[v3,0/2] initramfs: add support for xattrs in the initial ram disk

Message

Roberto Sassu May 17, 2019, 4:55 p.m. UTC

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 marshaling pathnames and xattrs in a file called
.xattr-list. They are unmarshaled by the CPIO parser after all files have
been extracted, or before the next ram disk is processed.

The difference from v1 (https://lkml.org/lkml/2018/11/22/1182) is that all
xattrs are stored in a single file and not per file (solves the file name
limitation issue, as it is not necessary to add a suffix to files
containing xattrs).

The difference with another proposal
(https://lore.kernel.org/patchwork/cover/888071/) is that xattrs can be
included in an image without changing the image format, as opposed to
defining a new one. As seen from the discussion, if a new format has to be
defined, it should fix the issues of the existing format, which requires
more time.

To fulfill both requirements, adding support for xattrs in a short time and
defining a new image format properly, this patch set takes an incremental
approach: it introduces a parser of xattrs that can be used either if
xattrs are in a regular file or directly added to the image (this patch set
reuses patch 9/15 of the existing proposal); in addition, it introduces a
wrapper of the xattr parser, to read xattrs from a file.

The changes introduced by this patch set don't cause any compatibility
issue: kernels without the xattr parser simply extracts .xattr-list and
don't unmarshal xattrs; kernels with the xattr parser don't unmarshal
xattrs if .xattr-list is not found in the image.

From the kernel space perspective, backporting this functionality to older
kernels should be very easy. It is sufficient to add two calls to the new
function do_readxattrs(). From the user space perspective, no change is
required for the use case. A new dracut module (module-setup.sh) will
execute:

getfattr --absolute-names -d -h -R -e hex -m security.ima \
    <file list> | xattr.awk -b > ${initdir}/.xattr-list

where xattr.awk is the script that marshals xattrs (see patch 3/3). The
same can be done with the initramfs-tools ram disk generator.

Changelog

v2:
- replace ksys_lsetxattr() with kern_path() and vfs_setxattr()
  (suggested by Jann Horn)
- replace ksys_open()/ksys_read()/ksys_close() with
  filp_open()/kernel_read()/fput()
  (suggested by Jann Horn)
- use path variable instead of name_buf in do_readxattrs()
- set last byte of str to 0 in do_readxattrs()
- call do_readxattrs() in do_name() before replacing an existing
  .xattr-list
- pass pathname to do_setxattrs()


Mimi Zohar (1):
  initramfs: set extended attributes

Roberto Sassu (1):
  initramfs: introduce do_readxattrs()

 init/initramfs.c | 170 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 168 insertions(+), 2 deletions(-)