CVE-2025-39913 - Linux Kernel eBPF SOCKMAP (UAF)

A comprehensive analysis of the Linux Kernel eBPF SOCKMAP Use-After-Free vulnerability

CVE-2025-39913
Published: Oct 18, 2025 Author: Byte Reaper Category: Kernel Security

CVE-2025-39913: Linux Kernel eBPF SOCKMAP Use-After-Free Vulnerability

This article provides a comprehensive analysis of the CVE-2025-39913 vulnerability in the Linux kernel's eBPF SOCKMAP component, based on detailed research of the kernel architecture and memory management mechanisms.

Executive Summary

CVE-2025-39913 is a memory management vulnerability residing in the Linux kernel's tcp_bpf_send_verdict() function, which is a key part of the BPF-TCP (or SOCKMAP) component.

The flaw's root cause is a failure in error handling:

  1. When a BPF program attempts to redirect an sk_msg (socket message), the kernel tries to allocate memory for the psock->cork structure.
  2. If this cork allocation fails (e.g., due to memory pressure or fault injection), the execution follows an incorrect cleanup path.
  3. The kernel fails to call sk_msg_free() for the associated sk_msg object.
  4. This omission leaves the sk_msg object improperly referenced in memory and causes an inconsistency in the socket's memory tracking (sk->sk_forward_alloc remains incorrectly allocated).

The failure to free the object results in a Reference Leak and creates a dangerous Use-After-Free (UAF) condition, which can be exploited by a local attacker to achieve privilege escalation or kernel information disclosure.

Basic Concepts

Before understanding the vulnerability, it is necessary to understand some basic concepts and terms:

1 - LSF (cBPF)

  • It is a technique in the Linux kernel aimed at filtering TCP packets
  • It was a small program in the Linux kernel for making decisions on packet filtering, for example:
    • Passing the package: to allow it to access the user application
    • Drop the packet: to discard it before it is processed by the kernel

2 - BPF (eBPF)

  • It is a more advanced version of the LSF technology because its goal is no longer just filtering packets and simple user customization
  • It has become a fully programmable and customizable virtual machine within the Linux kernel, providing a complete library for controlling packet flow, rejecting them, and handling them
  • It also supports JIT technology for translating code into binary "Native Machine Code" at the same time, making it much faster than LSF
  • It supports a Verifier, where the kernel verifier checks for errors or incorrect handling that could cause kernel crashes
  • It also supports controlling structures within the Linux kernel, specifically BPF Maps, allowing for data exchange and easy access from USER-space

3 - Hook Points

  • The idea of a HOOK POINT is that it sets an END POINT, which are predefined locations within the Linux kernel that allow the eBPF program to attach to them and complete the filtering
  • The function of these END POINTS is to:
    • Specify the layer that should be interacted with (Network layer, syscall (call system)...)
    • Determine the timing of executing a specific function or the arrival of a packet based on the purpose of this HOOK
    • Provide important information that helps a program recognize the HOOK, such as the type of packet, the type of function, or even the content of the packet
  • Types of HOOKs include:
    • XDP: An extremely early hook that operates when a packet is received directly from the network card, and is used for processing high-speed packets
    • tc BPF (Traffic Control): To control, modify, or direct packet traffic (before or after the routing decision)
    • Socket Hooks: To work at the socket level, such as sk_msg programs that operate within SOCKMAP or when connecting to a specific socket

4 - SOCKMAP

  • It consists of structures within the Linux kernel specialized in BPF MAPS, aimed at achieving network performance with packet differentiation through Network Stack Bypass
  • You can consider it a map for eBPF, and it works by attaching an EBPF program to an endpoint associated with this MAP
  • When data reaches the socket, instead of directing it to the Network layer, like firewall analysis or DPI, eBPF is used through the "bpf_msg_redirect_hash" function to redirect data from the input socket to the output stored in a map
  • This results in faster data and packet analysis, especially if the packet uses well-known and secure protocols
  • This technique is considered faster than complex analysis

5 - Corking

  • It is simply the suspension of data transmission in the socket structure in the Linux kernel until a certain size is gathered
  • You can consider it as a mechanism for collecting and waiting for chunk packets to be sent completely
  • By using the bpf_msg_cork_bytes function, this corking is enforced at the BPF program level, which requires allocating the SOCKET type to TCP_CORK and also allocating additional memory via psock->cork
  • Herein lies the flaw due to the failure to allocate memory for psock->cork, which led to the omission of the necessary cleanup step sk_msg_free() to revert the memory preallocation (sk->sk_forward_alloc)

Affected Component

The vulnerability affects the Linux kernel's networking subsystem, specifically the BPF-TCP component that handles message redirection within SOCKMAP.

Affected kernels: Versions ≤ 6.12.38 that have the necessary eBPF and SOCKMAP support enabled, regardless of the architecture (32-bit or 64-bit).

Type of Vulnerability

The vulnerability is fundamentally a Use-After-Free (UAF) condition. UAF occurs when a program continues to use a pointer to a block of memory after that memory block has been freed, allowing the kernel to potentially reuse the freed memory for a different object.

Since this UAF occurs in the kernel space, the goal of exploitation is to hijack the control flow by leveraging the kernel's memory management system (SLUB/SLAB allocator). The attacker would typically:

  1. Trigger the UAF flaw in the psock->cork path to create a reference to an already freed sk_msg object
  2. Use Memory Spraying techniques to inject a controlled data structure into the newly freed memory slot
  3. When the vulnerable code attempts to access the old, freed sk_msg object, it will instead access the attacker's injected structure, often leading to the execution of attacker-defined code (a "gadget") or the manipulation of critical kernel pointers

Successful exploitation of this UAF allows an attacker to bypass protections like KASLR (Kernel ASLR) and achieve Local Privilege Escalation (LPE).

Exploitation

The CVE-2025-39913 vulnerability is fundamentally a Use-After-Free (UAF) condition triggered during memory allocation for socket message aggregation (corking) within the Linux kernel's BPF-TCP component.

1 - Prerequisites and Activation

Exploiting this flaw requires specific eBPF program setup to ensure the vulnerable code path is hit:

  • Program Type: The attacker must utilize the BPF_SK_MSG_VERDICT (Attach Type). This type, attached via the bpf_prog_attach function, guarantees the BPF program executes when socket messages (sk_msg) are processed by mechanisms like SOCKMAP or SK_MSG.
  • Intervention & Verdict: The program's role is to intercept and intervene in the sk_msg lifecycle, allowing it to return a "Verdict" to the kernel, such as passing the message (BPF_MSG_PASS), dropping it (BPF_MSG_DROP), or redirecting it (BPF_MSG_REDIR).
  • Corking Activation: The eBPF program must specifically call the bpf_msg_cork_bytes() helper function. This activates the corking mechanism, instructing the kernel to aggregate parts of the message into one piece before sending.

2 - The Logical Error (Root Cause)

When the eBPF program is attached and corking is activated, the kernel calls the vulnerable function: tcp_bpf_send_verdict(), which in turn calls the corking logic:

  1. Initial Setup: The sk_msg is received. Memory is allocated for the message, the reference count for the associated memory increases, and the value of sk->sk_forward_alloc (which tracks memory allocated for future use—forward allocation) is updated accordingly.
  2. Allocation Failure: The logical error begins when the kernel attempts to allocate the psock->cork structure for a given message, but this stage fails (due to memory shortage, fault injection, or other transient errors).
  3. Missing Cleanup: The correct code path, upon failure, should recognize that the sk_msg was neither sent nor successfully aggregated. It must therefore roll back the setup by calling the crucial function sk_msg_free() to release the associated memory and reduce the value of sk->sk_forward_alloc.
  4. The Flaw: This essential cleanup logic is missing in the vulnerable versions. Consequently, the sk_msg object is treated as if the allocation succeeded, resulting in an incorrect object reference remaining in the kernel's memory space.

3 - Security Impact

The failure to call sk_msg_free() creates a Reference Leak and leaves the socket's memory tracking (sk->sk_forward_alloc) in an inconsistent state. This discrepancy leads directly to the UAF condition, enabling the use of random memory, which can be leveraged by an attacker to cause a kernel CRASH or achieve Local Privilege Escalation by injecting malicious data structures into the re-used memory slot.

Patch Analysis

The root cause of the CVE-2025-39913 vulnerability was the omission of a critical cleanup function in the error path of the tcp_bpf_send_verdict() routine. The patch directly addresses this missing logic.

1 - Goal of the Fix

The primary goal of the patch was to ensure that the associated sk_msg object is properly freed and dereferenced immediately when memory allocation for psock->cork fails. This action prevents the Reference Leak that leads to the UAF condition.

2 - Conceptual Code Snippet (Patched)

if (!psock->cork) //Failed attempts at allocation
{
sk_msg_free(sk, msg); //Free the sk_msg object
*copied = 0;
return -ENOMEM; //return ENOMEM (Insufficient memory)
}

3 - Final Impact of the Patch

The addition of sk_msg_free(msg) resolves the vulnerability completely by:

  • Preventing UAF: By freeing the sk_msg object and correctly decreasing its reference count, the patch ensures that the memory is properly returned to the SLUB allocator, eliminating the possibility of the kernel (or an attacker) later using a stale pointer to the freed memory.
  • Restoring Consistency: The sk_msg_free() call correctly updates the sk->sk_forward_alloc counter, removing the memory inconsistency that was misleading the kernel about the socket's reserved memory state.

Conclusion

The CVE-2025-39913 vulnerability serves as a critical example of how resource management errors in sensitive kernel code can translate into high-severity security flaws.

  1. Core Flaw and Severity: The vulnerability stems from a simple yet catastrophic error handling oversight within the tcp_bpf_send_verdict() function in the Linux kernel's BPF-TCP component. The failure to call the essential sk_msg_free() function upon psock->cork allocation failure leads directly to a Reference Leak and establishes the conditions for a Use-After-Free (UAF) state. This class of vulnerability is among the most dangerous, highly sought after by attackers for achieving Local Privilege Escalation (LPE).
  2. Context and eBPF Significance: This flaw highlights the security challenges inherent in modern networking components like eBPF and SOCKMAP. The exploit relies on using the advanced control provided by eBPF to force the kernel into the specific, vulnerable error path, underscoring that sophisticated in-kernel programming requires stringent memory safety auditing.
  3. Resolution: The fix itself was effective and straightforward, involving the addition of a single line of code—the call to sk_msg_free(msg)—in the failure path. This adheres to the fundamental kernel programming principle: acquired resources must be disposed of (freed) in both success and failure branches to ensure memory consistency and mitigate UAF vulnerabilities.