[PATCH 5/5] btrfs: replace: Use ref counts to avoid destroying target device when canceled

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

When dev-replace and scrub are run at the same time, dev-replace can be
canceled by scrub. It's quite common for btrfs/069.

The backtrace would be like:
general protection fault: 0000 [#1] SMP
Workqueue: btrfs-endio-raid56 btrfs_endio_raid56_helper [btrfs]
RIP: 0010:[<ffffffff813a2fa8>]  [<ffffffff813a2fa8>] generic_make_request_checks+0x198/0x5a0
Call Trace:
 [<ffffffff813a4cff>] ? generic_make_request+0xcf/0x290
 [<ffffffff813a4c54>] generic_make_request+0x24/0x290
 [<ffffffff813a4cff>] ? generic_make_request+0xcf/0x290
 [<ffffffff813a4f2e>] submit_bio+0x6e/0x120
 [<ffffffffa087279d>] ? page_in_rbio+0x4d/0x80 [btrfs]
 [<ffffffffa08737d0>] ? rbio_orig_end_io+0x80/0x80 [btrfs]
 [<ffffffffa0873e31>] finish_rmw+0x401/0x550 [btrfs]
 [<ffffffffa0874fc6>] validate_rbio_for_rmw+0x36/0x40 [btrfs]
 [<ffffffffa087504d>] raid_rmw_end_io+0x7d/0x90 [btrfs]
 [<ffffffff8139c536>] bio_endio+0x56/0x60
 [<ffffffffa07e6e5c>] end_workqueue_fn+0x3c/0x40 [btrfs]
 [<ffffffffa08285bf>] btrfs_scrubparity_helper+0xef/0x610 [btrfs]
 [<ffffffffa0828b9e>] btrfs_endio_raid56_helper+0xe/0x10 [btrfs]
 [<ffffffff810ec8df>] process_one_work+0x2af/0x720
 [<ffffffff810ec85b>] ? process_one_work+0x22b/0x720
 [<ffffffff810ecd9b>] worker_thread+0x4b/0x4f0
 [<ffffffff810ecd50>] ? process_one_work+0x720/0x720
 [<ffffffff810ecd50>] ? process_one_work+0x720/0x720
 [<ffffffff810f39d3>] kthread+0xf3/0x110
 [<ffffffff810f38e0>] ? kthread_park+0x60/0x60
 [<ffffffff81857647>] ret_from_fork+0x27/0x40

While in that case, target device can be destroyed at cancel time,
leading to a user-after-free bug:

     Process A (dev-replace)         |         Process B(scrub)
----------------------------------------------------------------------
                                     |(Any RW is OK)
                                     |scrub_setup_recheck_block()
                                     ||- btrfs_map_sblock()
                                     |   Got a bbio with tgtdev
btrfs_dev_replace_finishing()        |
|- btrfs_destory_dev_replace_tgtdev()|
   |- call_rcu(free_device)          |
      |- __free_device()             |
         |- kfree(device)            |
                                     | Scrub worker:
                                     | Access bbio->stripes[], which
                                     | contains tgtdev.
                                     | This triggers general protection.

The bug is mostly obvious for RAID5/6 since raid56 choose to keep old
rbio and rbio->bbio for later steal, this hugely enlarged the race
window and makes it much easier to trigger the bug.

This patch introduces 'tgtdev_refs' and 'tgtdev_wait' for btrfs_device
to wait for all its user released the target device.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
 fs/btrfs/dev-replace.c |  7 ++++++-
 fs/btrfs/volumes.c     | 36 +++++++++++++++++++++++++++++++++++-
 fs/btrfs/volumes.h     | 10 ++++++++++
 3 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index 5de280b9ad73..794a6a0bedf2 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -558,7 +558,6 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 			  rcu_str_deref(src_device->name),
 			  src_device->devid,
 			  rcu_str_deref(tgt_device->name));
-	tgt_device->is_tgtdev_for_dev_replace = 0;
 	tgt_device->devid = src_device->devid;
 	src_device->devid = BTRFS_DEV_REPLACE_DEVID;
 	memcpy(uuid_tmp, tgt_device->uuid, sizeof(uuid_tmp));
@@ -579,6 +578,12 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 
 	btrfs_dev_replace_unlock(dev_replace, 1);
 
+	/*
+	 * Only change is_tgtdev_for_dev_replace flag after all its
+	 * users get released.
+	 */
+	wait_target_device(tgt_device);
+	tgt_device->is_tgtdev_for_dev_replace = 0;
 	btrfs_rm_dev_replace_blocked(fs_info);
 
 	btrfs_rm_dev_replace_remove_srcdev(fs_info, src_device);
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 3c3c69c0eee4..84db9fb22b7d 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -2064,6 +2064,7 @@ void btrfs_destroy_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
 	WARN_ON(!tgtdev);
 	mutex_lock(&fs_info->fs_devices->device_list_mutex);
 
+	wait_target_device(tgtdev);
 	btrfs_sysfs_rm_device_link(fs_info->fs_devices, tgtdev);
 
 	if (tgtdev->bdev)
@@ -2598,6 +2599,8 @@ int btrfs_init_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
 	device->is_tgtdev_for_dev_replace = 1;
 	device->mode = FMODE_EXCL;
 	device->dev_stats_valid = 1;
+	atomic_set(&device->tgtdev_refs, 0);
+	init_waitqueue_head(&device->tgtdev_wait);
 	set_blocksize(device->bdev, 4096);
 	device->fs_devices = fs_info->fs_devices;
 	list_add(&device->dev_list, &fs_info->fs_devices->devices);
@@ -2624,6 +2627,8 @@ void btrfs_init_dev_replace_tgtdev_for_resume(struct btrfs_fs_info *fs_info,
 	tgtdev->sector_size = sectorsize;
 	tgtdev->fs_info = fs_info;
 	tgtdev->in_fs_metadata = 1;
+	atomic_set(&tgtdev->tgtdev_refs, 0);
+	init_waitqueue_head(&tgtdev->tgtdev_wait);
 }
 
 static noinline int btrfs_update_device(struct btrfs_trans_handle *trans,
@@ -5302,6 +5307,32 @@ static struct btrfs_bio *alloc_btrfs_bio(int total_stripes, int real_stripes)
 	return bbio;
 }
 
+static void pin_bbio_target_device(struct btrfs_bio *bbio)
+{
+	int i;
+
+	for (i = 0; i < bbio->num_stripes; i++) {
+		struct btrfs_device *device = bbio->stripes[i].dev;
+
+		if (device->is_tgtdev_for_dev_replace)
+			atomic_inc(&device->tgtdev_refs);
+	}
+}
+
+static void unpin_bbio_target_device(struct btrfs_bio *bbio)
+{
+	int i;
+
+	for (i = 0; i < bbio->num_stripes; i++) {
+		struct btrfs_device *device = bbio->stripes[i].dev;
+
+		if (device->is_tgtdev_for_dev_replace) {
+			atomic_dec(&device->tgtdev_refs);
+			wake_up(&device->tgtdev_wait);
+		}
+	}
+}
+
 void btrfs_get_bbio(struct btrfs_bio *bbio)
 {
 	WARN_ON(!atomic_read(&bbio->refs));
@@ -5312,8 +5343,10 @@ void btrfs_put_bbio(struct btrfs_bio *bbio)
 {
 	if (!bbio)
 		return;
-	if (atomic_dec_and_test(&bbio->refs))
+	if (atomic_dec_and_test(&bbio->refs)) {
+		unpin_bbio_target_device(bbio);
 		kfree(bbio);
+	}
 }
 
 static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
@@ -5868,6 +5901,7 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info,
 		bbio->stripes[0].physical = physical_to_patch_in_first_stripe;
 		bbio->mirror_num = map->num_stripes + 1;
 	}
+	pin_bbio_target_device(bbio);
 out:
 	if (dev_replace_is_ongoing) {
 		btrfs_dev_replace_clear_lock_blocking(dev_replace);
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 24ba6bc3ec34..702361182597 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -149,6 +149,10 @@ struct btrfs_device {
 	/* Counter to record the change of device stats */
 	atomic_t dev_stats_ccnt;
 	atomic_t dev_stat_values[BTRFS_DEV_STAT_VALUES_MAX];
+
+	/* To ensure we wait this target device before destroying it */
+	atomic_t tgtdev_refs;
+	wait_queue_head_t tgtdev_wait;
 };
 
 /*
@@ -538,4 +542,10 @@ struct list_head *btrfs_get_fs_uuids(void);
 void btrfs_set_fs_info_ptr(struct btrfs_fs_info *fs_info);
 void btrfs_reset_fs_info_ptr(struct btrfs_fs_info *fs_info);
 
+static inline void wait_target_device(struct btrfs_device *tgtdev)
+{
+	if (!tgtdev || !tgtdev->is_tgtdev_for_dev_replace)
+		return;
+	wait_event(tgtdev->tgtdev_wait, atomic_read(&tgtdev->tgtdev_refs) == 0);
+}
 #endif
-- 
2.11.0