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

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Date:   Mon, 01 Jul 2019 10:31:25 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        Rob Landley <rob@...dley.net>, viro@...iv.linux.org.uk
Cc:     linux-security-module@...r.kernel.org,
        linux-integrity@...r.kernel.org, initramfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, bug-cpio@....org,
        zohar@...ux.vnet.ibm.com, silviu.vlasceanu@...wei.com,
        dmitry.kasatkin@...wei.com, takondra@...co.com, kamensky@...co.com,
        hpa@...or.com, arnd@...db.de, james.w.mcmechan@...il.com,
        niveditas98@...il.com
Subject: Re: [PATCH v4 0/3] initramfs: add support for xattrs in the initial
 ram disk

On Mon, 2019-07-01 at 16:42 +0300, Roberto Sassu wrote:
> 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?
> >>>>
> >>>> Thanks
> >>>>
> >>>> Roberto
> >>>
> >>> 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.
> >>
> >> Ok.
> >>
> >> 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
> >> to set CONFIG_INITRAMFS_FILE_METADATA="xattr" and
> >> CONFIG_INITRAMFS_SOURCE="<file with xattr>" in the kernel configuration.
> >>
> >> To add xattrs to the external ram disk, it is necessary to patch cpio:
> >>
> >> https://github.com/euleros/cpio/commit/531cabc88e9ecdc3231fad6e4856869baa9a91ef
> >> (xattr-v1 branch)
> >>
> >> and dracut:
> >>
> >> https://github.com/euleros/dracut/commit/a2dee56ea80495c2c1871bc73186f7b00dc8bf3b
> >> (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
> >> to dracut.sh).
> >>
> >> 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/gen_initramfs_list.sh, 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.

True, but this alternative is for those who normally use dracut to
create an initramfs, but don't want to update cpio or dracut.

Mimi