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btrfs: fix race between quota rescan and disable leading to NULL pointer deref

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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.

Reported-by: syzbot+96977faa68092ad382c4@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/000000000000e5454b05f065a803@google.com/
Fixes: e804861bd4 ("btrfs: fix deadlock between quota disable and qgroup rescan worker")
CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2023-01-12 16:31:08 +00:00 committed by David Sterba
parent 1f55ee6d09
commit b7adbf9ada
1 changed files with 17 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
fs/btrfs/qgroup.c
View File

@ -3367,6 +3367,7 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
int err = -ENOMEM;
int ret = 0;
bool stopped = false;
bool did_leaf_rescans = false;
path = btrfs_alloc_path();
if (!path)
@ -3387,6 +3388,7 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
}
err = qgroup_rescan_leaf(trans, path);
did_leaf_rescans = true;
if (err > 0)
btrfs_commit_transaction(trans);
@ -3407,16 +3409,23 @@ out:
mutex_unlock(&fs_info->qgroup_rescan_lock);
/*
* only update status, since the previous part has already updated the
* qgroup info.
* Only update status, since the previous part has already updated the
* qgroup info, and only if we did any actual work. This also prevents
* race with a concurrent quota disable, which has already set
* fs_info->quota_root to NULL and cleared BTRFS_FS_QUOTA_ENABLED at
* btrfs_quota_disable().
*/
trans = btrfs_start_transaction(fs_info->quota_root, 1);
if (IS_ERR(trans)) {
err = PTR_ERR(trans);
if (did_leaf_rescans) {
trans = btrfs_start_transaction(fs_info->quota_root, 1);
if (IS_ERR(trans)) {
err = PTR_ERR(trans);
trans = NULL;
btrfs_err(fs_info,
"fail to start transaction for status update: %d",
err);
}
} else {
trans = NULL;
btrfs_err(fs_info,
"fail to start transaction for status update: %d",
err);
}
mutex_lock(&fs_info->qgroup_rescan_lock);