CVE-2023-54023 in Linux
Summary
by MITRE • 12/24/2025
In the Linux kernel, the following vulnerability has been resolved:
btrfs: fix race between balance and cancel/pause
Syzbot reported a panic that looks like this:
assertion failed: fs_info->exclusive_operation == BTRFS_EXCLOP_BALANCE_PAUSED, in fs/btrfs/ioctl.c:465 ------------[ cut here ]------------
kernel BUG at fs/btrfs/messages.c:259! RIP: 0010:btrfs_assertfail+0x2c/0x30 fs/btrfs/messages.c:259 Call Trace: <TASK> btrfs_exclop_balance fs/btrfs/ioctl.c:465 [inline]
btrfs_ioctl_balance fs/btrfs/ioctl.c:3564 [inline]
btrfs_ioctl+0x531e/0x5b30 fs/btrfs/ioctl.c:4632 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl fs/ioctl.c:856 [inline]
__x64_sys_ioctl+0x197/0x210 fs/ioctl.c:856 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x63/0xcd
The reproducer is running a balance and a cancel or pause in parallel. The way balance finishes is a bit wonky, if we were paused we need to save the balance_ctl in the fs_info, but clear it otherwise and cleanup. However we rely on the return values being specific errors, or having a cancel request or no pause request. If balance completes and returns 0, but we have a pause or cancel request we won't do the appropriate cleanup, and then the next time we try to start a balance we'll trip this ASSERT.
The error handling is just wrong here, we always want to clean up, unless we got -ECANCELLED and we set the appropriate pause flag in the exclusive op. With this patch the reproducer ran for an hour without tripping, previously it would trip in less than a few minutes.
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Analysis
by VulDB Data Team • 12/25/2025
The vulnerability identified as CVE-2023-54023 resides within the Linux kernel's Btrfs file system implementation, specifically addressing a race condition between balance operations and cancel/pause commands. This flaw manifests as a kernel panic due to an assertion failure in the exclusive operation handling mechanism. The issue was discovered through automated fuzzing by Syzbot, which revealed a critical timing vulnerability in the file system's operational state management. The panic occurs when the kernel encounters a scenario where the exclusive operation state does not match the expected BTRFS_EXCLOP_BALANCE_PAUSED value, indicating a fundamental mismatch in the operational state tracking logic.
The technical root cause stems from improper error handling within the balance operation completion logic. When a balance operation completes successfully with a return value of zero, the system should perform cleanup operations regardless of pending cancel or pause requests. However, the existing implementation fails to properly account for all possible state transitions, particularly when a balance operation concludes normally while simultaneous cancel or pause requests exist. The code path in fs/btrfs/ioctl.c at line 465 specifically checks for the assertion failure, where the exclusive operation state does not align with the expected paused state. This improper state management creates a window where cleanup operations are skipped, leading to inconsistent internal state that triggers subsequent assertion failures during subsequent balance operations.
The operational impact of this vulnerability extends beyond simple system instability to potentially compromise data integrity and system availability. When the race condition occurs, it results in kernel panics that require system reboot to recover, effectively causing denial of service conditions for systems utilizing Btrfs file systems. The vulnerability is particularly concerning because it can be triggered through standard Btrfs ioctl operations, making it exploitable by any process with appropriate permissions to interact with the file system. The reproducer demonstrates that under concurrent load with balance and cancel/pause operations, the system would crash within minutes, whereas the patched version maintains stability for extended periods, highlighting the severity of the timing issue.
Mitigation strategies for this vulnerability should focus on both immediate kernel updates and operational monitoring. The primary fix involves correcting the error handling logic to ensure proper cleanup operations occur under all conditions, specifically when balance operations complete with zero return values. This aligns with common security practices for race condition resolution and follows established patterns for state management in concurrent systems. The fix implements proper state transition handling that accounts for all possible combinations of operation completion and external requests, ensuring that exclusive operation state tracking remains consistent. Organizations should prioritize applying the kernel patch that resolves this issue, as it addresses a fundamental flaw in the file system's concurrency model. Additionally, system administrators should monitor for unusual system behavior or kernel panic messages that might indicate this vulnerability has been exploited, particularly in environments where Btrfs file systems are actively used with concurrent balance operations.