CVE-2023-20938: Understanding the Android Binder Use-After-Free Vulnerability in binder_transaction_buffer_release

CVE-2023-20938 is a security vulnerability discovered in the Android kernel's binder.c file, specifically in the binder_transaction_buffer_release function. This vulnerability could potentially lead to a local escalation of privilege without any additional execution privileges needed, and no user interaction is required for exploitation. In this blog post, we will delve into the details of this vulnerability, analyze the relevant code snippets, and explore how it can be exploited.

Vulnerability Details

The binder.c file contains the implementation of Android's inter-process communication (IPC) mechanism, which is used for processes to communicate with each other. This communication is done through Binder transactions, which involve a sender process filling a transaction buffer with data, and the recipient process reading the data from the buffer.

The vulnerability lies in the binder_transaction_buffer_release function, which is responsible for releasing the transaction buffers allocated for a binder transaction. The issue stems from improper input validation, which could potentially lead to a use-after-free scenario.

Below is the relevant code snippet from the binder.c file

void binder_transaction_buffer_release(
    struct binder_proc *proc,
    struct binder_transaction_buffer *t_buf,
    bool is_oneway) {

  binder_size_t *offsets = t_buf->data + t_buf->offsets_size;
  int num_valid = is_oneway ? t_buf->num_fds_in_array : t_buf->num_valid_fds;
  
  // ...

  for (i = ; i < num_valid; i++) {
    // ...
  }
  
  if (!is_oneway) {
    binder_free_proc_transaction(proc, t_buf);
  }
}

In the code above, we can see that the binder_free_proc_transaction function, which is responsible for freeing the transaction buffer, is invoked only if is_oneway is false. This implies that in one-way binder transactions, the transaction buffer will not be properly released, leading to a potential use-after-free situation.

Original References & Patch

The original discovery of this vulnerability and the subsequent patch can be found in the upstream kernel:

- Upstream commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8c33e6de245e6e565a6d112b529f7ab8f46c952d

The patch modifies the binder_transaction_buffer_release function to properly deallocate the transaction buffers in one-way transactions as well.

Exploit

To exploit this vulnerability, an attacker would need to initiate one-way binder transactions with carefully crafted data, causing the binder_transaction_buffer_release function to skip the proper deallocation of the transaction buffers. This could lead to a use-after-free condition, which could subsequently be leveraged by the attacker to escalate privileges on the device.

It is important to note that this vulnerability does not require user interaction for exploitation, and it can be leveraged by any process running on the Android device that initiates binder transactions.

Conclusion

CVE-2023-20938 is a significant security vulnerability in the Android kernel's binder.c file, potentially leading to local privilege escalation with no user interaction required. Developers and vendors must apply the appropriate patches to their Android kernel to prevent exploitation of this vulnerability. Users are advised to keep their devices updated and stay informed about security updates from their device manufacturers.

Timeline

Published on: 02/28/2023 17:15:00 UTC
Last modified on: 03/06/2023 19:32:00 UTC