CVE-2025-21791 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
vrf: use RCU protection in l3mdev_l3_out()
l3mdev_l3_out() can be called without RCU being held:
raw_sendmsg() ip_push_pending_frames() ip_send_skb() ip_local_out() __ip_local_out() l3mdev_ip_out()
Add rcu_read_lock() / rcu_read_unlock() pair to avoid a potential UAF.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability identified as CVE-2025-21791 resides within the Linux kernel's Virtual Routing and Forwarding (VRF) implementation, specifically in the l3mdev_l3_out() function. This issue represents a potential use-after-free condition that could be exploited to compromise system integrity. The vulnerability manifests when the l3mdev_l3_out() function is invoked without proper RCU (Read-Copy-Update) locking mechanisms, creating a race condition scenario where memory access occurs after the memory has been freed. The function is part of the network stack's packet output processing path and is called during the transmission of IP packets through virtual routing devices. The call chain leading to this vulnerability begins with raw_sendmsg() which eventually invokes ip_push_pending_frames(), followed by ip_send_skb(), ip_local_out(), __ip_local_out(), and finally l3mdev_ip_out() which triggers the problematic l3mdev_l3_out() function.
The technical flaw stems from insufficient synchronization primitives within the kernel's networking subsystem. When l3mdev_l3_out() is executed, it accesses data structures that may be modified or freed by concurrent processes without proper RCU locking. This creates a scenario where a thread might attempt to read from memory that has already been deallocated, resulting in undefined behavior that could manifest as system crashes, data corruption, or potentially privilege escalation. The vulnerability is particularly concerning because it occurs during packet transmission, which is a fundamental network operation that could be triggered by various network activities including network interface configuration changes, routing updates, or normal network traffic processing. The absence of rcu_read_lock() and rcu_read_unlock() calls around the critical section means that the function cannot safely access RCU-protected data structures that may be concurrently modified by other kernel threads.
The operational impact of this vulnerability extends beyond simple system instability to potentially enable sophisticated attack vectors. An attacker who can control network traffic or trigger specific network operations might exploit this race condition to cause system crashes or potentially gain elevated privileges. The vulnerability affects systems running Linux kernels that implement VRF functionality, which is commonly used in enterprise networking environments, cloud infrastructure, and network virtualization scenarios. The use-after-free condition could lead to memory corruption that might be leveraged for code execution, particularly if the freed memory is reallocated for malicious purposes. This vulnerability also impacts the overall reliability and security posture of network services that depend on kernel networking functionality, as it could be triggered through legitimate network operations or crafted network packets that cause the kernel to traverse the affected code path.
Mitigation strategies for CVE-2025-21791 focus on implementing proper RCU locking mechanisms within the affected kernel code. The primary fix involves adding rcu_read_lock() and rcu_read_unlock() calls around the critical section in l3mdev_l3_out() to ensure that the function properly synchronizes with concurrent modifications to the data structures it accesses. System administrators should prioritize applying kernel updates that contain the patched code, as this vulnerability requires kernel-level modifications to address. Organizations should also implement network monitoring to detect unusual packet processing patterns that might indicate exploitation attempts. The fix aligns with CWE-416, which addresses use-after-free vulnerabilities, and follows established kernel security practices for RCU synchronization. Additionally, implementing proper kernel hardening measures such as kernel address space layout randomization and stack canaries can provide additional defense-in-depth protections against potential exploitation of this vulnerability. The ATT&CK framework categorizes this vulnerability under T1068, which involves exploiting privileges, as the potential for privilege escalation makes this a significant concern for system security.