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

Jeff Xu
2024-07-16T15:18:51Z

On Tue, Jul 16, 2024 at 8:15 AM Mickaël Salaün <mic@digikod.net> wrote:
>
> 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@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
That looks good. Thanks for clarifying.