CVE-2026-31446

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> ext4: fix use-after-free in update_super_work when racing with umount<br /> <br /> Commit b98535d09179 ("ext4: fix bug_on in start_this_handle during umount<br /> filesystem") moved ext4_unregister_sysfs() before flushing s_sb_upd_work<br /> to prevent new error work from being queued via /proc/fs/ext4/xx/mb_groups<br /> reads during unmount. However, this introduced a use-after-free because<br /> update_super_work calls ext4_notify_error_sysfs() -&gt; sysfs_notify() which<br /> accesses the kobject&amp;#39;s kernfs_node after it has been freed by kobject_del()<br /> in ext4_unregister_sysfs():<br /> <br /> update_super_work ext4_put_super<br /> ----------------- --------------<br /> ext4_unregister_sysfs(sb)<br /> kobject_del(&amp;sbi-&gt;s_kobj)<br /> __kobject_del()<br /> sysfs_remove_dir()<br /> kobj-&gt;sd = NULL<br /> sysfs_put(sd)<br /> kernfs_put() // RCU free<br /> ext4_notify_error_sysfs(sbi)<br /> sysfs_notify(&amp;sbi-&gt;s_kobj)<br /> kn = kobj-&gt;sd // stale pointer<br /> kernfs_get(kn) // UAF on freed kernfs_node<br /> ext4_journal_destroy()<br /> flush_work(&amp;sbi-&gt;s_sb_upd_work)<br /> <br /> Instead of reordering the teardown sequence, fix this by making<br /> ext4_notify_error_sysfs() detect that sysfs has already been torn down<br /> by checking s_kobj.state_in_sysfs, and skipping the sysfs_notify() call<br /> in that case. A dedicated mutex (s_error_notify_mutex) serializes<br /> ext4_notify_error_sysfs() against kobject_del() in ext4_unregister_sysfs()<br /> to prevent TOCTOU races where the kobject could be deleted between the<br /> state_in_sysfs check and the sysfs_notify() call.