CVE-2026-97416btrfs: balance: fix potential bg lookup failure in btrfs_may_alloc_data_chunk()

Official description

In the Linux kernel, the following vulnerability has been resolved: btrfs: balance: fix potential bg lookup failure 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 */ 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() and print and error message for clarity.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/9f37a6d6ae9e998232e2ba091ab4999cdf967e1b
• https://git.kernel.org/stable/c/e479236a8f4d85c4fb6b3f7c2556c21f583e2bf2
• https://git.kernel.org/stable/c/322f891b4d4fe64acb3351a0ffa62a7fa5ba7256
• https://git.kernel.org/stable/c/81a664ed8b1a44c76b204c4bd0d751317a02c5cd
• https://git.kernel.org/stable/c/9740dd0c17a4be8cc47a06c5ca5293471a7e286e
• https://git.kernel.org/stable/c/59f75e0c8724d4adebc08c7dd3ff2358b80c21f7
• https://git.kernel.org/stable/c/18d32b0013efba19f7ad3e5b08d7aee813d604a6