btrfs: balance: fix null-ptr-deref in btrfs_may_alloc_data_chunk

[BUG]
Running btrfs balance can trigger a null-ptr-deref before relocating a data chunk when metadata corruption leaves a chunk in the chunk tree without a corresponding block group in the in-memory cache:

   KASAN: null-ptr-deref in range [0x0000000000000088-0x000000000000008f]
   RIP: 0010:btrfs_may_alloc_data_chunk+0x40/0x1c0 fs/btrfs/volumes.c:3601
   Call Trace:
     __btrfs_balance fs/btrfs/volumes.c:4217 [inline]
     btrfs_balance+0x2516/0x42b0 fs/btrfs/volumes.c:4604
     btrfs_ioctl_balance fs/btrfs/ioctl.c:3577 [inline]
     btrfs_ioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313
     ...

[CAUSE]
__btrfs_balance() iterates the on-disk chunk tree and passes the chunk logical bytenr to btrfs_may_alloc_data_chunk() before relocating a data chunk. That helper then queries the in-memory block group cache:

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

On a corrupt image can contain a chunk item whose matching block group item is missing, so no block group is ever inserted into the cache. In that case btrfs_lookup_block_group() returns NULL.

The code only guards this with ASSERT(cache), which becomes a no-op when CONFIG_BTRFS_ASSERT is disabled. The subsequent dereference of cache->flags therefore crashes the kernel.

[FIX]
Add a NULL check after btrfs_lookup_block_group() in btrfs_may_alloc_data_chunk(). If the lookup fails, emit a btrfs_err() message identifying the affected bytenr and return -EUCLEAN to report filesystem corruption instead of dereferencing NULL.

The caller already treats negative returns from btrfs_may_alloc_data_chunk() as fatal errors, so balance aborts cleanly and reports the corruption to userspace.

Fixes a6f93c71d412 ("Btrfs: avoid losing data raid profile when deleting a device")

Cc [email protected]

Signed-off-by ZhengYuan Huang <[email protected]>

---
  fs/btrfs/volumes.c | 6 +++++-
  1 file changed, 5 insertions(+), 1 deletion(-)

fs/btrfs/volumes.c +6 -1

index 4958e074d420..4657b826b48b 100644
@@ -3597,7 +3597,12 @@ static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
  	u64 bytes_used;
  	u64 chunk_type;
 
  	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
-	ASSERT(cache);
+	if (!cache) {
+		btrfs_err(fs_info,
+			  "balance: chunk at bytenr %llu has no corresponding block group",
+			  chunk_offset);
+		return -EUCLEAN;
+	}
  	chunk_type = cache->flags;
  	btrfs_put_block_group(cache);

I'd say adding a proper chunk/bg mapping check is the root fix.

Or you'll need to adhoc a lot of null pointer checks.