[v9,0/4] evm: Do HMAC of multiple per LSM xattrs for new inodes

Message

Roberto Sassu March 29, 2023, 1:04 p.m. UTC

From: Roberto Sassu <[e-mail]>

One of the major goals of LSM stacking is to run multiple LSMs side by side
without interfering with each other. The ultimate decision will depend on
individual LSM decision.

Several changes need to be made to the LSM infrastructure to be able to
support that. This patch set tackles one of them: gives to each LSM the
ability to specify one or multiple xattrs to be set at inode creation
time and, at the same time, gives to EVM the ability to access all those
xattrs and calculate the HMAC on them.

The first problem that this patch set addresses is to make the
inode_init_security hook definition suitable to use with EVM which, unlike
other LSMs, needs to have visibility of all xattrs and not only the one
that the LSM infrastructure passes to the LSM to be set.

The solution is to replace in the inode_init_security definition the
name/value/len parameters with the beginning of the array containing all
xattrs set by LSMs. Due to security_old_inode_init_security() API
limitation of setting only one xattr, it has been dropped and the remaining
users, ocfs2 and reiserfs, switch to security_inode_init_security().
However, due to the complexity of the changes required to fully exploit the
ability of security_inode_init_security() to set multiple xattrs, those
users can still set only one xattr (the first set in the xattr array) where
previously they called security_old_inode_init_security().

Furthermore, while EVM is invoked unlike before, its xattr will not be set
as it would not be the first set in the xattr array, or if it is the first,
there would not be protected xattrs to calculate the HMAC on.

Reiserfs, regardless of the switch had anyway another problem: it was
setting xattrs without the security prefix in the xattr name. That has been
solved too, by writing the full xattr name in a temporary buffer, before
passing it to the function which actually writes the xattr.

The second problem this patch set addresses is the limitation of the
call_int_hook() of stopping the loop when the return value from a hook
implementation is not zero. Unfortunately, for the inode_init_security hook
it is a legitimate case to return -EOPNOTSUPP, but this would not
necessarily mean that there is an error to report to the LSM infrastructure
but just that an LSM does not will to set an xattr. Other LSMs should be
still consulted as well.

The solution for this specific case is to replace the call_int_hook() with
the loop itself, so that -EOPNOTSUPP can be ignored. In addition, the
default return value of inode_init_security had also to be changed to
-EOPNOTSUPP, so that the whole security_inode_init_security() does not fail
due to BPF LSM returning zero and not providing an xattr.

Next, this patch set removes the limitation of creating only two xattrs,
one by an active LSM and another by EVM. This patch set extends the
reservation mechanism of the LSM infrastructure, to allow each LSM to
request one or multiple xattrs. While this could potentially lead to
reaching the filesystem limits of number/size of the xattrs, it seems not
an issue that need to be solved by the LSM infrastructure but by the
filesystems themselves. Currently, if the limit is reached, the only
workaround would be to use fewer LSMs.

The reservation mechanism concept would have made it very easy for LSMs to
position themselves correctly in the xattr array, as the LSM infrastructure
at initialization time changes the number of xattrs requested by each LSM
with an offset.

However, this opens for the possibility of having gaps in the xattr array,
due to the fact that an LSM can request xattr slots to the LSM
infrastructure but not fill them (if it was loaded but not initialized).

Instead, the decision was to add an additional parameter to the
inode_init_security_hook, the number of filled slots in the xattr array,
which each LSM is expected to update for each xattr it provides. In this
way, the next LSM starts to fill after the last filled slot, regardless of
whether previous LSMs were initialized or not. SELinux, Smack and EVM have
been updated to use this new mechanism.

Finally, this patch set modifies the evm_inode_init_security() definition
to be compatible with the inode_init_security hook definition and adds
support for scanning the whole xattr array and for calculating the HMAC
on all xattrs provided by LSMs.

This patch set has been tested by introducing several instances of a
TestLSM (some providing an xattr, some not, one with a wrong implementation
to see how the LSM infrastructure handles it, one providing multiple xattrs
and another providing an xattr but in a disabled state). The patch is not
included in this set but it is available here:

https://github.com/robertosassu/linux/commit/8fba3224e3f7698114ae721fb8e899d322cc1f4c

The test, added to ima-evm-utils, is available here:

https://github.com/robertosassu/ima-evm-utils/blob/evm-multiple-lsms-v9-devel-v2/tests/evm_multiple_lsms.test

The test takes a UML kernel built by Github Actions and launches it several
times, each time with a different combination of LSMs and filesystems (ext4,
reiserfs, ocfs2). After boot, it first checks that there is an xattr for each
LSM providing it (for reiserfs and ocfs2 just the first LSM), and then (for
ext4) calculates the HMAC in user space and compares it with the HMAC
calculated by EVM in kernel space.

A test report can be obtained here:

https://github.com/robertosassu/ima-evm-utils/actions/runs/4553749294/jobs/8030902168

The patch set has been tested with both the SElinux and Smack test suites.
Below, there is the summary of the test results:

SELinux Test Suite result (without patches):
All tests successful.
Files=76, Tests=1357, 229 wallclock secs ( 0.45 usr  0.12 sys + 12.76 cusr 16.03 csys = 29.36 CPU)
Result: PASS

SELinux Test Suite result (with patches):
All tests successful.
Files=76, Tests=1357, 231 wallclock secs ( 0.48 usr  0.11 sys + 12.44 cusr 16.21 csys = 29.24 CPU)
Result: PASS

Smack Test Suite result (without patches):
95 Passed, 0 Failed, 100% Success rate

Smack Test Suite result (with patches):
95 Passed, 0 Failed, 100% Success rate

Changelog

v8:
- Add a new reiserfs patch to write the full xattr name
- Add num_filled_xattrs parameter to inode_init_security hook (suggested by
  Paul) and evm_inode_init_security()
- Change default return value of inode_init_security hook to -EOPNOTSUPP
- Rename lbs_xattr field of lsm_blob_sizes to lbs_xattr_count
- Introduce lsm_find_xattr_slot() helper
- Rename lsm_xattr parameter of evm_init_hmac() to xattrs
- Retrieve the EVM xattr slot with lsm_find_xattr_slot() and double check
  with the xattr array terminator
- Remove security_check_compact_filled_xattrs() (suggested by Paul)
- Update security_inode_init_security() documentation
- Ensure that inode_init_security hook incremented the number of filled
  slots if it returned zero
- Ensure that xattr name and value are non-NULL in the filled slots
- Add the xattr name assignment after the xattr value one (suggested by
  Paul)
- Drop patches 1 - 3 (already in lsm/next)

v7:
- Add a patch dependency comment in patch 1 (suggested by Mimi)
- Restore check of -EOPNOTSUPP status in ocfs2_mknod() and ocfs2_symlink()
  (reported by Mimi)
- Add explanation in evm_inode_init_security() why walking through the
  xattrs array is safe (suggested by Mimi)
- Document the lbs_xattr field of struct lsm_blob_sizes (suggested by
  Casey)
- Move documentation changes of the inode_init_security hook to security.c,
  after LSM documentation reorganization by Paul
- Use attributes in plural form in the description of the xattrs parameter
  of smack_inode_init_security()
- Check xattr name instead of xattr value in evm_inode_init_security(),
  for consistency with evm_init_hmac(); equivalent, since
  security_check_compact_filled_xattrs() rejects xattrs with xattr name
  NULL and value not NULL, and viceversa

v6:
- Add a comment in Smack to introduce its xattrs (suggested by Casey)
- Document the overloaded meaning of -EOPNOTSUPP in
  security_inode_init_security() (suggested by Mimi)

v5:
- Modify the cover letter to explain that the goal of this patch set is
  supporting multiple per LSM xattrs in EVM, and not moving IMA and EVM to
  the LSM infrastructure (suggested by Mimi)
- Remove references in the patches description about moving IMA and EVM
  to the LSM infrastructure (suggested by Mimi)
- Explain that the additional EVM invocation due to the switch to
  security_inode_init_security() will not cause the EVM xattr to be added
  (suggested by Mimi)

v4:
- Remove patch to call reiserfs_security_free(), already queued
- Switch ocfs2 and reiserfs to security_inode_init_security() (suggested by
  Mimi)
- Remove security_old_inode_init_security() (suggested by Paul)
- Rename security_check_compact_xattrs() to
  security_check_compact_filled_xattrs() and add function description
  (suggested by Mimi)
- Rename checked_xattrs parameter of security_check_compact_filled_xattrs()
  to num_filled_xattrs (suggested by Mimi)
- Rename cur_xattrs variable in security_inode_init_security() to
  num_filled_xattrs (suggested by Mimi)

v3:
- Don't free the xattr name in reiserfs_security_free()
- Don't include fs_data parameter in inode_init_security hook
- Don't change evm_inode_init_security(), as it will be removed if EVM is
  stacked
- Fix inode_init_security hook documentation
- Drop lsm_find_xattr_slot(), use simple xattr reservation mechanism and
  introduce security_check_compact_xattrs() to compact the xattr array
- Don't allocate xattr array if LSMs didn't reserve any xattr
- Return zero if initxattrs() is not provided to
  security_inode_init_security(), -EOPNOTSUPP if value is not provided to
  security_old_inode_init_security()
- Request LSMs to fill xattrs if only value (not the triple) is provided to
  security_old_inode_init_security(), to avoid unnecessary memory
  allocation

v2:
- rewrite selinux_old_inode_init_security() to use
  security_inode_init_security()
- add lbs_xattr field to lsm_blob_sizes structure, to give the ability to
  LSMs to reserve slots in the xattr array (suggested by Casey)
- add new parameter base_slot to inode_init_security hook definition

v1:
- add calls to reiserfs_security_free() and initialize sec->value to NULL
  (suggested by Tetsuo and Mimi)
- change definition of inode_init_security hook, replace the name, value
  and len triple with the xattr array (suggested by Casey)
- introduce lsm_find_xattr_slot() helper for LSMs to find an unused slot in
  the passed xattr array

Roberto Sassu (4):
  reiserfs: Add security prefix to xattr name in
    reiserfs_security_write()
  security: Allow all LSMs to provide xattrs for inode_init_security
    hook
  evm: Align evm_inode_init_security() definition with LSM
    infrastructure
  evm: Support multiple LSMs providing an xattr

 fs/reiserfs/xattr_security.c        |  8 ++-
 include/linux/evm.h                 | 14 +++--
 include/linux/lsm_hook_defs.h       |  6 +-
 include/linux/lsm_hooks.h           | 14 +++++
 security/integrity/evm/evm.h        |  4 +-
 security/integrity/evm/evm_crypto.c | 11 +++-
 security/integrity/evm/evm_main.c   | 41 ++++++++++---
 security/security.c                 | 94 ++++++++++++++++++++++-------
 security/selinux/hooks.c            | 18 +++---
 security/smack/smack_lsm.c          | 33 ++++++----
 10 files changed, 179 insertions(+), 64 deletions(-)