Re: CVE-2024-26821: fs: relax mount_setattr() permission checks

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, May 15, 2024 at 06:58:38PM +0200, Christian Brauner wrote:
> On Tue, May 14, 2024 at 02:49:39PM +0200, Anthony Iliopoulos wrote:
> > On Wed, Apr 17, 2024 at 11:44:04AM +0200, Greg Kroah-Hartman wrote:
> > > Description
> > > ===========
> > > 
> > > In the Linux kernel, the following vulnerability has been resolved:
> > > 
> > > fs: relax mount_setattr() permission checks
> > > 
> > > When we added mount_setattr() I added additional checks compared to the
> > > legacy do_reconfigure_mnt() and do_change_type() helpers used by regular
> > > mount(2). If that mount had a parent then verify that the caller and the
> > > mount namespace the mount is attached to match and if not make sure that
> > > it's an anonymous mount.
> > > 
> > > The real rootfs falls into neither category. It is neither an anoymous
> > > mount because it is obviously attached to the initial mount namespace
> > > but it also obviously doesn't have a parent mount. So that means legacy
> > > mount(2) allows changing mount properties on the real rootfs but
> > > mount_setattr(2) blocks this. I never thought much about this but of
> > > course someone on this planet of earth changes properties on the real
> > > rootfs as can be seen in [1].
> > > 
> > > Since util-linux finally switched to the new mount api in 2.39 not so
> > > long ago it also relies on mount_setattr() and that surfaced this issue
> > > when Fedora 39 finally switched to it. Fix this.
> > > 
> > > The Linux kernel CVE team has assigned CVE-2024-26821 to this issue.
> > 
> > This one probably needs to be disputed as it isn't an actual
> > vulnerability, but rather a fix for the mount_setattr which previously
> > didn't allow reconfiguring the real rootfs similar to what the mount
> > syscall always allowed to do.
> > 
> > So it merely brings mount_attr up to par with mount in terms of allowing
> > the real rootfs to be reconfigured.
> > 
> > Christian, what do you think ?
> 
> Yeah, it's not security related at all. It just allows _additional_
> functionality. Not sure how that ended up on the CVE list. Thanks for
> pinging about this!

Now rejected, thanks all for reviewing this and letting us know.

greg k-h