CVE-2024-40938: landlock: Fix d_parent walk

[Mickaël Salaün]

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?

> 
> > Could you please Cc me for future CVE related to my changes or to
> > Landlock?  For kernel CVEs, I think it would be good to Cc at least
> > maintainers, reviewers, authors, and committers for the related commits.
> 
> I suggest setting up lei to watch the linux-cve-announce mailing list if
> you wish to do this (just filter for landlock stuff).  Automatically
> mailing cve stuff to maintainers has been deemed too "noisy" which is
> why we do not do this by default.

Well, it might be too noisy for some but I guess/hope not for most.
Email filtering should be easy for the few receiving too many of these
emails though.