Re: [Linux-ima-user] Initramfs and IMA Appraisal

Mimi Zohar
2014-12-29T21:46:57Z

On Mon, 2014-12-29 at 14:34 -0600, Rob Landley wrote:
> On 12/29/2014 07:45 AM, Mimi Zohar wrote:
> > On Thu, 2014-11-27 at 10:15 +0100, Christophe Fillot wrote:
> >>>
> >>> Are you using an initrd not an initramfs? According to
> >>> Documentation/filesystems/ramfs-rootfs-initramfs.txt, "If
> >> CONFIG_TMPFS
> >>> is enabled, rootfs will use tmpfs instead of ramfs by default".
> >>>
> >> Yes, that what I thought too, but it seems that it is not really the
> >> case because of this test:
> >>
> >> if (IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
> >> (!root_fs_names || strstr(root_fs_names, "tmpfs"))) {
> >> err = shmem_init();
> >> is_tmpfs = true;
> >> } else {
> >> err = init_ramfs_fs();
> >> }
> >
> > [CC'ing Rob Landley, lsm, lkml]
> >
> > Thanks! "saved_root_name" is set to the boot command line "root="
> > option, which in my case is the UUID. I'm not sure why real root should
> > impact the initramfs tmpfs/ramfs decision.
> >
> > Unless there is a good explanation, did you want to post a patch to
> > remove the test?
>
> I added support last year, here's the start of the patch series:
>
> https://lkml.org/lkml/2013/6/29/101
>
> The logic is that if you specify a fallback root via root=, then you're
> not staying on rootfs (that's what root= _means_, "here is the root
> filesystem the kernel is to mount over rootfs"), and thus the extra
> infrastructure for tmpfs instead of ramfs is unnecessary.

Thanks Rob for the explanation. The problem is that ramfs does not
support extended attributes, while tmpfs does. The boot loader could
"measure" (trusted boot) the initramfs, but as the initramfs is
generated on the target system, the initramfs is not signed, preventing
it from being appraised (secure Boot). To close the initramfs integrity
appraisal gap requires verifying the individual initramfs file
signatures, which are stored as extended attributes.

> I keep encountering people who set root=/dev/ram0 because they think
> that means initrd (it doesn't), and then they feed in a cpio archive
> (that's a third state even before you get to the ramfs/tmpfs
> distinction), and they always want to change the code to make what they
> asked it to do not be crazy...
>
> Possibly the documentation needs to elaborate, but I expect what we
> really need is a CONFIG_VERBOSE_ROOT_SETUP that sticks in a bunch of
> printfs so the /dev/console output explains what it's doing. ("could not
> exec /init out of initramfs (errno %d, file %s), falling back to
> root=\nAdd blather=1 to kernel cmdline to see cpio
> filenames/permissions.", and so on. Where "actual exec" shows where your
> dynamic linker is when that's what wasn't there.)

To add to the confusion
Documentation/filesystems/ramfs-rootfs-initramfs.txt says, "If
CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
default." This statement should be modified to reflect the actual code.

Mimi