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

[Kees Cook]

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