CVE-2026-23234 in Linux
Summary
by MITRE • 03/04/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix to avoid UAF in f2fs_write_end_io()
As syzbot reported an use-after-free issue in f2fs_write_end_io().
It is caused by below race condition:
loop device umount - worker_thread - loop_process_work - do_req_filebacked - lo_rw_aio - lo_rw_aio_complete - blk_mq_end_request - blk_update_request - f2fs_write_end_io - dec_page_count - folio_end_writeback - kill_f2fs_super - kill_block_super - f2fs_put_super : free(sbi) : get_pages(, F2FS_WB_CP_DATA) accessed sbi which is freed
In kill_f2fs_super(), we will drop all page caches of f2fs inodes before call free(sbi), it guarantee that all folios should end its writeback, so it should be safe to access sbi before last folio_end_writeback().
Let's relocate ckpt thread wakeup flow before folio_end_writeback() to resolve this issue.
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Analysis
by VulDB Data Team • 06/24/2026
The vulnerability CVE-2026-23234 represents a use-after-free condition in the F2FS (Flash-Friendly File System) implementation within the Linux kernel. This critical flaw manifests in the f2fs_write_end_io() function where a race condition allows for memory access after deallocation. The issue was identified through automated fuzzing by syzbot, which revealed that concurrent operations involving loop device processing and filesystem unmounting can lead to dangling pointer references. The vulnerability stems from improper ordering of operations during the filesystem shutdown process, creating a window where memory structures become invalid while still being referenced by ongoing I/O operations.
The technical root cause involves a complex race condition between multiple kernel subsystems working in parallel. When a loop device processes asynchronous I/O requests through worker threads, the sequence of operations leads to a scenario where blk_mq_end_request() invokes f2fs_write_end_io() which decrements page counts and calls folio_end_writeback(). Concurrently, the umount operation triggers kill_f2fs_super() which calls kill_block_super() and eventually f2fs_put_super(), leading to the freeing of the superblock information (sbi) structure. However, the folio_end_writeback() operation still attempts to access this freed memory structure, creating a classic use-after-free vulnerability. This race condition aligns with CWE-416, specifically the use-after-free pattern, and demonstrates improper resource management in concurrent kernel operations.
The operational impact of this vulnerability is significant as it can lead to system instability, potential privilege escalation, and denial of service conditions. Attackers could potentially exploit this race condition to execute arbitrary code with kernel privileges by carefully orchestrating the timing of I/O operations and filesystem unmounting. The vulnerability affects systems running Linux kernels with F2FS filesystem support, particularly those utilizing loop devices for file-backed I/O operations. The issue is particularly concerning in environments where filesystem operations and unmounting occur concurrently, as the race window can be triggered through normal system operations or through maliciously crafted I/O patterns that force the specific sequence of events leading to memory corruption.
The mitigation strategy involves reordering the checkpoint thread wakeup flow to occur before folio_end_writeback() in the kill_f2fs_super() call chain. This temporal adjustment ensures that all checkpoint operations complete and necessary cleanup occurs before the superblock structure is freed, preventing the access to freed memory. The fix addresses the fundamental ordering issue that creates the race condition, aligning with ATT&CK technique T1068 by targeting privilege escalation through kernel memory corruption. This remediation approach follows secure coding practices for concurrent systems, ensuring proper synchronization between I/O completion and resource deallocation operations. The solution maintains the integrity of the filesystem shutdown process while preserving the functionality of both loop device operations and F2FS filesystem management.