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.

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2024-26821
https://www.cve.org/CVERecord?id=CVE-2024-26821
https://git.kernel.org/stable/c/2a7a31e1fb9717845d9d5e2a8c6e48848147801e
https://git.kernel.org/stable/c/31f71f2d7a081fc6c6bdf06865beedf6db5b0ca4
https://git.kernel.org/stable/c/46f5ab762d048dad224436978315cbc2fa79c630
https://git.kernel.org/stable/c/95de4ad173ca0e61034f3145d66917970961c210
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2024/CVE-2024-26821.mbox
https://bugzilla.redhat.com/show_bug.cgi?id=2275616