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

[Mickaël Salaün]

On Wed, Jul 10, 2024 at 09:26:14AM -0700, Kees Cook wrote:
> 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

Yes, that's the idea.

> 
> 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.

Indeed. We don't want AT_CHECK's behavior to change according to
securebits.

> 
> > - 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!

Good! Jeff, Steve, Florian, Matt, others, what do you think?