Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

Kees Cook
2024-07-10T16:26:14Z

On Wed, Jul 10, 2024 at 11:58:25AM +0200, Mickaël Salaün wrote:
> Here is another proposal:
>
> We can change a bit the semantic by making it the norm to always check
> file executability with AT_CHECK, and using the securebits to restrict
> file interpretation and/or command injection (e.g. user supplied shell
> commands). Non-executable checked files can be reported/logged at the
> kernel level, with audit, configured by sysadmins.
>
> New securebits (feel free to propose better names):
>
> - SECBIT_EXEC_RESTRICT_FILE: requires AT_CHECK to pass.

Would you want the enforcement of this bit done by userspace or the
kernel?

IIUC, userspace would always perform AT_CHECK regardless of
SECBIT_EXEC_RESTRICT_FILE, and then which would happen?

1) userspace would ignore errors from AT_CHECK when
SECBIT_EXEC_RESTRICT_FILE is unset

or

2) kernel would allow all AT_CHECK when SECBIT_EXEC_RESTRICT_FILE is
unset

I suspect 1 is best and what you intend, given that
SECBIT_EXEC_DENY_INTERACTIVE can only be enforced by userspace.

> - SECBIT_EXEC_DENY_INTERACTIVE: deny any command injection via
> command line arguments, environment variables, or configuration files.
> This should be ignored by dynamic linkers. We could also have an
> allow-list of shells for which this bit is not set, managed by an
> LSM's policy, if the native securebits scoping approach is not enough.
>
> Different modes for script interpreters:
>
> 1. RESTRICT_FILE=0 DENY_INTERACTIVE=0 (default)
> Always interpret scripts, and allow arbitrary user commands.
> => No threat, everyone and everything is trusted, but we can get
> ahead of potential issues with logs to prepare for a migration to a
> restrictive mode.
>
> 2. RESTRICT_FILE=1 DENY_INTERACTIVE=0
> Deny script interpretation if they are not executable, and allow
> arbitrary user commands.
> => Threat: (potential) malicious scripts run by trusted (and not
> fooled) users. That could protect against unintended script
> executions (e.g. sh /tmp/*.sh).
> ==> Makes sense for (semi-restricted) user sessions.
>
> 3. RESTRICT_FILE=1 DENY_INTERACTIVE=1
> Deny script interpretation if they are not executable, and also deny
> any arbitrary user commands.
> => Threat: malicious scripts run by untrusted users.
> ==> Makes sense for system services executing scripts.
>
> 4. RESTRICT_FILE=0 DENY_INTERACTIVE=1
> Always interpret scripts, but deny arbitrary user commands.
> => Goal: monitor/measure/assess script content (e.g. with IMA/EVM) in
> a system where the access rights are not (yet) ready. Arbitrary
> user commands would be much more difficult to monitor.
> ==> First step of restricting system services that should not
> directly pass arbitrary commands to shells.

I like these bits!

--
Kees Cook