A recently discovered vulnerability in the Linux kernel, specified as CVE-2023-0597, exposes the possibility of a memory leak in the mapping of X86 CPU data to memory within the Linux kernel's cpu_entry_area. This flaw enables a local user to potentially gain access to critical data by predicting the location of exception stacks or other significant data in memory.

This blog post will provide an in-depth look at this vulnerability, including insights into its details, how it can be exploited, and potential mitigations. Additionally, we will provide code snippets and links to the original references for further understanding.

Vulnerability Details

The cpu_entry_area is a critical part of the Linux kernel responsible for managing exception stacks, interrupt handler stacks, and other vital CPU data structures. It contains a set of mappings, each of which corresponds to a separate CPU feature or data structure, and is a powerful mechanism for managing the access to these protected areas of memory.

The vulnerability arises in the way the Linux kernel maps X86 CPU data to memory within the cpu_entry_area. This mapping accidentally allows users to predict the location of exception stacks or other important data erroneously. The following code snippet demonstrates the issue:

 /* Kernel source: arch/x86/kernel/cpu_entry_area.c */
 
 static void __init cea_init_percpu_pages(void)
 {
     int cpu;
 
     for_each_possible_cpu(cpu) {
         void *cea_start;
 
         cea_init_percpu_entry(cpu);
 
         /* ... */
 
         /* The bug is in this line */
         cea_start = __this_cpu_early_ptr(cea_exception_stacks_vaddr);
 
         /* ... */
 
         /* Copy the exception stacks */
         cpumask_set_cpu(cpu, &cpu_entry_area_initialized);
     }
 }

In this snippet, the flaw's origin can be traced to the line where cea_start is assigned the value of __this_cpu_early_ptr(cea_exception_stacks_vaddr). This assignment does not take into consideration proper sanitization, which might result in the leakage of important memory locations to users.

Exploitation Details

A local attacker could exploit the vulnerability by predicting the location of the exception stacks or other relevant data structures in memory. This prediction can be achieved through trial and error, by requesting access to specific memory locations and observing the resulting behavior. If an attacker successfully identifies the correct memory location, they might be able to access sensitive data.

Here's a simplified example of how an attacker might attempt to exploit the vulnerability

 #include <stdio.h>
 #include <stdlib.h>
 
 int main(void)
 {
     char *guess_memory_location;
     void *critical_memory_address;
 
     /* Attempt to predict the critical memory location */
     for (int i = ; i < MAX_TRIES; i++) {
         guess_memory_location = make_guess();
 
         /* Test the guessed memory location */
         if (access_critical_data(guess_memory_location)) {
             critical_memory_address = guess_memory_location;
             break;
         }
     }
 
     if (critical_memory_address) {
         printf("Found critical data at address: %p\n", critical_memory_address);
     } else {
         printf("Could not find critical data\n");
     }
 
     return ;
 }

Mitigation

Currently, a potential solution is available in the form of a proposed patch that changes how the mapping occurs within the cpu_entry_area. This patch aims to eliminate the predictability of important memory locations and improve overall kernel security. The proposed changes can be followed in the main Linux kernel repository.

For more information and references, please refer to the following sources

1. CVE-2023-0597 - Original CVE Record
2. Linux Kernel Mailing List - Patch Proposal (PDF)

Conclusion

In this post, we've examined the details of CVE-2023-0597, a vulnerability that potentially exposes memory leaks in the Linux kernel's cpu_entry_area. We explored the nature of the flaw, how it could be exploited, and potential mitigations. The vulnerability serves as a reminder of the importance of diligent code review in ensuring that software remains secure and resilient to threats.

Timeline

Published on: 02/23/2023 20:15:00 UTC
Last modified on: 03/03/2023 15:59:00 UTC