CVE-2024-26685 in Linux
Summary
by MITRE • 04/03/2024
In the Linux kernel, the following vulnerability has been resolved:
nilfs2: fix potential bug in end_buffer_async_write
According to a syzbot report, end_buffer_async_write(), which handles the completion of block device writes, may detect abnormal condition of the buffer async_write flag and cause a BUG_ON failure when using nilfs2.
Nilfs2 itself does not use end_buffer_async_write(). But, the async_write flag is now used as a marker by commit 7f42ec394156 ("nilfs2: fix issue with race condition of competition between segments for dirty blocks") as a means of resolving double list insertion of dirty blocks in nilfs_lookup_dirty_data_buffers() and nilfs_lookup_node_buffers() and the resulting crash.
This modification is safe as long as it is used for file data and b-tree node blocks where the page caches are independent. However, it was irrelevant and redundant to also introduce async_write for segment summary and super root blocks that share buffers with the backing device. This led to the possibility that the BUG_ON check in end_buffer_async_write would fail as described above, if independent writebacks of the backing device occurred in parallel.
The use of async_write for segment summary buffers has already been removed in a previous change.
Fix this issue by removing the manipulation of the async_write flag for the remaining super root block buffer.
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Analysis
by VulDB Data Team • 07/17/2025
The vulnerability described in CVE-2024-26685 represents a critical flaw in the Linux kernel's nilfs2 filesystem implementation that stems from improper handling of asynchronous write operations. This issue manifests in the end_buffer_async_write() function which serves as a completion handler for block device writes and contains a BUG_ON failure condition that can be triggered under specific circumstances. The vulnerability arises from the inappropriate use of the async_write flag as a synchronization marker across different types of filesystem buffers, creating a race condition scenario that leads to kernel panic.
The technical root cause involves the modification of commit 7f42ec394156 which introduced the async_write flag as a means to resolve double list insertion issues in dirty block handling functions. While this approach was initially safe for file data and b-tree node blocks that maintain independent page caches, it became problematic when applied to segment summary and super root blocks that share buffers with the backing device. The super root blocks in particular represent a critical edge case because they operate in a shared buffer context where independent writebacks can occur in parallel, creating the conditions for the BUG_ON check to fail. This represents a violation of the principle of proper buffer management where flags should only be used in contexts where their semantics are well-defined and consistent.
The operational impact of this vulnerability is severe as it can cause kernel panics and system crashes when the nilfs2 filesystem is in use, particularly under concurrent write operations. The vulnerability affects systems running Linux kernels with nilfs2 filesystem support and can lead to complete system instability, requiring manual intervention to restore normal operation. The issue is particularly concerning because it occurs in kernel space where such failures can compromise system integrity and availability. This vulnerability aligns with CWE-691, which addresses inadequate protection of code against excessive resource consumption, and can be categorized under ATT&CK technique T1490 for data destruction and T1566 for credential access through exploitation of kernel vulnerabilities.
The fix implemented addresses this by removing the manipulation of the async_write flag specifically for super root block buffers, thereby eliminating the conflict between the flag's intended use and its actual behavior in shared buffer contexts. This resolution ensures that the async_write flag is only used in contexts where its semantics are properly defined and consistent with the underlying buffer sharing model. The fix maintains the original intent of preventing double list insertions while eliminating the race condition that led to the BUG_ON failure, demonstrating proper kernel security engineering practices. This approach aligns with the principle of least privilege and proper resource management in kernel code, ensuring that kernel-level flags and synchronization primitives are only applied where they can be safely and predictably utilized, preventing the cascading failures that can occur when such fundamental kernel components are misapplied.