CVE-2025-68367 in Linux
Summary
by MITRE • 12/24/2025
In the Linux kernel, the following vulnerability has been resolved:
macintosh/mac_hid: fix race condition in mac_hid_toggle_emumouse
The following warning appears when running syzkaller, and this issue also exists in the mainline code.
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list_add double add: new=ffffffffa57eee28, prev=ffffffffa57eee28, next=ffffffffa5e63100. WARNING: CPU: 0 PID: 1491 at lib/list_debug.c:35 __list_add_valid_or_report+0xf7/0x130 Modules linked in: CPU: 0 PID: 1491 Comm: syz.1.28 Not tainted 6.6.0+ #3 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014 RIP: 0010:__list_add_valid_or_report+0xf7/0x130 RSP: 0018:ff1100010dfb7b78 EFLAGS: 00010282 RAX: 0000000000000000 RBX: ffffffffa57eee18 RCX: ffffffff97fc9817 RDX: 0000000000040000 RSI: ffa0000002383000 RDI: 0000000000000001 RBP: ffffffffa57eee28 R08: 0000000000000001 R09: ffe21c0021bf6f2c R10: 0000000000000001 R11: 6464615f7473696c R12: ffffffffa5e63100 R13: ffffffffa57eee28 R14: ffffffffa57eee28 R15: ff1100010dfb7d48 FS: 00007fb14398b640(0000) GS:ff11000119600000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 000000010d096005 CR4: 0000000000773ef0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 PKRU: 80000000 Call Trace: <TASK> input_register_handler+0xb3/0x210 mac_hid_start_emulation+0x1c5/0x290 mac_hid_toggle_emumouse+0x20a/0x240 proc_sys_call_handler+0x4c2/0x6e0 new_sync_write+0x1b1/0x2d0 vfs_write+0x709/0x950 ksys_write+0x12a/0x250 do_syscall_64+0x5a/0x110 entry_SYSCALL_64_after_hwframe+0x78/0xe2
The WARNING occurs when two processes concurrently write to the mac-hid emulation sysctl, causing a race condition in mac_hid_toggle_emumouse(). Both processes read old_val=0, then both try to register the input handler, leading to a double list_add of the same handler.
CPU0 CPU1 ------------------------- ------------------------- vfs_write() //write 1 vfs_write() //write 1 proc_sys_write() proc_sys_write() mac_hid_toggle_emumouse() mac_hid_toggle_emumouse() old_val = *valp // old_val=0 old_val = *valp // old_val=0 mutex_lock_killable() proc_dointvec() // *valp=1 mac_hid_start_emulation() input_register_handler() mutex_unlock() mutex_lock_killable() proc_dointvec() mac_hid_start_emulation() input_register_handler() //Trigger Warning mutex_unlock()
Fix this by moving the old_val read inside the mutex lock region.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 01/12/2026
The vulnerability described in CVE-2025-68367 resides within the macintosh/mac_hid subsystem of the Linux kernel, specifically in the mac_hid_toggle_emumouse function. This flaw manifests as a race condition that can lead to a double list_add operation, resulting in system instability and potential security implications. The issue is particularly concerning because it occurs during concurrent access to a sysctl interface that controls mac_hid emulation, making it exploitable under certain multi-threaded conditions. The warning message indicates a critical kernel consistency error where the same list element is added twice to a linked list structure, which can corrupt kernel data structures and potentially lead to arbitrary code execution or system crashes.
The technical root cause of this vulnerability stems from improper synchronization within the mac_hid_toggle_emumouse function. The race condition occurs when multiple processes attempt to write to the mac-hid emulation sysctl simultaneously. Both processes read the initial value of the control variable before any locking mechanism is engaged, allowing them both to proceed with attempting to register the input handler. This results in duplicate list additions when the input_register_handler function is called, as the system attempts to add the same handler to the input subsystem's list twice. The problem is exacerbated by the fact that the read operation for the old_val variable occurs outside of the critical section protected by mutex locks, creating a window where concurrent access can lead to inconsistent state.
The operational impact of this vulnerability extends beyond simple system instability, as it represents a potential pathway for privilege escalation or denial of service attacks. The race condition can be triggered through concurrent writes to the sysctl interface, which is accessible to unprivileged users, making exploitation relatively straightforward. When the double list_add occurs, it can corrupt kernel memory structures, leading to kernel oops messages, system crashes, or more severe consequences that could allow an attacker to gain elevated privileges. The vulnerability is particularly dangerous in environments where multiple processes might simultaneously attempt to modify the mac_hid emulation state, such as in virtualized environments or systems with automated monitoring tools that frequently access sysctl interfaces.
The fix for this vulnerability involves a straightforward but critical code modification that moves the old_val read operation inside the mutex-protected region. This ensures that only one process can read the control variable and proceed with handler registration at a time, eliminating the race condition entirely. The solution aligns with standard kernel development practices for preventing race conditions and follows the principle of minimizing the critical section to only include operations that require synchronization. This fix addresses the core issue identified in CWE-362, which describes a race condition where two or more processes access shared resources concurrently without proper synchronization, and it also relates to ATT&CK technique T1068, which covers privilege escalation through kernel vulnerabilities. The implementation of this fix ensures that the mac_hid subsystem maintains data integrity under concurrent access while preserving the intended functionality of the emulation toggle mechanism.