Meltdown Attack: Data Integrity vs. Secrecy

TL;DR

A Meltdown attack primarily violates data secrecy, but it can also indirectly lead to violations of data integrity in other processes under certain conditions. This happens because the attack allows reading arbitrary memory locations, potentially including code and data belonging to other running programs.

Understanding the Attack

Meltdown exploits a flaw in processors that allow user-level applications to access kernel memory directly. Normally, this is prevented by hardware mechanisms like page tables and permissions checks. Meltdown bypasses these protections by exploiting speculative execution – a technique where the processor tries to predict future instructions to improve performance.

Data Secrecy Violation

The core of the Meltdown attack involves reading sensitive data from kernel memory that shouldn’t be accessible to the user process. This directly compromises confidentiality. For example, it can reveal passwords, encryption keys, or other confidential information stored in the kernel.

Data Integrity Violation – How It Happens

While not its primary goal, Meltdown can lead to data integrity issues. Here’s how:

1. Reading Code Sections: The attack can read executable code from other processes’ memory spaces.
2. Modifying Process State (Indirectly): Although Meltdown doesn’t directly allow writing to arbitrary memory locations, reading critical process state information (like function pointers or data structures used for control flow) could enable an attacker to manipulate the target process indirectly. This is more complex and requires further exploitation steps.
3. Cache Side-Channel Attacks: The timing of memory accesses during speculative execution can reveal information about the contents of other processes’ memory, which might be used to infer or even influence their behaviour.

Example Scenario

Imagine two processes running on a system: Process A and Process B.

• Process A is vulnerable to Meltdown.
• Process B contains sensitive data and critical code sections.

An attacker using Meltdown against Process A could:

1. Read the memory of Process B, including its code.
2. Identify a function pointer within Process B’s memory space.
3. If possible (and this is the tricky part), find a way to overwrite that function pointer with a malicious address, effectively hijacking control flow in Process B. This requires additional vulnerabilities or exploitation techniques beyond just Meltdown itself.

Mitigation

Several mitigations have been developed to address Meltdown and related attacks:

• Kernel Page-Table Isolation (KPTI): Separates kernel memory from user processes, making it harder to access sensitive data.
• Software Patches: Updates to the operating system and hypervisor to prevent speculative execution vulnerabilities.
• Hardware Changes: Newer processors include hardware protections against these types of attacks (e.g., Intel’s Retpoline).

Example KPTI check on Linux:

cat /proc/cmdline | grep mitigations

This command will show if KPTI and other mitigations are enabled.

Conclusion

Meltdown is primarily a data secrecy attack, but it can create opportunities for indirect data integrity violations. While directly overwriting memory isn’t possible with Meltdown alone, reading critical process state information opens the door to further exploitation that could compromise the integrity of other processes.