CVE-2024-40938: landlock: Fix d_parent walk

[Greg Kroah-Hartman]

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?

People like to have their devices/servers rebooted if _anything_ is seen
to go wrong.  A few billion phones have this enabled, as do most all
cloud servers it seems :(

> It literally transforms a warning message into a system DoS (i.e.
> WARN_ON into BUG_ON).

Yes :(

> We should explicitly use BUG_ON() if this is a critical unhandled
> case, right?

Personally I would use neither, handle the error properly and clean up
correctly.  Only if the continuing to run would cause unrecoverable harm
(i.e. data loss or corruption or compromise) would I ever call BUG_ON().

> > > 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.

For now we have decided to not cc: maintainers as these are all sent to
a public list that can be subscribed to if they wish to.

thanks,

greg k-h