[PATCH RFC 0/3] security: allow a LSM to specify NO-OP return code

[Paolo Abeni]

This is another attempt to solve the current problem with eBPF LSM,
already discussed at least in [1].

The basic idea is to introduce the minimum amount of changes to let
the core consider a no-op any LSM hooks returning the
LSM_RET_DEFAULT [2].

AFAICS that is already the case for most int hooks with LSM_RET_DEFAULT
equal to 0 due to the current call_int_hook implementation. Even most
int hook with non zero LSM_RET_DEFAULT are not problematic. Specifically
the hooks [3]:

fs_context_parse_param
dentry_init_security
inode_getsecurity
inode_setsecurity
inode_copy_up_xattr
task_prctl
security_secid_to_secctx 

already have special handling for to basically ignore default return
value from the LSMs, while:

security_getprocattr
security_setprocattr

only operate on the specified LSM.

The only hooks that need some love are:

* hooks that have a 0 LSM_RET_DEFAULT, but with no LSM loaded returns a
  non zero value to the security_<hook> caller:
sb_set_mnt_opts
inode_init_security
inode_getsecctx
socket_getpeersec_stream
socket_getpeersec_dgram

* hooks that have a 0 LSM_RET_DEFAULT, but internally security_<hook>
  uses a non zero return value as a selector to perform a default
  action:
inode_setxattr
inode_removexattr

* hooks the somehow have to reconciliate multiple, non-zero, LSM return
  values to take a single decision:
vm_enough_memory
xfrm_state_pol_flow_match

This series introduces a new variant of the call_int_hook macro and
changes the LSM_RET_DEFAULT for the mentioned hooks, to achieve the
goal [2].

The patches have been split according to the above grouping with the
hope to simplify the reviews, but I guess could be squashed in a single
one.

A simple follow-up would be extend the new hook usage to the hooks [3]
to reduce the code duplication.

Sharing as an early RFC (with almost no testing) to try to understand if
this path is a no go or instead is somewhat viable.

[1] https://patchwork.kernel.org/project/netdevbpf/patch/20220609234601.2026362-1-kpsingh@kernel.org/

Paolo Abeni (3):
  security: introduce and use call_int_hook_ignore_default()
  security: two more call_int_hook_ignore_default use-cases
  security: more call_int_hook_ignore_default use-cases

 include/linux/lsm_hook_defs.h | 16 +++----
 security/security.c           | 83 ++++++++++++++++++-----------------
 2 files changed, 51 insertions(+), 48 deletions(-)

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2.41.0