Re: [PATCH v2 00/25] security: Move IMA and EVM to the LSM infrastructure

[Casey Schaufler]

On 8/31/2023 3:41 AM, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
>
> IMA and EVM are not effectively LSMs, especially due the fact that in the
> past they could not provide a security blob while there is another LSM
> active.
>
> That changed in the recent years, the LSM stacking feature now makes it
> possible to stack together multiple LSMs, and allows them to provide a
> security blob for most kernel objects. While the LSM stacking feature has
> some limitations being worked out, it is already suitable to make IMA and
> EVM as LSMs.
>
> In short, while this patch set is big, it does not make any functional
> change to IMA and EVM. IMA and EVM functions are called by the LSM
> infrastructure in the same places as before (except ima_post_path_mknod()),
> rather being hardcoded calls, and the inode metadata pointer is directly
> stored in the inode security blob rather than in a separate rbtree.
>
> More specifically, patches 1-11 make IMA and EVM functions suitable to
> be registered to the LSM infrastructure, by aligning function parameters.
>
> Patches 12-20 add new LSM hooks in the same places where IMA and EVM
> functions are called, if there is no LSM hook already.

I've commented on the individual patches, but it seems like a general comment
might be in order. When a new LSM hook is proposed we want to see more than
"project XYZZY needs this hook" to justify it. We want to know how it is useful
for XYZZY and how it could be used in another LSM. If I were creating a new LSM
it could be useful to understand the difference between security_inode_setattr()
and security_inode_post_setattr(). As a reviewer who has had only incidental
exposure to the IMA code it's important to understand why it doesn't use the
existing hooks.

>
> Patches 21-24 do the bulk of the work, remove hardcoded calls to IMA, EVM
> and integrity functions, register those functions in the LSM
> infrastructure, and let the latter call them. In addition, they also
> reserve one slot for EVM to supply an xattr to the inode_init_security
> hook.
>
> Finally, patch 25 removes the rbtree used to bind metadata to the inodes,
> and instead reserve a space in the inode security blob to store the pointer
> to metadata. This also brings performance improvements due to retrieving
> metadata in constant time, as opposed to logarithmic.
>
> The patch set applies on top of lsm/next, commit 8e4672d6f902 ("lsm:
> constify the 'file' parameter in security_binder_transfer_file()")
>
> Changelog:
>
> v1:
>  - Drop 'evm: Complete description of evm_inode_setattr()', 'fs: Fix
>    description of vfs_tmpfile()' and 'security: Introduce LSM_ORDER_LAST',
>    they were sent separately (suggested by Christian Brauner)
>  - Replace dentry with file descriptor parameter for
>    security_inode_post_create_tmpfile()
>  - Introduce mode_stripped and pass it as mode argument to
>    security_path_mknod() and security_path_post_mknod()
>  - Use goto in do_mknodat() and __vfs_removexattr_locked() (suggested by
>    Mimi)
>  - Replace __lsm_ro_after_init with __ro_after_init
>  - Modify short description of security_inode_post_create_tmpfile() and
>    security_inode_post_set_acl() (suggested by Stefan)
>  - Move security_inode_post_setattr() just after security_inode_setattr()
>    (suggested by Mimi)
>  - Modify short description of security_key_post_create_or_update()
>    (suggested by Mimi)
>  - Add back exported functions ima_file_check() and
>    evm_inode_init_security() respectively to ima.h and evm.h (reported by
>    kernel robot)
>  - Remove extern from prototype declarations and fix style issues
>  - Remove unnecessary include of linux/lsm_hooks.h in ima_main.c and
>    ima_appraise.c
>
> Roberto Sassu (25):
>   ima: Align ima_inode_post_setattr() definition with LSM infrastructure
>   ima: Align ima_post_path_mknod() definition with LSM infrastructure
>   ima: Align ima_post_create_tmpfile() definition with LSM
>     infrastructure
>   ima: Align ima_file_mprotect() definition with LSM infrastructure
>   ima: Align ima_inode_setxattr() definition with LSM infrastructure
>   ima: Align ima_inode_removexattr() definition with LSM infrastructure
>   ima: Align ima_post_read_file() definition with LSM infrastructure
>   evm: Align evm_inode_post_setattr() definition with LSM infrastructure
>   evm: Align evm_inode_setxattr() definition with LSM infrastructure
>   evm: Align evm_inode_post_setxattr() definition with LSM
>     infrastructure
>   security: Align inode_setattr hook definition with EVM
>   security: Introduce inode_post_setattr hook
>   security: Introduce inode_post_removexattr hook
>   security: Introduce file_post_open hook
>   security: Introduce file_pre_free_security hook
>   security: Introduce path_post_mknod hook
>   security: Introduce inode_post_create_tmpfile hook
>   security: Introduce inode_post_set_acl hook
>   security: Introduce inode_post_remove_acl hook
>   security: Introduce key_post_create_or_update hook
>   ima: Move to LSM infrastructure
>   ima: Move IMA-Appraisal to LSM infrastructure
>   evm: Move to LSM infrastructure
>   integrity: Move integrity functions to the LSM infrastructure
>   integrity: Switch from rbtree to LSM-managed blob for
>     integrity_iint_cache
>
>  fs/attr.c                             |   5 +-
>  fs/file_table.c                       |   3 +-
>  fs/namei.c                            |  18 +-
>  fs/nfsd/vfs.c                         |   3 +-
>  fs/open.c                             |   1 -
>  fs/posix_acl.c                        |   5 +-
>  fs/xattr.c                            |   9 +-
>  include/linux/evm.h                   | 103 ----------
>  include/linux/ima.h                   | 136 -------------
>  include/linux/integrity.h             |  26 ---
>  include/linux/lsm_hook_defs.h         |  21 +-
>  include/linux/security.h              |  65 +++++++
>  security/integrity/evm/evm_main.c     | 104 ++++++++--
>  security/integrity/iint.c             |  92 +++------
>  security/integrity/ima/ima.h          |  11 ++
>  security/integrity/ima/ima_appraise.c |  37 +++-
>  security/integrity/ima/ima_main.c     |  76 ++++++--
>  security/integrity/integrity.h        |  44 ++++-
>  security/keys/key.c                   |  10 +-
>  security/security.c                   | 265 ++++++++++++++++----------
>  security/selinux/hooks.c              |   3 +-
>  security/smack/smack_lsm.c            |   4 +-
>  22 files changed, 540 insertions(+), 501 deletions(-)
>