btrfs: fix race between quota enable and quota rescan ioctl

When enabling quotas, at btrfs_quota_enable(), after committing the
transaction, we change fs_info->quota_root to point to the quota root we
created and set BTRFS_FS_QUOTA_ENABLED at fs_info->flags. Then we try
to start the qgroup rescan worker, first by initializing it with a call
to qgroup_rescan_init() - however if that fails we end up freeing the
quota root but we leave fs_info->quota_root still pointing to it, this
can later result in a use-after-free somewhere else.

We have previously set the flags BTRFS_FS_QUOTA_ENABLED and
BTRFS_QGROUP_STATUS_FLAG_ON, so we can only fail with -EINPROGRESS at
btrfs_quota_enable(), which is possible if someone already called the
quota rescan ioctl, and therefore started the rescan worker.

So fix this by ignoring an -EINPROGRESS and asserting we can't get any
other error.

Reported-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/linux-btrfs/20220823015931.421355-1-yebin10@huawei.com/
CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index db723c0..ba323dc 100644 (file)
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1174,6 +1174,21 @@ out_add_root:
                fs_info->qgroup_rescan_running = true;
                btrfs_queue_work(fs_info->qgroup_rescan_workers,
                                 &fs_info->qgroup_rescan_work);
+       } else {
+               /*
+                * We have set both BTRFS_FS_QUOTA_ENABLED and
+                * BTRFS_QGROUP_STATUS_FLAG_ON, so we can only fail with
+                * -EINPROGRESS. That can happen because someone started the
+                * rescan worker by calling quota rescan ioctl before we
+                * attempted to initialize the rescan worker. Failure due to
+                * quotas disabled in the meanwhile is not possible, because
+                * we are holding a write lock on fs_info->subvol_sem, which
+                * is also acquired when disabling quotas.
+                * Ignore such error, and any other error would need to undo
+                * everything we did in the transaction we just committed.
+                */
+               ASSERT(ret == -EINPROGRESS);
+               ret = 0;
        }
 
 out_free_path: