ima: Attach CREDS_CHECK IMA hook to bprm_creds_from_file LSM hook

Since commit 56305aa9b6fa ("exec: Compute file based creds only once"), the
credentials to be applied to the process after execution are not calculated
anymore for each step of finding intermediate interpreters (including the
final binary), but only after the final binary to be executed without
interpreter has been found.

In particular, that means that the bprm_check_security LSM hook will not
see the updated cred->e[ug]id for the intermediate and for the final binary
to be executed, since the function doing this task has been moved from
prepare_binprm(), which calls the bprm_check_security hook, to
bprm_creds_from_file().

This breaks the IMA expectation for the CREDS_CHECK hook, introduced with
commit d906c10d8a31 ("IMA: Support using new creds in appraisal policy"),
which expects to evaluate "the credentials that will be committed when the
new process is started". This is clearly not the case for the CREDS_CHECK
IMA hook, which is attached to bprm_check_security.

This issue does not affect systems which load a policy with the BPRM_CHECK
hook with no other criteria, as is the case with the built-in "tcb" and/or
"appraise_tcb" IMA policies. The "tcb" built-in policy measures all
executions regardless of the new credentials, and the "appraise_tcb" policy
is written in terms of the file owner, rather than IMA hooks.

However, it does affect systems without a BPRM_CHECK policy rule or with a
BPRM_CHECK policy rule that does not include what CREDS_CHECK evaluates. As
an extreme example, taking a standalone rule like:

measure func=CREDS_CHECK euid=0

This will not measure for example sudo (because CREDS_CHECK still sees the
bprm->cred->euid set to the regular user UID), but only the subsequent
commands after the euid was applied to the children.

Make set[ug]id programs measured/appraised again by splitting
ima_bprm_check() in two separate hook implementations (CREDS_CHECK now
being implemented by ima_creds_check()), and by attaching CREDS_CHECK to
the bprm_creds_from_file LSM hook.

The limitation of this approach is that CREDS_CHECK will not be invoked
anymore for the intermediate interpreters, like it was before, but only for
the final binary. This limitation can be removed only by reverting commit
56305aa9b6fa ("exec: Compute file based creds only once").

Link: https://github.com/linux-integrity/linux/issues/3
Fixes: 56305aa9b6fa ("exec: Compute file based creds only once")
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Jann Horn <jannh@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

0 files changed, 0 insertions, 0 deletions