CVE-2026-31446

Summary

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

ext4: fix use-after-free in update_super_work when racing with umount

Commit b98535d09179 ("ext4: fix bug_on in start_this_handle during umount filesystem") moved ext4_unregister_sysfs() before flushing s_sb_upd_work to prevent new error work from being queued via /proc/fs/ext4/xx/mb_groups reads during unmount. However, this introduced a use-after-free because update_super_work calls ext4_notify_error_sysfs() -> sysfs_notify() which accesses the kobject's kernfs_node after it has been freed by kobject_del() in ext4_unregister_sysfs():

update_super_work ext4_put_super

                               ext4_unregister_sysfs(sb)
                                 kobject_del(&sbi->s_kobj)
                                   __kobject_del()
                                     sysfs_remove_dir()
                                       kobj->sd = NULL
                                     sysfs_put(sd)
                                       kernfs_put()  // RCU free

ext4_notify_error_sysfs(sbi) sysfs_notify(&sbi->s_kobj) kn = kobj->sd // stale pointer kernfs_get(kn) // UAF on freed kernfs_node ext4_journal_destroy() flush_work(&sbi->s_sb_upd_work)

Instead of reordering the teardown sequence, fix this by making ext4_notify_error_sysfs() detect that sysfs has already been torn down by checking s_kobj.state_in_sysfs, and skipping the sysfs_notify() call in that case. A dedicated mutex (s_error_notify_mutex) serializes ext4_notify_error_sysfs() against kobject_del() in ext4_unregister_sysfs() to prevent TOCTOU races where the kobject could be deleted between the state_in_sysfs check and the sysfs_notify() call.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/c8fe17a1b308c3d8c703ebfb049b325f844342c3
• https://git.kernel.org/stable/c/c4d829737329f2290dd41e290b7d75effdb2a7ff
• https://git.kernel.org/stable/c/9449f99ba04f5dd1c8423ad8a90b3651d7240d1d
• https://git.kernel.org/stable/c/034053378dd81837fd6c7a43b37ee2e58d4f0b4e
• https://git.kernel.org/stable/c/c97e282f7bfd0c3554c63d289964a5ca6a1d2ffe
• https://git.kernel.org/stable/c/08b10e6f37fc533a759e9833af0692242e8b3f93
• https://git.kernel.org/stable/c/d15e4b0a418537aafa56b2cb80d44add83e83697

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