CVE-2026-97416 in Linux
Summary
by MITRE • 09/24/2026
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.
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Analysis
by VulDB Data Team • 09/24/2026
The vulnerability identified as CVE-2024-something related to the Btrfs filesystem within the Linux kernel represents a critical stability issue triggered by metadata corruption during balance operations. The core of this flaw lies in the function btrfs_may_alloc_data_chunk, which is invoked during the execution of the btrfs_balance command. This command is designed to redistribute data and metadata across devices to optimize storage usage or replace failing hardware. However, under specific conditions where the on-disk filesystem contains corrupted metadata, specifically a chunk item present in the chunk tree without a corresponding block group entry in the in-memory cache, the kernel encounters an unhandled null pointer dereference. This scenario arises because the balance operation iterates through the persistent chunk tree and attempts to allocate data chunks based on logical byte numbers found there. The helper function responsible for this allocation relies on looking up the associated block group structure from memory to determine flags such as whether the space is reserved or free. When metadata corruption results in a mismatch between the disk state and the kernel's internal cache, the lookup fails silently by returning NULL instead of creating an entry or handling the absence gracefully.
The technical root cause stems from insufficient defensive programming within btrfs_may_alloc_data_chunk. The code previously relied on an ASSERT macro to verify that the returned pointer was not null. While assertions are useful for debugging during development, they are typically compiled out in production kernels unless CONFIG_BTRFS_ASSERT is explicitly enabled. Consequently, when running a standard kernel configuration, this check disappears entirely, leaving the subsequent dereference of cache->flags unprotected. This leads directly to a kernel panic or system crash, as indicated by KASAN reports showing a null pointer dereference at offset 0x88 within the function. The impact is severe because it allows local users with permission to execute balance operations on corrupted filesystems to cause denial of service through system instability. In environments where high availability is critical, such crashes can lead to significant downtime and potential data inaccessibility if the crash occurs during a write operation or causes memory corruption that persists beyond the immediate fault.
From a security perspective, this vulnerability aligns with CWE-476: NULL Pointer Dereference, which describes situations where code attempts to use a pointer that it expects to be valid but is actually null. Furthermore, given that the trigger involves manipulating filesystem state through ioctl calls like btrfs_balance, it can also be associated with CWE-20: Improper Input Validation, as the kernel fails to adequately validate the consistency between on-disk structures and in-memory representations before proceeding with operations. In terms of MITRE ATT&CK mapping, this could fall under T1498 Network Denial of Service or more specifically local denial of service vectors where an attacker exploits a flaw in system resource management. Although typically requiring root privileges to run balance commands on most systems, the existence of such vulnerabilities highlights risks for privileged processes and potential escalation paths if combined with other weaknesses that allow unauthorized access to filesystem administration tools.
The resolution implemented by the Linux kernel maintainers involves adding explicit NULL checks after calling btrfs_lookup_block_group within btrfs_may_alloc_data_chunk. This ensures that if a block group is not found in memory, likely due to metadata inconsistency or corruption, the function handles it gracefully rather than crashing. The fix includes printing an error message to aid administrators in diagnosing underlying filesystem issues, which serves as both a technical safeguard and an operational diagnostic tool. To mitigate this risk, system administrators should ensure that their Linux kernels are updated to versions containing this patch. Additionally, regular integrity checks using tools like btrfs check can help identify and repair metadata corruption before it triggers such crashes during maintenance operations. It is also advisable to maintain recent backups of critical data stored on Btrfs volumes, as the presence of corrupted chunks indicates deeper filesystem health issues that may require manual intervention or restoration from backup if automatic repairs fail.