CVE-2023-52896: In the Linux kernel, the following vulnerability has been resolved:  btrfs: fix race between quota rescan and disable leading to NULL pointer deref  If we have one task trying to start the quota rescan worker while another one is trying to disable quotas, we can end up hitting a race that results in the quota rescan worker doing a NULL pointer dereference. The steps for this are the following:  1) Quotas are enabled;  2) Task A calls the quota rescan ioctl and enters btrfs_qgroup_rescan().    It calls qgroup_rescan_init() which returns 0 (success) and then joins a    transaction and commits it;  3) Task B calls the quota disable ioctl and enters btrfs_quota_disable().    It clears the bit BTRFS_FS_QUOTA_ENABLED from fs_info->flags and calls    btrfs_qgroup_wait_for_completion(), which returns immediately since the    rescan worker is not yet running.    Then it starts a transaction and locks fs_info->qgroup_ioctl_lock;  4) Task A queues the rescan worker, by calling btrfs_queue_work();  5) The rescan worker starts, and calls rescan_should_stop() at the start    of its while loop, which results in 0 iterations of the loop, since    the flag BTRFS_FS_QUOTA_ENABLED was cleared from fs_info->flags by    task B at step 3);  6) Task B sets fs_info->quota_root to NULL;  7) The rescan worker tries to start a transaction and uses    fs_info->quota_root as the root argument for btrfs_start_transaction().    This results in a NULL pointer dereference down the call chain of    btrfs_start_transaction(). The stack trace is something like the one    reported in Link tag below:     general protection fault, probably for non-canonical address 0xdffffc0000000041: 0000 [#1] PREEMPT SMP KASAN    KASAN: null-ptr-deref in range [0x0000000000000208-0x000000000000020f]    CPU: 1 PID: 34 Comm: kworker/u4:2 Not tainted 6.1.0-syzkaller-13872-gb6bb9676f216 #0    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022    Workqueue: btrfs-qgroup-rescan btrfs_work_helper    RIP: 0010:start_transaction+0x48/0x10f0 fs/btrfs/transaction.c:564    Code: 48 89 fb 48 (...)    RSP: 0018:ffffc90000ab7ab0 EFLAGS: 00010206    RAX: 0000000000000041 RBX: 0000000000000208 RCX: ffff88801779ba80    RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000    RBP: dffffc0000000000 R08: 0000000000000001 R09: fffff52000156f5d    R10: fffff52000156f5d R11: 1ffff92000156f5c R12: 0000000000000000    R13: 0000000000000001 R14: 0000000000000001 R15: 0000000000000003    FS:  0000000000000000(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033    CR2: 00007f2bea75b718 CR3: 000000001d0cc000 CR4: 00000000003506e0    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400    Call Trace:     <TASK>     btrfs_qgroup_rescan_worker+0x3bb/0x6a0 fs/btrfs/qgroup.c:3402     btrfs_work_helper+0x312/0x850 fs/btrfs/async-thread.c:280     process_one_work+0x877/0xdb0 kernel/workqueue.c:2289     worker_thread+0xb14/0x1330 kernel/workqueue.c:2436     kthread+0x266/0x300 kernel/kthread.c:376     ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:308     </TASK>    Modules linked in:  So fix this by having the rescan worker function not attempt to start a transaction if it didn't do any rescan work.