[PATCH v2 02/15] lsm: Add the bprm_apply_policy_object LSM hook

[Justin Suess]

Add the first policy object operation, applying a policy to the
credentials prepared for an execution:

  bprm_apply_policy_object(bprm, object, flags)

The hook is only called between the preparation and the commitment of
the bprm's credentials, i.e. from a bprm_creds_for_exec() or
bprm_creds_from_file() context, where the executed task can still be
arranged to start confined by the policy.

How the policy composes with restrictions the credentials already
carry, and the meaning of @flags, are defined by the implementing
LSM, which must reject unsupported flags with -EINVAL.  An LSM with
no notion of applying a policy object to an execution does not
implement the hook, and the calling kfunc fails with -EOPNOTSUPP.

Like the lifetime hooks, this hook is excluded from the "bpf" LSM's
attachment points, as the targeted dispatch makes an attachment there
unreachable.

Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Signed-off-by: Justin Suess <utilityemal77@xxxxxxxxx>
---
 include/linux/lsm_hook_defs.h | 2 ++
 kernel/bpf/bpf_lsm.c          | 1 +
 2 files changed, 3 insertions(+)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index d7684407737a..ddb15bea383e 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -452,6 +452,8 @@ LSM_HOOK(int, 0, bpf_token_create, struct bpf_token *token, union bpf_attr *attr
 LSM_HOOK(void, LSM_RET_VOID, bpf_token_free, struct bpf_token *token)
 LSM_HOOK(int, 0, bpf_token_cmd, const struct bpf_token *token, enum bpf_cmd cmd)
 LSM_HOOK(int, 0, bpf_token_capable, const struct bpf_token *token, int cap)
+LSM_HOOK(int, -EOPNOTSUPP, bprm_apply_policy_object, struct linux_binprm *bprm,
+	 struct lsm_policy_object *object, u32 flags)
 LSM_HOOK(int, -EOPNOTSUPP, policy_object_from_fd, int fd,
 	 struct lsm_policy_object **object)
 LSM_HOOK(int, -EOPNOTSUPP, policy_object_get, struct lsm_policy_object *object)
diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
index d06744d72e04..d5859553f7f8 100644
--- a/kernel/bpf/bpf_lsm.c
+++ b/kernel/bpf/bpf_lsm.c
@@ -56,6 +56,7 @@ BTF_ID(func, bpf_lsm_xfrm_decode_session)
 #endif
 BTF_ID(func, bpf_lsm_ismaclabel)
 BTF_ID(func, bpf_lsm_file_alloc_security)
+BTF_ID(func, bpf_lsm_bprm_apply_policy_object)
 BTF_ID(func, bpf_lsm_policy_object_from_fd)
 BTF_ID(func, bpf_lsm_policy_object_get)
 BTF_ID(func, bpf_lsm_policy_object_put)
-- 
2.55.0