early in
the boot process
by the bootloader.
The patch applies to latest version of the mainline kernel 2.6.35-rc4.
Yes, the initramfs measurements are therefore redundant, as they're
already included in the initramfs measurement, but perhaps, as the
number of initramfs is very limited and the individual file measurements
supplies additional information, it wouldn't hurt to keep the individual
file measurements as well. These measurements could potentially help in
identifying initramfs changes.

Would appreciate other opinions before accepting this change.
The hash value of the initramfs is unstable since it was generated
at the time of kernel installation.
So still I want to check the individual used file in initramfs.
If initrd is measured by boot loader then changes to individual files should
not be measured as this IS redundant. Use the new hash of the initrd as an
integrity metric. Why would this not be enough?
This depends on remote verifier.
Creating the initramfs is client side task and the hash value of initramfs
will vary each clients.

For me, validation of current measurements is easier than validation of
initramfs. And it seems the overhead of this redundancy is less painful.

But some system can validate (or trust) the initramfs measured by IPL.
So, I would suggest that add Kconfig option to change the default policy.

IMHO, if the eventlog contains fsmagic information for each measurements.
Verifier can skip the validation of RAMFS measurement easily.

This is true, the initramfs's digest cannot be validated by a remote verifier. But in my opinion there are three main reasons for don't include those files in the measurement list.
First, this is a readonly system and measures don't change in time; so if you create the image under a controlled environment and its digest doesn't change you can assert it will behave correctly.
Second, including those measurements may be very confusing for a verifier since there may be multiple versions of the same object (the initramfs changes very rarely in respect to other files).
Lastly, a pratical use of IMA is to load a custom policy. The better place to do that is the initramfs but measurements cannot be taken until the policy is loaded. The only way, as Shaz mentioned in a previous email, to keep track of all actions made during the boot process is that you have the initramfs image measured early by the boot loader.


Roberto

--
Seiji

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