[PATCH 4/9 v2.1] btrfs: fix UAF due to race between replace start and cancel

[Anand Jain <anand.jain@oracle.com>]

replace cancel thread can race with the replace start thread and if
fs_info::scrubs_running is not yet set the btrfs_scrub_cancel() will fail
to stop the scrub thread, so the scrub thread continue with the scrub for
replace which then shall try to write to the target device and which is
already freed by the cancel thread. Please ref to the logs below.

So scrub_setup_ctx() warns as tgtdev is null.

static noinline_for_stack
struct scrub_ctx *scrub_setup_ctx(struct btrfs_device *dev, int
is_dev_replace)
{
::
        if (is_dev_replace) {
                WARN_ON(!fs_info->dev_replace.tgtdev);  <===
                sctx->pages_per_wr_bio = SCRUB_PAGES_PER_WR_BIO;
                sctx->wr_tgtdev = fs_info->dev_replace.tgtdev;
                sctx->flush_all_writes = false;
        }

[ 6724.497655] BTRFS info (device sdb): dev_replace from /dev/sdb (devid 1) to /dev/sdc started
[ 6753.945017] BTRFS info (device sdb): dev_replace from /dev/sdb (devid 1) to /dev/sdc canceled
[ 6852.426700] WARNING: CPU: 0 PID: 4494 at fs/btrfs/scrub.c:622 scrub_setup_ctx.isra.19+0x220/0x230 [btrfs]
::
[ 6852.428928] RIP: 0010:scrub_setup_ctx.isra.19+0x220/0x230 [btrfs]
::
[ 6852.432970] Call Trace:
[ 6852.433202]  btrfs_scrub_dev+0x19b/0x5c0 [btrfs]
[ 6852.433471]  btrfs_dev_replace_start+0x48c/0x6a0 [btrfs]
[ 6852.433800]  btrfs_dev_replace_by_ioctl+0x3a/0x60 [btrfs]
[ 6852.434097]  btrfs_ioctl+0x2476/0x2d20 [btrfs]
[ 6852.434365]  ? do_sigaction+0x7d/0x1e0
[ 6852.434623]  do_vfs_ioctl+0xa9/0x6c0
[ 6852.434865]  ? syscall_trace_enter+0x1c8/0x310
[ 6852.435124]  ? syscall_trace_enter+0x1c8/0x310
[ 6852.435387]  ksys_ioctl+0x60/0x90
[ 6852.435663]  __x64_sys_ioctl+0x16/0x20
[ 6852.435907]  do_syscall_64+0x50/0x180
[ 6852.436150]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Further, as the replace thread enters the
scrub_write_page_to_dev_replace() without the target device it panics

static int scrub_add_page_to_wr_bio(struct scrub_ctx *sctx,
                                    struct scrub_page *spage)
{
::
      bio_set_dev(bio, sbio->dev->bdev); <======

[ 6929.715145] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
::
[ 6929.717106] Workqueue: btrfs-scrub btrfs_scrub_helper [btrfs]
[ 6929.717420] RIP: 0010:scrub_write_page_to_dev_replace+0xb4/0x260
[btrfs]
::
[ 6929.721430] Call Trace:
[ 6929.721663]  scrub_write_block_to_dev_replace+0x3f/0x60 [btrfs]
[ 6929.721975]  scrub_bio_end_io_worker+0x1af/0x490 [btrfs]
[ 6929.722277]  normal_work_helper+0xf0/0x4c0 [btrfs]
[ 6929.722552]  process_one_work+0x1f4/0x520
[ 6929.722805]  ? process_one_work+0x16e/0x520
[ 6929.723063]  worker_thread+0x46/0x3d0
[ 6929.723313]  kthread+0xf8/0x130
[ 6929.723544]  ? process_one_work+0x520/0x520
[ 6929.723800]  ? kthread_delayed_work_timer_fn+0x80/0x80
[ 6929.724081]  ret_from_fork+0x3a/0x50

Fix this by letting the btrfs_dev_replace_finishing() to do the job of
cleaning after the cancel, including freeing of the target device.
btrfs_dev_replace_finishing() is called when btrfs_scub_dev() returns
along with the scrub return status.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v2->v2.1: Fix compiler warning. (I couldn't reproduce on gcc 4.8.5)

fs/btrfs/dev-replace.c: In function ‘btrfs_dev_replace_cancel’:
fs/btrfs/dev-replace.c:865:9: warning: ‘result’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return result;
         ^~~~~~

v1->v2: pls ref to the cover letter

 fs/btrfs/dev-replace.c | 61 ++++++++++++++++++++++++++++++++------------------
 1 file changed, 39 insertions(+), 22 deletions(-)

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index 35ce10f18607..d32d696d931c 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -797,39 +797,56 @@ int btrfs_dev_replace_cancel(struct btrfs_fs_info *fs_info)
 	case BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED:
 		result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED;
 		btrfs_dev_replace_write_unlock(dev_replace);
-		goto leave;
+		break;
 	case BTRFS_IOCTL_DEV_REPLACE_STATE_STARTED:
+		result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
+		tgt_device = dev_replace->tgtdev;
+		src_device = dev_replace->srcdev;
+		btrfs_dev_replace_write_unlock(dev_replace);
+		btrfs_scrub_cancel(fs_info);
+		/*
+		 * btrfs_dev_replace_finishing() will handle the cleanup part
+		 */
+		btrfs_info_in_rcu(fs_info,
+			"dev_replace from %s (devid %llu) to %s canceled",
+			btrfs_dev_name(src_device), src_device->devid,
+			btrfs_dev_name(tgt_device));
+		break;
 	case BTRFS_IOCTL_DEV_REPLACE_STATE_SUSPENDED:
+		/*
+		 * scrub doing the replace isn't running so do the cleanup here
+		 */
 		result = BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR;
 		tgt_device = dev_replace->tgtdev;
 		src_device = dev_replace->srcdev;
 		dev_replace->tgtdev = NULL;
 		dev_replace->srcdev = NULL;
-		break;
-	}
-	dev_replace->replace_state = BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED;
-	dev_replace->time_stopped = ktime_get_real_seconds();
-	dev_replace->item_needs_writeback = 1;
-	btrfs_dev_replace_write_unlock(dev_replace);
-	btrfs_scrub_cancel(fs_info);
+		dev_replace->replace_state = BTRFS_IOCTL_DEV_REPLACE_STATE_CANCELED;
+		dev_replace->time_stopped = ktime_get_real_seconds();
+		dev_replace->item_needs_writeback = 1;
 
-	trans = btrfs_start_transaction(root, 0);
-	if (IS_ERR(trans)) {
-		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
-		return PTR_ERR(trans);
-	}
-	ret = btrfs_commit_transaction(trans);
-	WARN_ON(ret);
+		btrfs_dev_replace_write_unlock(dev_replace);
 
-	btrfs_info_in_rcu(fs_info,
-		"dev_replace from %s (devid %llu) to %s canceled",
-		btrfs_dev_name(src_device), src_device->devid,
-		btrfs_dev_name(tgt_device));
+		btrfs_scrub_cancel(fs_info);
+
+		trans = btrfs_start_transaction(root, 0);
+		if (IS_ERR(trans)) {
+			mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
+			return PTR_ERR(trans);
+		}
+		ret = btrfs_commit_transaction(trans);
+		WARN_ON(ret);
 
-	if (tgt_device)
-		btrfs_destroy_dev_replace_tgtdev(tgt_device);
+		btrfs_info_in_rcu(fs_info,
+		"suspended dev_replace from %s (devid %llu) to %s canceled",
+			btrfs_dev_name(src_device), src_device->devid,
+			btrfs_dev_name(tgt_device));
+
+		if (tgt_device)
+			btrfs_destroy_dev_replace_tgtdev(tgt_device);
+		break;
+	}
 
-leave:
 	mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
 	return result;
 }
-- 
1.8.3.1