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

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