Overlaybd-tcmu Crash Investigation On Ubuntu 24.04 A Buffer Overflow Analysis

by Sebastian Müller 78 views

#overlaybd #ubuntu24.04 #bufferoverflow #crashinvestigation #containers #containerd #tcmu #bugfix #debugging #opensource

We've got a critical issue on our hands, guys! The overlaybd-tcmu service is crashing on Ubuntu 24.04 due to a buffer overflow. This is a serious problem that can lead to system instability and data loss. Let's dive into the details and figure out what's going on and how we can fix it.

The Problem: Buffer Overflow Crash on Ubuntu 24.04

Our team encountered a crash with overlaybd-tcmu specifically on Ubuntu 24.04 while using the v1.0.15 release. The crash manifests as a buffer overflow, leading to the termination of the service. This issue seems to be unique to Ubuntu 24.04, as other builds appear to be unaffected. To reproduce this, simply install the Ubuntu 24.04 build from the specified GitHub release on an Ubuntu 24.04 server. We need to get to the bottom of this and ensure a stable experience for our users.

Detailed Error Analysis

Let's break down the error. The core dump provides valuable clues. Here's the stack trace we observed:

(gdb) where
#0  __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3  0x00007171bd44527e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007171bd4288ff in __GI_abort () at ./stdlib/abort.c:79
#5  0x00007171bd4297b6 in __libc_message_impl (fmt=fmt@entry=0x7171bd5ce765 "*** %s ***: terminated\n") at ../sysdeps/posix/libc_fatal.c:134
#6  0x00007171bd536c19 in __GI___fortify_fail (msg=msg@entry=0x7171bd5ce74c "buffer overflow detected") at ./debug/fortify_fail.c:24
#7  0x00007171bd5365d4 in __GI___chk_fail () at ./debug/chk_fail.c:28
#8  0x00007171bd537db5 in ___snprintf_chk (s=<optimized out>, maxlen=<optimized out>, flag=<optimized out>, slen=<optimized out>, format=<optimized out>)
    at ./debug/snprintf_chk.c:29
#9  0x00005d387a0c3cbc in tcmu_emulate_evpd_inquiry(tcmu_device*, tgt_port*, unsigned char*, iovec*, unsigned long) ()
#10 0x00005d387a037f2a in cmd_handler(tcmu_device*, tcmulib_cmd*) ()
#11 0x00005d387a038074 in handle(void*) ()
#12 0x00005d387a03f069 in photon::ThreadPoolBase::stub(void*) ()
#13 0x00005d387a04390f in _photon_thread_stub ()
#14 0x0000000000000000 in ?? ()

The stack trace clearly points to a buffer overflow detected within the tcmu_emulate_evpd_inquiry function. This function is responsible for emulating SCSI inquiry commands, and it seems like it's writing data beyond the allocated buffer's boundaries. This is a classic security vulnerability and needs immediate attention. The ___snprintf_chk function, a fortified version of snprintf, detected the overflow, preventing further damage but also terminating the service.

Examining the Logs

The journalctl logs provide additional context. Here’s a snippet:

overlaybd-tcmu[18728]: 2025/07/30 05:33:01.930697|INFO |th=00006214CF87A700|image_service.cpp:195|read_global_config_and_set:[global_conf.logConfig().logPath()=/var/log/overlaybd.log]
Jul 30 05:33:01 aks-np15114u24-85235044-vmss000001 overlaybd-tcmu[18728]: 2025/07/30 05:33:01.930723|INFO |th=00006214CF87A700|image_service.cpp:209|read_global_config_and_set:set log_level: 0
Jul 30 05:33:01 aks-np15114u24-85235044-vmss000001 overlaybd-tcmu[18728]: 2025/07/30 05:33:01.930727|INFO |th=00006214CF87A700|image_service.cpp:212|read_global_config_and_set:set log_path: /var/log/overlaybd.log, log_size: 10485760, log_num: 3
Jul 30 05:33:02 aks-np15114u24-85235044-vmss000001 overlaybd-tcmu[18728]: *** buffer overflow detected ***: terminated
Jul 30 05:33:02 aks-np15114u24-85235044-vmss000001 systemd[1]: overlaybd-tcmu.service: Main process exited, code=dumped, status=6/ABRT
Jul 30 05:33:02 aks-np15114u24-85235044-vmss000001 systemd[1]: overlaybd-tcmu.service: Failed with result 'core-dump'.
Jul 30 05:33:03 aks-np15114u24-85235044-vmss000001 systemd[1]: overlaybd-tcmu.service: Scheduled restart job, restart counter is at 6.
Jul 30 05:33:03 aks-np15114u24-85235044-vmss000001 systemd[1]: overlaybd-tcmu.service: Start request repeated too quickly.
Jul 30 05:33:03 aks-np15114u24-85235044-vmss000001 systemd[1]: overlaybd-tcmu.service: Failed with result 'core-dump'.
Jul 30 05:33:03 aks-np15114u24-85235044-vmss000001 systemd[1]: Failed to start overlaybd-tcmu.service - overlaybd-tcmu service.

The logs confirm the buffer overflow detection and the subsequent termination of the overlaybd-tcmu service. The systemd logs show that the service is attempting to restart, but failing due to the persistent crash. This rapid restart cycle can put a strain on system resources, making it even more critical to address the root cause.

Reproducing the Issue

To reproduce the issue, follow these steps:

  1. Install Ubuntu 24.04 on a server.
  2. Download the v1.0.15 release of overlaybd from https://github.com/containerd/overlaybd/releases/tag/v1.0.15.
  3. Install and configure overlaybd-tcmu according to the documentation.
  4. Attempt to use overlaybd-tcmu in a containerized environment. The crash should occur during SCSI inquiry emulation.

By consistently reproducing the issue, we can validate any potential fixes and ensure the problem is truly resolved. This also helps in identifying the specific conditions that trigger the overflow, which can provide valuable insights for debugging.

Potential Causes and Investigation Steps

Given the stack trace and the nature of the error, here are some potential causes we need to investigate:

  1. Incorrect Buffer Size Calculation: The tcmu_emulate_evpd_inquiry function might be calculating the required buffer size incorrectly, leading to an undersized buffer allocation.
  2. Unvalidated Input Lengths: Input data lengths might not be validated before being copied into the buffer, allowing more data to be written than the buffer can hold.
  3. Format String Vulnerability: Although less likely with ___snprintf_chk, there's a slim chance of a format string vulnerability if the format string itself is derived from user input.
  4. Compiler Optimization Differences: Ubuntu 24.04 might be using a different compiler version or optimization level that exposes an existing bug in the code.

To investigate these causes, we should:

  • Examine the tcmu_emulate_evpd_inquiry function: Carefully review the code for buffer size calculations and data copying operations.
  • Analyze Input Data: Inspect the input data being passed to the function to identify any patterns or unusual lengths that might trigger the overflow.
  • Use Static Analysis Tools: Employ static analysis tools to detect potential buffer overflows and other vulnerabilities in the code.
  • Debug with GDB: Step through the code with GDB, paying close attention to buffer allocations and memory writes.
  • Compare with Working Builds: Compare the code and build environment with versions that are not exhibiting the issue to identify any differences.

Next Steps and Call to Action

This buffer overflow in overlaybd-tcmu on Ubuntu 24.04 is a critical issue that requires immediate attention. We need to thoroughly investigate the code, identify the root cause, and implement a robust fix. We've already started the initial analysis, but we need the community's help to expedite the process.

If you have experience with overlaybd, SCSI emulation, or buffer overflow debugging, your expertise would be invaluable. Please consider:

  • Reviewing the Code: Take a look at the tcmu_emulate_evpd_inquiry function and related code to identify potential issues.
  • Analyzing the Core Dump: Dive deeper into the core dump to understand the state of the program at the time of the crash.
  • Testing Potential Fixes: If you have ideas for a fix, please test them and share your results.
  • Submitting a PR: If you can implement a fix, please submit a pull request with your changes.

We're committed to resolving this issue and ensuring the stability of overlaybd. Your contributions will help us achieve this goal. Let's work together to make overlaybd even better!

Key Information

  • Issue: overlaybd-tcmu crash due to buffer overflow on Ubuntu 24.04
  • Version: v1.0.15
  • Affected OS: Ubuntu 24.04
  • Root Cause (Suspected): Buffer overflow in tcmu_emulate_evpd_inquiry function
  • Call to Action: Community involvement needed for code review, analysis, and testing.

Are you willing to submit PRs to fix it?

Yes, I am willing to fix it. We believe in open source and are dedicated to contributing back to the community.