CVE-2025-38234 in Linux
Summary
by MITRE • 07/04/2025
In the Linux kernel, the following vulnerability has been resolved:
sched/rt: Fix race in push_rt_task
Overview ======== When a CPU chooses to call push_rt_task and picks a task to push to another CPU's runqueue then it will call find_lock_lowest_rq method which would take a double lock on both CPUs' runqueues. If one of the locks aren't readily available, it may lead to dropping the current runqueue lock and reacquiring both the locks at once. During this window it is possible that the task is already migrated and is running on some other CPU. These cases are already handled. However, if the task is migrated and has already been executed and another CPU is now trying to wake it up (ttwu) such that it is queued again on the runqeue (on_rq is 1) and also if the task was run by the same CPU, then the current checks will pass even though the task was migrated out and is no longer in the pushable tasks list.
Crashes ======= This bug resulted in quite a few flavors of crashes triggering kernel panics with various crash signatures such as assert failures, page faults, null pointer dereferences, and queue corruption errors all coming from scheduler itself.
Some of the crashes: -> kernel BUG at kernel/sched/rt.c:1616! BUG_ON(idx >= MAX_RT_PRIO) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? pick_next_task_rt+0x6e/0x1d0 ? do_error_trap+0x64/0xa0 ? pick_next_task_rt+0x6e/0x1d0 ? exc_invalid_op+0x4c/0x60 ? pick_next_task_rt+0x6e/0x1d0 ? asm_exc_invalid_op+0x12/0x20 ? pick_next_task_rt+0x6e/0x1d0 __schedule+0x5cb/0x790 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: kernel NULL pointer dereference, address: 00000000000000c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? __warn+0x8a/0xe0 ? exc_page_fault+0x3d6/0x520 ? asm_exc_page_fault+0x1e/0x30 ? pick_next_task_rt+0xb5/0x1d0 ? pick_next_task_rt+0x8c/0x1d0 __schedule+0x583/0x7e0 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: unable to handle page fault for address: ffff9464daea5900 kernel BUG at kernel/sched/rt.c:1861! BUG_ON(rq->cpu != task_cpu(p))
-> kernel BUG at kernel/sched/rt.c:1055! BUG_ON(!rq->nr_running) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? dequeue_top_rt_rq+0xa2/0xb0 ? do_error_trap+0x64/0xa0 ? dequeue_top_rt_rq+0xa2/0xb0 ? exc_invalid_op+0x4c/0x60 ? dequeue_top_rt_rq+0xa2/0xb0 ? asm_exc_invalid_op+0x12/0x20 ? dequeue_top_rt_rq+0xa2/0xb0 dequeue_rt_entity+0x1f/0x70 dequeue_task_rt+0x2d/0x70 __schedule+0x1a8/0x7e0 ? blk_finish_plug+0x25/0x40 schedule+0x3c/0xb0 futex_wait_queue_me+0xb6/0x120 futex_wait+0xd9/0x240 do_futex+0x344/0xa90 ? get_mm_exe_file+0x30/0x60 ? audit_exe_compare+0x58/0x70 ? audit_filter_rules.constprop.26+0x65e/0x1220 __x64_sys_futex+0x148/0x1f0 do_syscall_64+0x30/0x80 entry_SYSCALL_64_after_hwframe+0x62/0xc7
-> BUG: unable to handle page fault for address: ffff8cf3608bc2c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? spurious_kernel_fault+0x171/0x1c0 ? exc_page_fault+0x3b6/0x520 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? asm_exc_page_fault+0x1e/0x30 ? _cond_resched+0x15/0x30 ? futex_wait_queue_me+0xc8/0x120 ? futex_wait+0xd9/0x240 ? try_to_wake_up+0x1b8/0x490 ? futex_wake+0x78/0x160 ? do_futex+0xcd/0xa90 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? plist_del+0x6a/0xd0 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? dequeue_pushable_task+0x20/0x70 ? __schedule+0x382/0x7e0 ? asm_sysvec_reschedule_i ---truncated---
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Analysis
by VulDB Data Team • 08/01/2026
The vulnerability described in CVE-2025-38234 resides within the Linux kernel's real-time scheduling subsystem, specifically in the `sched/rt` component. This flaw manifests as a race condition during the execution of the `push_rt_task` function, which is responsible for migrating real-time tasks between CPU runqueues to balance load. The issue arises from improper locking mechanisms when attempting to acquire locks on multiple runqueues, leading to potential inconsistencies in task state management. The race condition occurs when a CPU attempts to push a task to another CPU’s runqueue, and during this process, the system may temporarily drop and reacquire locks, creating a window where the task state can change unexpectedly.
The technical root cause involves the `find_lock_lowest_rq` method, which attempts to lock two runqueues simultaneously. When one of the locks cannot be acquired immediately, the system drops the first lock and reacquires both locks in sequence. During this window, if the task has already been migrated to another CPU, the system may incorrectly proceed with operations assuming the task is still in the original runqueue. This leads to inconsistencies in the scheduler's internal data structures, particularly in the handling of task queues and priority lists. The race condition is further exacerbated when the migrated task is woken up again, causing it to be queued on a different runqueue while the original code path assumes it remains in the pushable list. According to CWE-362, this vulnerability maps directly to a race condition, and from an ATT&CK perspective, it could be leveraged for privilege escalation or system instability by an attacker who can control task scheduling or trigger specific workload patterns.
The operational impact of this vulnerability is severe, as it results in kernel panics and system crashes, with multiple crash signatures reported. These include assertions failing, null pointer dereferences, and page faults originating from scheduler code paths. The crashes occur at specific locations within the real-time scheduler such as `kernel/sched/rt.c` lines 1616, 1861, and 1055, indicating that the system attempts to access invalid memory or perform operations on corrupted data structures. The instability manifests through various kernel traps and fault handlers, ultimately leading to system-wide crashes. The vulnerability is particularly dangerous in real-time systems or environments where predictable task scheduling is critical, as it can lead to missed deadlines or complete system lockups.
Mitigation strategies for this vulnerability involve applying the latest kernel patches that address the race condition in the real-time scheduler. The fix typically includes tightening the locking mechanisms to prevent the temporary release of locks and ensuring that task state checks are performed atomically. System administrators should also consider monitoring for scheduler-related kernel crashes and implementing proper system hardening practices. In environments where real-time performance is critical, it is recommended to upgrade to kernel versions that contain the patched code. Additionally, runtime monitoring tools can help detect anomalous scheduling behavior that may indicate the presence of this vulnerability. The vulnerability demonstrates the importance of careful lock management in concurrent systems and the need for thorough testing of race conditions in kernel-level code.