[PATCH v3 2/2] btrfs: fix the unsafe access in btrfs_lookup_first_ordered_range()

Please fold this fix into patch "btrfs: introduce btrfs_lookup_first_ordered_range()".

[BUG]
David reported a failure in generic/521 which
btrfs_lookup_first_ordered_range() got a poisoned pointer:

 run fstests generic/521 at 2021-05-14 00:33:06
 general protection fault, probably for non-canonical address 0x6b6b6b6b6b6b6a9b: 0000 [#1] PREEMPT SMP
 CPU: 0 PID: 20046 Comm: fsx Not tainted 5.13.0-rc1-default+ #1463
 RIP: 0010:btrfs_lookup_first_ordered_range+0x46/0x140 [btrfs]
 RAX: 6b6b6b6b6b6b6b6b RBX: 6b6b6b6b6b6b6b6b RCX: ffffffffffffffff
 RDX: 6b6b6b6b6b6b6b6b RSI: ffffffffc01b3e09 RDI: ffff93c444e397d0
 Call Trace:
  btrfs_invalidatepage+0xd3/0x390 [btrfs]
  truncate_cleanup_page+0xda/0x170
  truncate_inode_pages_range+0x131/0x5a0
  ? trace_btrfs_space_reservation+0x33/0xf0 [btrfs]
  ? lock_acquire+0xa0/0x150
  ? unmap_mapping_pages+0x4d/0x130
  ? do_raw_spin_unlock+0x4b/0xa0
  ? unmap_mapping_pages+0x5e/0x130
  btrfs_punch_hole_lock_range+0xc5/0x130 [btrfs]
  btrfs_zero_range+0x1d7/0x4b0 [btrfs]
  btrfs_fallocate+0x6b4/0x890 [btrfs]
  ? __x64_sys_fallocate+0x3e/0x70
  ? __do_sys_newfstatat+0x40/0x70
  vfs_fallocate+0x12e/0x420
  __x64_sys_fallocate+0x3e/0x70
  do_syscall_64+0x3f/0xb0
  entry_SYSCALL_64_after_hwframe+0x44/0xae

[CAUSE]
Although I can't reproduce, according to the line number, it's in the btree
search code, and just lines before that, I use some copied code from
tree_search():

	struct rb_node *node = tree->tree.rb_node;

But that assignment is out of spinlock, which is not safe to access,
thus lead to above poisoned pointer.

Unlike tree_search(), which callers have already hold the spinlock.

[FIX]
Fix it by only assign @node after we have hold the spinlock.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/ordered-data.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ordered-data.c b/fs/btrfs/ordered-data.c
index 4fa377da40e4..b1b377ad99a0 100644
--- a/fs/btrfs/ordered-data.c
+++ b/fs/btrfs/ordered-data.c

@@ -943,13 +943,14 @@ struct btrfs_ordered_extent *btrfs_lookup_first_ordered_range(
 			struct btrfs_inode *inode, u64 file_offset, u64 len)
 {
 	struct btrfs_ordered_inode_tree *tree = &inode->ordered_tree;
-	struct rb_node *node = tree->tree.rb_node;
+	struct rb_node *node;
 	struct rb_node *cur;
 	struct rb_node *prev;
 	struct rb_node *next;
 	struct btrfs_ordered_extent *entry = NULL;
 
 	spin_lock_irq(&tree->lock);
+	node = tree->tree.rb_node;
 	/*
 	 * Here we don't want to use tree_search() which will use tree->last
 	 * and screw up the search order.

-- 
2.31.1