{"id":"DEBIAN-CVE-2026-64378","details":"In the Linux kernel, the following vulnerability has been resolved:  writeback: fix race between cgroup_writeback_umount() and inode_switch_wbs()  When a container exits, the following BUG_ON() is occasionally triggered:  ==================================================================  VFS: Busy inodes after unmount of sdb (ext4)  ------------[ cut here ]------------  kernel BUG at fs/super.c:695!  CPU: 3 PID: 6 Comm: containerd-shim Tainted: G OE K 6.6 #1  pstate: 63400009 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)  pc : generic_shutdown_super+0xf0/0x100  lr : generic_shutdown_super+0xf0/0x100  Call trace:   generic_shutdown_super+0xf0/0x100   kill_block_super+0x20/0x48   ext4_kill_sb+0x28/0x60   deactivate_locked_super+0x54/0x130   deactivate_super+0x84/0xa0   cleanup_mnt+0xa4/0x140   __cleanup_mnt+0x18/0x28   task_work_run+0x78/0xe0   do_notify_resume+0x204/0x240 ==================================================================  The root cause is a race between cgroup_writeback_umount() and inode_switch_wbs()/cleanup_offline_cgwb(). There is a window between inode_prepare_wbs_switch() returning true and the subsequent wb_queue_isw() call. Following is the process that triggers the issue:        CPU A (umount)           |          CPU B (writeback) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                  inode_switch_wbs/cleanup_offline_cgwb                                   atomic_inc(&isw_nr_in_flight)                                   inode_prepare_wbs_switch                                    -\u003e passes SB_ACTIVE check                                    __iget(inode)  generic_shutdown_super   sb-\u003es_flags &= ~SB_ACTIVE   cgroup_writeback_umount(sb)    smp_mb()    atomic_read(&isw_nr_in_flight)    rcu_barrier()     -\u003e no pending RCU callbacks    flush_workqueue(isw_wq)     -\u003e nothing queued, returns   evict_inodes(sb)    -\u003e Inode skipped as isw still holds a ref.   sop-\u003eput_super(sb)    /* destroys percpu counters */   -\u003e VFS: Busy inodes after unmount!                                   wb_queue_isw()                                    queue_work(isw_wq, ...)                                   /* later in work function */                                   inode_switch_wbs_work_fn                                    process_inode_switch_wbs                                     iput() -\u003e evict                                      percpu_counter_dec() // UAF!  Fix this by extending the RCU read-side critical section in inode_switch_wbs() and cleanup_offline_cgwb() to cover from inode_prepare_wbs_switch() through wb_queue_isw().  Since there is no sleep in this window, rcu_read_lock() can be used.  Then add a synchronize_rcu() in cgroup_writeback_umount() before the existing rcu_barrier(), so that all in-flight switchers that have passed the SB_ACTIVE check have completed queue_work() before flush_workqueue() is called.  The existing rcu_barrier() is intentionally retained so this fix can be backported unchanged to stable kernels (5.10.y, 6.6.y, ...) that still queue switches via queue_rcu_work(). It is a no-op on current mainline (since commit e1b849cfa6b6 (\"writeback: Avoid contention on wb-\u003elist_lock when switching inodes\")) and is removed in a follow-up patch.","modified":"2026-09-09T08:47:27.245718422Z","published":"2026-07-25T10:17:20.940Z","upstream":["CVE-2026-64378"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2026-64378"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:12","purl":"pkg:deb/debian/linux?arch=source&distro=bookworm"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.1.180-1"}]}],"versions":["6.1.106-1","6.1.106-2","6.1.106-3","6.1.112-1","6.1.115-1","6.1.119-1","6.1.123-1","6.1.124-1","6.1.128-1","6.1.129-1","6.1.133-1","6.1.135-1","6.1.137-1","6.1.139-1","6.1.140-1","6.1.147-1","6.1.148-1","6.1.153-1","6.1.158-1","6.1.159-1","6.1.162-1","6.1.164-1","6.1.170-1","6.1.170-2","6.1.170-3","6.1.172-1","6.1.174-1","6.1.176-1","6.1.177-1","6.1.27-1","6.1.37-1","6.1.38-1","6.1.38-2","6.1.38-2~bpo11+1","6.1.38-3","6.1.38-4","6.1.38-4~bpo11+1","6.1.52-1","6.1.55-1","6.1.55-1~bpo11+1","6.1.64-1","6.1.66-1","6.1.67-1","6.1.69-1","6.1.69-1~bpo11+1","6.1.76-1","6.1.76-1~bpo11+1","6.1.82-1","6.1.85-1","6.1.90-1","6.1.90-1~bpo11+1","6.1.94-1","6.1.94-1~bpo11+1","6.1.98-1","6.1.99-1"],"ecosystem_specific":{"urgency":"not 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