CVE-2024-49867: In the Linux kernel, the following vulnerability has been resolved:  btrfs: wait for fixup workers before stopping cleaner kthread during umount  During unmount, at close_ctree(), we have the following steps in this order:  1) Park the cleaner kthread - this doesn't destroy the kthread, it basically    halts its execution (wake ups against it work but do nothing);  2) We stop the cleaner kthread - this results in freeing the respective    struct task_struct;  3) We call btrfs_stop_all_workers() which waits for any jobs running in all    the work queues and then free the work queues.  Syzbot reported a case where a fixup worker resulted in a crash when doing a delayed iput on its inode while attempting to wake up the cleaner at btrfs_add_delayed_iput(), because the task_struct of the cleaner kthread was already freed. This can happen during unmount because we don't wait for any fixup workers still running before we call kthread_stop() against the cleaner kthread, which stops and free all its resources.  Fix this by waiting for any fixup workers at close_ctree() before we call kthread_stop() against the cleaner and run pending delayed iputs.  The stack traces reported by syzbot were the following:    BUG: KASAN: slab-use-after-free in __lock_acquire+0x77/0x2050 kernel/locking/lockdep.c:5065   Read of size 8 at addr ffff8880272a8a18 by task kworker/u8:3/52    CPU: 1 UID: 0 PID: 52 Comm: kworker/u8:3 Not tainted 6.12.0-rc1-syzkaller #0   Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024   Workqueue: btrfs-fixup btrfs_work_helper   Call Trace:    <TASK>    __dump_stack lib/dump_stack.c:94 [inline]    dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120    print_address_description mm/kasan/report.c:377 [inline]    print_report+0x169/0x550 mm/kasan/report.c:488    kasan_report+0x143/0x180 mm/kasan/report.c:601    __lock_acquire+0x77/0x2050 kernel/locking/lockdep.c:5065    lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5825    __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]    _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162    class_raw_spinlock_irqsave_constructor include/linux/spinlock.h:551 [inline]    try_to_wake_up+0xb0/0x1480 kernel/sched/core.c:4154    btrfs_writepage_fixup_worker+0xc16/0xdf0 fs/btrfs/inode.c:2842    btrfs_work_helper+0x390/0xc50 fs/btrfs/async-thread.c:314    process_one_work kernel/workqueue.c:3229 [inline]    process_scheduled_works+0xa63/0x1850 kernel/workqueue.c:3310    worker_thread+0x870/0xd30 kernel/workqueue.c:3391    kthread+0x2f0/0x390 kernel/kthread.c:389    ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147    ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244    </TASK>    Allocated by task 2:    kasan_save_stack mm/kasan/common.c:47 [inline]    kasan_save_track+0x3f/0x80 mm/kasan/common.c:68    unpoison_slab_object mm/kasan/common.c:319 [inline]    __kasan_slab_alloc+0x66/0x80 mm/kasan/common.c:345    kasan_slab_alloc include/linux/kasan.h:247 [inline]    slab_post_alloc_hook mm/slub.c:4086 [inline]    slab_alloc_node mm/slub.c:4135 [inline]    kmem_cache_alloc_node_noprof+0x16b/0x320 mm/slub.c:4187    alloc_task_struct_node kernel/fork.c:180 [inline]    dup_task_struct+0x57/0x8c0 kernel/fork.c:1107    copy_process+0x5d1/0x3d50 kernel/fork.c:2206    kernel_clone+0x223/0x880 kernel/fork.c:2787    kernel_thread+0x1bc/0x240 kernel/fork.c:2849    create_kthread kernel/kthread.c:412 [inline]    kthreadd+0x60d/0x810 kernel/kthread.c:765    ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147    ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244    Freed by task 61:    kasan_save_stack mm/kasan/common.c:47 [inline]    kasan_save_track+0x3f/0x80 mm/kasan/common.c:68    kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:579    poison_slab_object mm/kasan/common.c:247 [inline]    __kasan_slab_free+0x59/0x70 mm/kasan/common.c:264    kasan_slab_free include/linux/kasan.h:230 [inline]    slab_free_h ---truncated---