linux-hardening - Re: [PATCH v2 0/2] Introduce the pkill_on

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Message-ID: <202111181226.D4538E2F@keescook>
Date:   Thu, 18 Nov 2021 12:29:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Alexander Popov <alex.popov@...ux.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
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Subject: Re: [PATCH v2 0/2] Introduce the pkill_on_warn parameter

On Thu, Nov 18, 2021 at 10:30:32AM -0800, Casey Schaufler wrote:
> On 11/18/2021 9:32 AM, Kees Cook wrote:
> > On Tue, Nov 16, 2021 at 11:00:23AM -0800, Casey Schaufler wrote:
> > > On 11/16/2021 10:41 AM, Kees Cook wrote:
> > > > On Tue, Nov 16, 2021 at 12:12:16PM +0300, Alexander Popov wrote:
> > > > > What if the Linux kernel had a LSM module responsible for error handling policy?
> > > > > That would require adding LSM hooks to BUG*(), WARN*(), KERN_EMERG, etc.
> > > > > In such LSM policy we can decide immediately how to react on the kernel error.
> > > > > We can even decide depending on the subsystem and things like that.
> > > > That would solve the "atomicity" issue the WARN tracepoint solution has,
> > > > and it would allow for very flexible userspace policy.
> > > > 
> > > > I actually wonder if the existing panic_on_* sites should serve as a
> > > > guide for where to put the hooks. The current sysctls could be replaced
> > > > by the hooks and a simple LSM.
> > > Do you really want to make error handling a "security" issue?
> > > If you add security_bug(), security_warn_on() and the like
> > > you're begging that they be included in SELinux (AppArmor) policy.
> > > BPF, too, come to think of it. Is that what you want?
> > Yeah, that is what I was thinking. This would give the LSM a view into
> > kernel state, which seems a reasonable thing to do. If system integrity
> > is compromised, an LSM may want to stop trusting things.
> 
> How are you planning to communicate the security relevance of the
> warning to the LSM? I don't think that __FILE__, __LINE__ or __func__
> is great information to base security policy on. Nor is a backtrace.

I think that would be part of the design proposal. Initially, the known
parts are "warn or bug" and "pid".

> > A dedicated error-handling LSM could be added for those hooks that
> > implemented the existing default panic_on_* sysctls, and could expand on
> > that logic for other actions.
> 
> I can see having an interface like LSM for choosing a bug/warn policy.
> I worry about expanding the LSM hook list for a case where I would
> hope no existing LSM would use them, and the new LSM doesn't use any
> of the existing hooks.

Yeah, I can see that, though we've got a history of the "specialized"
hooks getting used by other LSMs. (e.g. loadpin's stuff got hooked up to
other LSMs, etc.)

-- 
Kees Cook