CVE-2025-21753: btrfs: fix use-after-free when attempting to join an aborted transaction
In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix use-after-free when attempting to join an aborted transaction

When we are trying to join the current transaction and if it's aborted,
we read its 'aborted' field after unlocking fs_info->trans_lock and
without holding any extra reference count on it. This means that a
concurrent task that is aborting the transaction may free the transaction
before we read its 'aborted' field, leading to a use-after-free.

Fix this by reading the 'aborted' field while holding fs_info->trans_lock
since any freeing task must first acquire that lock and set
fs_info->running_transaction to NULL before freeing the transaction.

This was reported by syzbot and Dmitry with the following stack traces
from KASAN:

   ==================================================================
   BUG: KASAN: slab-use-after-free in join_transaction+0xd9b/0xda0 fs/btrfs/transaction.c:278
   Read of size 4 at addr ffff888011839024 by task kworker/u4:9/1128

   CPU: 0 UID: 0 PID: 1128 Comm: kworker/u4:9 Not tainted 6.13.0-rc7-syzkaller-00019-gc45323b7560e #0
   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
   Workqueue: events_unbound btrfs_async_reclaim_data_space
   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:378 [inline]
    print_report+0x169/0x550 mm/kasan/report.c:489
    kasan_report+0x143/0x180 mm/kasan/report.c:602
    join_transaction+0xd9b/0xda0 fs/btrfs/transaction.c:278
    start_transaction+0xaf8/0x1670 fs/btrfs/transaction.c:697
    flush_space+0x448/0xcf0 fs/btrfs/space-info.c:803
    btrfs_async_reclaim_data_space+0x159/0x510 fs/btrfs/space-info.c:1321
    process_one_work kernel/workqueue.c:3236 [inline]
    process_scheduled_works+0xa66/0x1840 kernel/workqueue.c:3317
    worker_thread+0x870/0xd30 kernel/workqueue.c:3398
    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 5315:
    kasan_save_stack mm/kasan/common.c:47 [inline]
    kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
    poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
    __kasan_kmalloc+0x98/0xb0 mm/kasan/common.c:394
    kasan_kmalloc include/linux/kasan.h:260 [inline]
    __kmalloc_cache_noprof+0x243/0x390 mm/slub.c:4329
    kmalloc_noprof include/linux/slab.h:901 [inline]
    join_transaction+0x144/0xda0 fs/btrfs/transaction.c:308
    start_transaction+0xaf8/0x1670 fs/btrfs/transaction.c:697
    btrfs_create_common+0x1b2/0x2e0 fs/btrfs/inode.c:6572
    lookup_open fs/namei.c:3649 [inline]
    open_last_lookups fs/namei.c:3748 [inline]
    path_openat+0x1c03/0x3590 fs/namei.c:3984
    do_filp_open+0x27f/0x4e0 fs/namei.c:4014
    do_sys_openat2+0x13e/0x1d0 fs/open.c:1402
    do_sys_open fs/open.c:1417 [inline]
    __do_sys_creat fs/open.c:1495 [inline]
    __se_sys_creat fs/open.c:1489 [inline]
    __x64_sys_creat+0x123/0x170 fs/open.c:1489
    do_syscall_x64 arch/x86/entry/common.c:52 [inline]
    do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

   Freed by task 5336:
    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:582
    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:233 [inline]
    slab_free_hook mm/slub.c:2353 [inline]
    slab_free mm/slub.c:4613 [inline]
    kfree+0x196/0x430 mm/slub.c:4761
    cleanup_transaction fs/btrfs/transaction.c:2063 [inline]
    btrfs_commit_transaction+0x2c97/0x3720 fs/btrfs/transaction.c:2598
    insert_balance_item+0x1284/0x20b0 fs/btrfs/volumes.c:3757
    btrfs_balance+0x992/
---truncated---
CVSS v3.1: 7.8