linux-hardening - Re: CVE-2024-40938: landlock: Fix d

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Message-ID: <202407150908.34E00AAD1@keescook>
Date: Mon, 15 Jul 2024 09:11:35 -0700
From: Kees Cook <kees@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, cve@...nel.org,
	linux-kernel@...r.kernel.org,
	Günther Noack <gnoack@...gle.com>,
	linux-security-module@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: CVE-2024-40938: landlock: Fix d_parent walk

On Mon, Jul 15, 2024 at 02:20:59PM +0200, Mickaël Salaün wrote:
> On Mon, Jul 15, 2024 at 01:16:38PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Jul 15, 2024 at 12:37:53PM +0200, Mickaël Salaün wrote:
> > > Hello,
> > > 
> > > AFAIK, commit 88da52ccd66e ("landlock: Fix d_parent walk") doesn't fix a
> > > security issue but an unexpected case.  The triggered WARN_ON_ONCE() is
> > > just a canary, and this case was correctly handled with defensive
> > > programming and didn't allow to bypass the security policy nor to harm
> > > the kernel.  However, this fix should indeed be backported.
> > 
> > If a WARN_ON() is hit, a machine with panic_on_warn enabled will reboot,
> > hence if there is any way that userspace can hit this, it needs to be
> > issued a CVE, sorry.
> 
> OK, I didn't know about this panic_on_warn rule for CVE.  Out of
> curiosity, panic_on_warn is definitely useful for fuzzing and testing,
> but what is the rational to enable panic_on_warn on production systems?
> It literally transforms a warning message into a system DoS (i.e.
> WARN_ON into BUG_ON).  We should explicitly use BUG_ON() if this is a
> critical unhandled case, right?

We need a way to raise WARN to panic for deployments that have tested
their workloads and want FORTIFY_SOURCE and UBSAN_BOUNDS to actually
perform mitigations instead of just warning. Linus rejected all prior
knobs for this and panic_on_warn (or better yet, kernel.warn_limit
syscall) is used for this purpose.

Userspace actions must never be able to reach a WARN or BUG state:
https://docs.kernel.org/process/deprecated.html#bug-and-bug-on

-- 
Kees Cook