Re: [PATCH v4 0/3] initramfs: add support for xattrs in the initial ram disk

From: Roberto Sassu
Date: Mon Jul 01 2019 - 09:42:23 EST

On 6/30/2019 6:39 PM, Mimi Zohar wrote:
On Wed, 2019-06-26 at 10:15 +0200, Roberto Sassu wrote:
On 6/3/2019 8:32 PM, Rob Landley wrote:
On 6/3/19 4:31 AM, Roberto Sassu wrote:
This patch set aims at solving the following use case: appraise files from
the initial ram disk. To do that, IMA checks the signature/hash from the
security.ima xattr. Unfortunately, this use case cannot be implemented
currently, as the CPIO format does not support xattrs.

This proposal consists in including file metadata as additional files named
METADATA!!!, for each file added to the ram disk. The CPIO parser in the
kernel recognizes these special files from the file name, and calls the
appropriate parser to add metadata to the previously extracted file. It has
been proposed to use bit 17:16 of the file mode as a way to recognize files
with metadata, but both the kernel and the cpio tool declare the file mode
as unsigned short.

Any opinion on this patch set?



Sorry, I've had the window open since you posted it but haven't gotten around to
it. I'll try to build it later today.

It does look interesting, and I have no objections to the basic approach. I
should be able to add support to toybox cpio over a weekend once I've got the
kernel doing it to test against.


Let me give some instructions so that people can test this patch set.

To add xattrs to the ram disk embedded in the kernel it is sufficient
CONFIG_INITRAMFS_SOURCE="<file with xattr>" in the kernel configuration.

To add xattrs to the external ram disk, it is necessary to patch cpio:
(xattr-v1 branch)

and dracut:
(digest-lists branch)

The same modification can be done for mkinitramfs (add '-e xattr' to the
cpio command line).

To simplify the test, it would be sufficient to replace only the cpio
binary and the dracut script with the modified versions. For dracut, the
patch should be applied to the local dracut (after it has been renamed

Then, run:

dracut -e xattr -I <file with xattr> (add -f to overwrite the ram disk)

Xattrs can be seen by stopping the boot process for example by adding
rd.break to the kernel command line.

A simple way of testing, without needing any changes other than the
kernel patches, is to save the dracut temporary directory by supplying
"--keep" on the dracut command line, calling
usr/, followed by usr/gen_init_cpio with the "-e
xattr" option.

Alternatively, follow the instructions to create the embedded ram disk
with xattrs, and use the existing external ram disk created with dracut
to check if xattrs are created.


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