[PATCH v3 2/4] btrfs: balance: fix null-ptr-deref in chunk_usage_range_filter

[ZhengYuan Huang <gality369@gmail.com>]

[BUG]
Running btrfs balance with a usage range filter (-dusage=min..max) can
trigger a null-ptr-deref when metadata corruption causes a chunk to have
no corresponding block group in the in-memory cache:

  KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077]
  RIP: 0010:chunk_usage_range_filter fs/btrfs/volumes.c:3845 [inline]
  RIP: 0010:should_balance_chunk fs/btrfs/volumes.c:4031 [inline]
  RIP: 0010:__btrfs_balance fs/btrfs/volumes.c:4182 [inline]
  RIP: 0010:btrfs_balance+0x249e/0x4320 fs/btrfs/volumes.c:4618
  ...
  Call Trace:
    btrfs_ioctl_balance fs/btrfs/ioctl.c:3577 [inline]
    btrfs_ioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313
    vfs_ioctl fs/ioctl.c:51 [inline]
    ...

The bug is reproducible on next-20260312.

[CAUSE]
Two separate data structures are involved:

1. The on-disk chunk tree, which records every chunk (logical address
   space region) and is iterated by __btrfs_balance().
2. The in-memory block group cache (fs_info->block_group_cache_tree),
   which is built at mount time by btrfs_read_block_groups() and holds
   a struct btrfs_block_group for each chunk. This cache is what the
   usage range filter queries.

On a well-formed filesystem, these two are kept in 1:1 correspondence.
However, btrfs_read_block_groups() builds the cache from block group
items in the extent tree, not directly from the chunk tree. A corrupted
image can therefore contain a chunk item in the chunk tree whose
corresponding block group item is absent from the extent tree; that
chunk's block group is then never inserted into the in-memory cache.

When balance iterates the chunk tree and reaches such an orphaned chunk,
should_balance_chunk() calls chunk_usage_range_filter(), which queries
the block group cache:

  cache = btrfs_lookup_block_group(fs_info, chunk_offset);
  chunk_used = cache->used;   /* cache may be NULL */

btrfs_lookup_block_group() returns NULL silently when no cached entry
covers chunk_offset. chunk_usage_range_filter() does not check the return
value, so the immediately following dereference of cache->used triggers
the crash.

[FIX]
Add a NULL check after btrfs_lookup_block_group() in
chunk_usage_range_filter(). When the lookup fails, emit a btrfs_err()
message identifying the affected bytenr and return -EUCLEAN to indicate
filesystem corruption.

Since chunk_usage_range_filter() now has an error path, change its
return type from bool to int: negative errno on error, 0 if the chunk
matches the usage range, and 1 if it should be filtered out. Update the
BTRFS_BALANCE_ARGS_USAGE_RANGE branch in should_balance_chunk() to
propagate negative errors from chunk_usage_range_filter() instead of
treating them as a normal filter result.

After the fix, the same corruption is correctly detected and reported
by the filter, and the null-ptr-deref is no longer triggered.

Fixes: bc3094673f22 ("btrfs: extend balance filter usage to take minimum and maximum")
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
---
 fs/btrfs/volumes.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 1eca5fa6bdaa..65df8652d760 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3832,16 +3832,22 @@ static bool chunk_profiles_filter(u64 chunk_type, struct btrfs_balance_args *bar
 	return true;
 }
 
-static bool chunk_usage_range_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
-				     struct btrfs_balance_args *bargs)
+static int chunk_usage_range_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
+				    struct btrfs_balance_args *bargs)
 {
 	struct btrfs_block_group *cache;
 	u64 chunk_used;
 	u64 user_thresh_min;
 	u64 user_thresh_max;
-	bool ret = true;
+	int ret = 1;
 
 	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
+	if (!cache) {
+		btrfs_err(fs_info,
+			  "balance: chunk at bytenr %llu has no corresponding block group",
+			  chunk_offset);
+		return -EUCLEAN;
+	}
 	chunk_used = cache->used;
 
 	if (bargs->usage_min == 0)
@@ -3857,7 +3863,7 @@ static bool chunk_usage_range_filter(struct btrfs_fs_info *fs_info, u64 chunk_of
 		user_thresh_max = mult_perc(cache->length, bargs->usage_max);
 
 	if (user_thresh_min <= chunk_used && chunk_used < user_thresh_max)
-		ret = false;
+		ret = 0;
 
 	btrfs_put_block_group(cache);
 	return ret;
@@ -4027,9 +4033,13 @@ static int should_balance_chunk(struct extent_buffer *leaf, struct btrfs_chunk *
 			return ret2;
 		if (ret2)
 			return false;
-	} else if ((bargs->flags & BTRFS_BALANCE_ARGS_USAGE_RANGE) &&
-	    chunk_usage_range_filter(fs_info, chunk_offset, bargs)) {
-		return false;
+	} else if (bargs->flags & BTRFS_BALANCE_ARGS_USAGE_RANGE) {
+		int ret2 = chunk_usage_range_filter(fs_info, chunk_offset, bargs);
+
+		if (ret2 < 0)
+			return ret2;
+		if (ret2)
+			return false;
 	}
 
 	/* devid filter */
-- 
2.43.0