Is printk() a Security Issue on Linux?
This article aims to explore the potential security implications of the printk() function in the Linux operating system. Specifically, we will address the concerns related to kernel pointers and their impact on system security.
Understanding the Functionality
The function printk() is commonly used in Linux for printing messages to the kernel log buffer. While this functionality is essential for debugging and logging purposes, it raises questions regarding its potential to expose sensitive information and facilitate privilege escalation attacks.
Sanitization of Pointers
The security of printk() largely depends on whether unsanitized pointers are printed. In general, pointers are sanitized when the kernel.kptr_restrict configuration flag is set to a non-zero value. This is achieved through the use of the %pK identifier, which automatically sanitizes pointers by default. However, mistakes can happen, resulting in accidental pointer leakage. For example, a typo can lead to the use of %pk, which does not go through the same sanitization process. These infoleaks are not uncommon and can also occur in virtual filesystems like proc and sysfs.
User-Level Access and Kernel Pointers
When an attacker gains user-level access to the system, having access to kernel pointers can potentially aid in their exploitation efforts. However, the impact is limited. The most significant risk lies in the discovery of the base offset of the kernel to bypass Kernel Address Space Layout Randomization (KASLR). It is worth noting that KASLR is not foolproof against local attackers and can be circumvented through infoleaks or timing attacks on CPU architectures.
Securing Highly Sensitive Systems
Disabling the printk() function entirely by setting the CONFIG_PRINTK configuration flag to ‘n’ is not recommended for highly secured systems. Instead, a more effective approach is to restrict access to the kernel log buffer. This can be achieved by setting the kernel.dmesg_restrict configuration flag to ‘1’, which ensures that only users with high privileges can access the kernel log buffer. This not only protects against unsanitized pointers leaking into the syslog but also prevents the exposure of other potentially sensitive information that may be printed there.
It is important to note that the information provided in this article applies to stock kernels. If you compile your own kernel, especially when using RANDSTRUCT, knowledge of pointers becomes necessary for an attacker as it can aid in identifying the offset of sensitive kernel functions.