CVE-2026-72080: fs/resctrl: Fix use-after-free during unmount

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

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

fs/resctrl: Fix use-after-free during unmount

During unmount or failure teardown all mon_data structures that contain
monitoring event file private data are freed after which kernfs nodes are
removed. However, the RDT_DELETED flag is never set for the statically
allocated default resource group.

A concurrent reader of an event file associated with the default resource
group may, after dropping kernfs active protection, block on rdtgroup_mutex
while unmount proceeds to free the file private data and destroy the kernfs
node without waiting for the reader.

When the mutex is released, the reader wakes up, observes that RDT_DELETED
is not set for the default group, and dereferences the already-freed
file private data.

The scenario can be depicted as follows:
  CPU0                                      CPU1
   /*
    * Default resource group's
    * monitoring data accessible via
    * kernfs file with kernfs_node::priv
    * pointing to a struct mon_data.
    * User opens the file for reading.
    */
   rdtgroup_mondata_show()                 /* arch encounters fatal error */
    rdtgroup_kn_lock_live()                 resctrl_exit()
     atomic_inc(&rdtgroup_default.waitcount) cpus_read_lock()
     kernfs_break_active_protection(kn)      mutex_lock(&rdtgroup_mutex)
     cpus_read_lock()                        resctrl_fs_teardown()
     mutex_lock(&rdtgroup_mutex)              rmdir_all_sub()
                                              mon_put_kn_priv()
                                               /* Delete all mon_data structures */
                                              rdtgroup_destroy_root()
                                               kernfs_destroy_root()
                                               rdtgroup_default.kn = NULL
                                             mutex_unlock(&rdtgroup_mutex)
     /*
      * rdtgroup_default.flags is empty so
      * rdtgroup_kn_lock_live() returns
      * &rdtgroup_default
      */
     md = of->kn->priv;

     /* md points to freed mon_data */

Set RDT_DELETED for the default group unconditionally since the flag does
not lead to the freeing of this statically allocated group.

Do not allow a new resctrl mount if there are any waiters on default group
of previous mount. A new mount will re-initialize the default group that
would appear to waiters from previous mount as though the default group is
accessible causing them to access the mon_data structures from the previous
mount that have been removed.

The Linux kernel CVE team has assigned CVE-2026-72080 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.16 with commit 2a65660385444e9d9deffe995c71ee20443ef76e and fixed in 6.18.42 with commit 7b7bb07efe41bb646a93c4624aa2bb35df190342
	Issue introduced in 6.16 with commit 2a65660385444e9d9deffe995c71ee20443ef76e and fixed in 7.1.5 with commit 7d330a1d663381579b9d5dafa642b1b3158a0ce2
	Issue introduced in 6.16 with commit 2a65660385444e9d9deffe995c71ee20443ef76e and fixed in 7.2-rc3 with commit 52fce648607e0d6a76eeb443d78708c49df1c554

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-72080
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/resctrl/rdtgroup.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/7b7bb07efe41bb646a93c4624aa2bb35df190342
	https://git.kernel.org/stable/c/7d330a1d663381579b9d5dafa642b1b3158a0ce2
	https://git.kernel.org/stable/c/52fce648607e0d6a76eeb443d78708c49df1c554