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

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, Feb 03, 2017 at 04:20:23PM +0800, Qu Wenruo wrote:
> 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.
>

We already have bio_counter to block device replace to avoid
use-after-free problem, is there any particular reason of not using
it?

Thanks,

-liubo

> 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
> 
> 
> 
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