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 btrfsqgrouprescan(). It calls qgrouprescaninit() which returns 0 (success) and then joins a transaction and commits it;

3) Task B calls the quota disable ioctl and enters btrfsquotadisable(). It clears the bit BTRFSFSQUOTAENABLED from fsinfo->flags and calls btrfsqgroupwaitforcompletion(), which returns immediately since the rescan worker is not yet running. Then it starts a transaction and locks fsinfo->qgroupioctl_lock;

4) Task A queues the rescan worker, by calling btrfsqueuework();

5) The rescan worker starts, and calls rescanshouldstop() at the start of its while loop, which results in 0 iterations of the loop, since the flag BTRFSFSQUOTAENABLED was cleared from fsinfo->flags by task B at step 3);

6) Task B sets fsinfo->quotaroot to NULL;

7) The rescan worker tries to start a transaction and uses fsinfo->quotaroot as the root argument for btrfsstarttransaction(). This results in a NULL pointer dereference down the call chain of btrfsstarttransaction(). 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 btrfsworkhelper RIP: 0010:starttransaction+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> btrfsqgrouprescanworker+0x3bb/0x6a0 fs/btrfs/qgroup.c:3402 btrfsworkhelper+0x312/0x850 fs/btrfs/async-thread.c:280 processonework+0x877/0xdb0 kernel/workqueue.c:2289 workerthread+0xb14/0x1330 kernel/workqueue.c:2436 kthread+0x266/0x300 kernel/kthread.c:376 retfromfork+0x1f/0x30 arch/x86/entry/entry64.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.