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

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

On Mon, Mar 20, 2017 at 02:30:48PM +0800, Qu Wenruo wrote:
> 
> 
> At 03/18/2017 10:12 AM, Liu Bo wrote:
> > 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 for pointing this out.
> 
> And I found raid56 is already using bio_counter().
> But we still trigger such use-after-free bug because bio_counter only ensure
> we won't delete/replace a device when there is still bio running.
> 
> Here we are facing a difference situation, where we holds a pointer to
> btrfs_dev which can be freed since there is no pending bios for the whole
> fs.
>

It's not just for in-flight bios, check btrfs_discard_extent(), anyone
who expects target devices to be alive can use bio_counter to avoid
the race against device replace.

You could inc/dec bio_counter along with scrub_recover as it takes a
pointer of bbio and maintains a reference.

Thanks,

-liubo

> So here I'm afraid we need refcount based solution for each btrfs_dev
> structure, other than global bio_counter.
> 
> Thanks,
> Qu
> > 
> > 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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> > 
> > 
> 
>