CVE-2026-97416: btrfs: balance: fix potential bg lookup failure in btrfs_may_alloc_data_chunk()
In the Linux kernel, the following vulnerability has been resolved:
btrfs: balance: fix potential bg lookup failure in btrfsmayallocdatachunk()
[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:btrfsmayallocdatachunk+0x40/0x1c0 fs/btrfs/volumes.c:3601 Call Trace: btrfsbalance fs/btrfs/volumes.c:4217 [inline] btrfsbalance+0x2516/0x42b0 fs/btrfs/volumes.c:4604 btrfsioctlbalance fs/btrfs/ioctl.c:3577 [inline] btrfsioctl+0x25cf/0x5b90 fs/btrfs/ioctl.c:5313 ...
[CAUSE] btrfsbalance() iterates the on-disk chunk tree and passes the chunk logical bytenr to btrfsmayallocdatachunk() before relocating a data chunk. That helper then queries the in-memory block group cache:
cache = btrfslookupblockgroup(fsinfo, chunkoffset); chunktype = 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 btrfslookupblockgroup() returns NULL.
The code only guards this with ASSERT(cache), which becomes a no-op when CONFIGBTRFSASSERT is disabled. The subsequent dereference of cache->flags therefore crashes the kernel.
[FIX] Add a NULL check after btrfslookupblockgroup() in btrfsmayallocdatachunk() and print and error message for clarity.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
In btrfs_may_alloc_data_chunk(), add a NULL check after btrfs_lookup_block_group() and print an error message before dereferencing cache->flags.
Event History
Frequently Asked Questions
What conditions are required to trigger the crash?
The filesystem image must be corrupted such that the on-disk chunk tree contains a chunk item without a corresponding block-group item in the in-memory block-group cache. Running btrfs balance can then dereference a NULL block-group lookup result before relocating a data chunk.
Who is exposed in practice?
Systems that run btrfs balance on a Btrfs filesystem with the described metadata corruption are exposed to the null-pointer dereference. The provided information does not indicate that a normal, non-corrupt filesystem triggers the issue.
How can administrators identify the affected corruption pattern?
The relevant condition is a chunk item in the on-disk chunk tree whose matching block-group item is missing, causing btrfs_lookup_block_group() to return NULL. A crash during btrfs balance with a stack trace involving btrfs_may_alloc_data_chunk(), __btrfs_balance(), and btrfs_balance is also consistent with this issue.