btrfs: check block group before marking it unused in balance_remap_chunks()

Fix a potential segfault in balance_remap_chunks(): if we quit early
because btrfs_inc_block_group_ro() fails, all the remaining items in the
chunks list will still have their bg value set to NULL. It's thus not
safe to dereference this pointer without checking first.

Reported-by: Chris Mason <clm@fb.com>
Link: https://lore.kernel.org/linux-btrfs/20260125120717.1578828-1-clm@meta.com/
Fixes: 81e5a45 ("btrfs: allow balancing remap tree")
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: maharmstone

fs/btrfs/volumes.c

@@ -4277,20 +4277,29 @@ static int balance_remap_chunks(struct btrfs_fs_info *fs_info, struct btrfs_path
 end:
 	while (!list_empty(chunks)) {
 		bool is_unused;
+		struct btrfs_block_group *bg;
 
 		rci = list_first_entry(chunks, struct remap_chunk_info, list);
 
-		spin_lock(&rci->bg->lock);
-		is_unused = !btrfs_is_block_group_used(rci->bg);
-		spin_unlock(&rci->bg->lock);
+		bg = rci->bg;
+		if (bg) {
+			/*
+			 * This is a bit racy and the 'used' status can change
+			 * but this is not a problem as later functions will
+			 * verify it again.
+			 */
+			spin_lock(&bg->lock);
+			is_unused = !btrfs_is_block_group_used(bg);
+			spin_unlock(&bg->lock);
 
-		if (is_unused)
-			btrfs_mark_bg_unused(rci->bg);
+			if (is_unused)
+				btrfs_mark_bg_unused(bg);
 
-		if (rci->made_ro)
-			btrfs_dec_block_group_ro(rci->bg);
+			if (rci->made_ro)
+				btrfs_dec_block_group_ro(bg);
 
-		btrfs_put_block_group(rci->bg);
+			btrfs_put_block_group(bg);
+		}
 
 		list_del(&rci->list);
 		kfree(rci);