CVE-2025-38305 in Linux
Summary
by MITRE • 07/10/2025
In the Linux kernel, the following vulnerability has been resolved:
ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()
There is no disagreement that we should check both ptp->is_virtual_clock and ptp->n_vclocks to check if the ptp virtual clock is in use.
However, when we acquire ptp->n_vclocks_mux to read ptp->n_vclocks in ptp_vclock_in_use(), we observe a recursive lock in the call trace starting from n_vclocks_store().
============================================ WARNING: possible recursive locking detected 6.15.0-rc6 #1 Not tainted -------------------------------------------- syz.0.1540/13807 is trying to acquire lock: ffff888035a24868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
ptp_vclock_in_use drivers/ptp/ptp_private.h:103 [inline]
ffff888035a24868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
ptp_clock_unregister+0x21/0x250 drivers/ptp/ptp_clock.c:415
but task is already holding lock: ffff888030704868 (&ptp->n_vclocks_mux){+.+.}-{4:4}, at:
n_vclocks_store+0xf1/0x6d0 drivers/ptp/ptp_sysfs.c:215
other info that might help us debug this: Possible unsafe locking scenario:
CPU0 ---- lock(&ptp->n_vclocks_mux); lock(&ptp->n_vclocks_mux);
*** DEADLOCK *** .... ============================================
The best way to solve this is to remove the logic that checks ptp->n_vclocks in ptp_vclock_in_use().
The reason why this is appropriate is that any path that uses ptp->n_vclocks must unconditionally check if ptp->n_vclocks is greater than 0 before unregistering vclocks, and all functions are already written this way. And in the function that uses ptp->n_vclocks, we already get ptp->n_vclocks_mux before unregistering vclocks.
Therefore, we need to remove the redundant check for ptp->n_vclocks in ptp_vclock_in_use() to prevent recursive locking.
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Analysis
by VulDB Data Team • 12/19/2025
The vulnerability identified as CVE-2025-38305 resides within the Linux kernel's Precision Time Protocol implementation, specifically affecting the ptp_vclock_in_use() function. This issue manifests as a recursive locking condition that can lead to system deadlock during concurrent operations involving PTP virtual clock management. The root cause stems from improper lock acquisition patterns where the function attempts to acquire the ptp->n_vclocks_mux lock while already holding it, creating a circular dependency that prevents system progress.
The technical flaw occurs in the context of PTP (Precision Time Protocol) subsystem operations where virtual clocks are managed through the ptp_clock_unregister function. When n_vclocks_store() attempts to modify the virtual clock count, it already holds the n_vclocks_mux lock, but ptp_vclock_in_use() subsequently tries to acquire the same lock for reading the n_vclocks value. This creates a classic deadlock scenario as documented in the kernel's locking subsystem where the same lock is acquired twice by the same thread in the same call stack. The recursive locking detection mechanism properly identifies this condition and logs the warning message showing the conflicting lock acquisition paths.
This vulnerability directly impacts system stability and availability by potentially causing kernel deadlocks during PTP clock operations, particularly when virtual clocks are being registered or unregistered. The operational impact extends to network time synchronization services that rely on PTP functionality, potentially leading to service disruption or complete system hang conditions. The issue affects systems running Linux kernel versions where the PTP subsystem is active, particularly those utilizing virtual clock functionality for precise timing operations.
The resolution strategy involves removing the redundant ptp->n_vclocks check logic from the ptp_vclock_in_use() function as the existing code paths already properly handle the virtual clock count validation through other mechanisms. This approach aligns with the principle of avoiding redundant checks that can interfere with existing lock management patterns. The fix addresses the issue at its source by eliminating the problematic lock acquisition sequence rather than attempting to restructure the locking hierarchy, which would be more complex and error-prone. The solution maintains the existing safety mechanisms while preventing the recursive locking condition that was causing system instability.
This vulnerability demonstrates a classic example of improper lock ordering and recursive locking patterns that can occur in kernel subsystems where multiple functions access shared data structures through locks. The issue relates to CWE-367 which describes Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities, though in this case it manifests as a lock ordering problem. From an ATT&CK perspective, this represents a system stability compromise that could potentially be leveraged to cause denial of service conditions. The fix aligns with best practices for kernel lock management and demonstrates the importance of careful lock acquisition ordering in concurrent systems. The vulnerability highlights the complexity of maintaining proper locking semantics in kernel subsystems where multiple entry points access shared resources, and proper lock ordering is crucial for system stability.