btrfs: subpage: fix wild pointer access during metadata read failure

[BUG]
When running fstests for btrfs subpage read-write test, it has a very
high chance to crash at generic/475 with the following stack:

 BTRFS warning (device dm-8): direct IO failed ino 510 rw 1,34817 sector 0xcdf0 len 94208 err no 10
 Unable to handle kernel paging request at virtual address ffff80001157e7c0
 CPU: 2 PID: 687125 Comm: kworker/u12:4 Tainted: G        WC        5.12.0-rc2-custom+ #5
 Hardware name: Khadas VIM3 (DT)
 Workqueue: btrfs-endio-meta btrfs_work_helper [btrfs]
 pc : queued_spin_lock_slowpath+0x1a0/0x390
 lr : do_raw_spin_lock+0xc4/0x11c
 Call trace:
  queued_spin_lock_slowpath+0x1a0/0x390
  _raw_spin_lock+0x68/0x84
  btree_readahead_hook+0x38/0xc0 [btrfs]
  end_bio_extent_readpage+0x504/0x5f4 [btrfs]
  bio_endio+0x170/0x1a4
  end_workqueue_fn+0x3c/0x60 [btrfs]
  btrfs_work_helper+0x1b0/0x1b4 [btrfs]
  process_one_work+0x22c/0x430
  worker_thread+0x70/0x3a0
  kthread+0x13c/0x140
  ret_from_fork+0x10/0x30
 Code: 910020e0 8b0200c2 f861d884 aa0203e1 (f8246827)

[CAUSE]
In end_bio_extent_readpage(), if we hit an error during read, we will
handle the error differently for data and metadata.
For data we queue a repair, while for metadata, we record the error and
let the caller choose what to do.

But the code is still using page->private to grab extent buffer, which
no longer points to extent buffer for subpage metadata pages.

Thus this wild pointer access leads to above crash.

[FIX]
Introduce a helper, find_extent_buffer_readpage(), to grab extent
buffer.

The difference against find_extent_buffer_nospinlock() is:

- Also handles regular sectorsize == PAGE_SIZE case
- No extent buffer refs increase/decrease
  As extent buffer under IO must have non-zero refs, so this is safe

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -2885,6 +2885,35 @@ static void end_page_read(struct page *page, bool uptodate, u64 start, u32 len)
 		btrfs_subpage_end_reader(fs_info, page, start, len);
 }
 
+/*
+ * Find extent buffer for a givne bytenr.
+ *
+ * This is for end_bio_extent_readpage(), thus we can't do any unsafe locking
+ * in endio context.
+ */
+static struct extent_buffer *find_extent_buffer_readpage(
+		struct btrfs_fs_info *fs_info, struct page *page, u64 bytenr)
+{
+	struct extent_buffer *eb;
+
+	/*
+	 * For regular sectorsize, we can use page->private to grab extent
+	 * buffer
+	 */
+	if (fs_info->sectorsize == PAGE_SIZE) {
+		ASSERT(PagePrivate(page) && page->private);
+		return (struct extent_buffer *)page->private;
+	}
+
+	/* For subpage case, we need to lookup buffer radix tree */
+	rcu_read_lock();
+	eb = radix_tree_lookup(&fs_info->buffer_radix,
+			       bytenr >> fs_info->sectorsize_bits);
+	rcu_read_unlock();
+	ASSERT(eb);
+	return eb;
+}
+
 /*
  * after a readpage IO is done, we need to:
  * clear the uptodate bits on error
@@ -2996,7 +3025,7 @@ static void end_bio_extent_readpage(struct bio *bio)
 		} else {
 			struct extent_buffer *eb;
 
-			eb = (struct extent_buffer *)page->private;
+			eb = find_extent_buffer_readpage(fs_info, page, start);
 			set_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
 			eb->read_mirror = mirror;
 			atomic_dec(&eb->io_pages);