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

[Mickaël Salaün]

On Tue, Jul 16, 2024 at 08:02:37AM -0700, Jeff Xu wrote:
> On Thu, Jul 11, 2024 at 1:57 AM Mickaël Salaün <mic at digikod.net> wrote:
> >
> > 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.
> >
> One bit is good.
> 
> > >
> > > > - 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?
> 
> For below two cases: will they be restricted by one (or some) mode above ?
> 
> 1> cat /tmp/a.sh | sh
> 
> 2> sh -c "$(cat /tmp/a.sh)"

Yes, DENY_INTERACTIVE=1 is to deny both of these cases (i.e. arbitrary
user command).

These other examples should be allowed with AT_CHECK and RESTRICT_FILE=1
if a.sh is executable though:
* sh /tmp/a.sh
* sh < /tmp/a.sh