Row Hammer Attack: Virtual Machine Security

TL;DR

Yes, virtualized environments are vulnerable to the Row Hammer attack, though it’s more complex than attacking bare metal. The shared physical memory creates a potential path for attackers to exploit weaknesses in DRAM and gain access to other VMs on the same host. Mitigation involves hardware features (like DRAM scrubbing), hypervisor protections, and careful VM configuration.

Understanding Row Hammer

The Row Hammer attack exploits a weakness in Dynamic Random Access Memory (DRAM). Repeatedly accessing (hammering) a single row of memory can cause bit flips in adjacent rows. This is because DRAM relies on electrical charges to store data, and frequent access can weaken the charge in nearby cells.

Why Virtual Machines are Affected

Virtual machines share the same physical hardware – including the RAM – as other VMs running on the host server. This means a malicious actor gaining control of one VM could potentially attempt to flip bits in memory used by another VM, leading to privilege escalation or data theft.

How an Attack Might Work

1. Gain Access: An attacker first needs to compromise a virtual machine. This could be through malware, exploiting a software vulnerability within the guest OS, or social engineering.
2. Identify Shared Memory: The attacker then attempts to identify memory regions shared between VMs (if any exist). This is harder in modern systems with strong isolation but possible depending on configuration.
3. Hammering: Using specially crafted code, the attacker repeatedly accesses a specific row of DRAM within their VM. This ‘hammering’ aims to induce bit flips in adjacent rows.
4. Exploitation: If successful, these bit flips can alter data or code within another VM’s memory space, potentially allowing the attacker to gain control.

Mitigation Steps

Here’s how to protect your virtualized environments:

1. Hardware Protections

• DRAM Scrubbing: Modern DRAM chips often include built-in scrubbing features that periodically refresh memory cells, reducing the likelihood of bit flips. Ensure this is enabled in your server BIOS/UEFI settings.
• Targeted Row Hammer Mitigation (TRM): Newer CPUs and chipsets offer specific hardware mitigations for Row Hammer attacks. Check your CPU documentation to see if TRM or similar features are available and enable them.

2. Hypervisor Level Protections

• Memory Isolation: Strong memory isolation is crucial. Configure the hypervisor (e.g., VMware ESXi, KVM, Hyper-V) to minimise shared memory pages between VMs. Use features like nested page tables and address space layout randomisation (ASLR).
• Page Table Isolation (PTI): PTI helps prevent attackers from accessing kernel memory in other VMs.
• Virtual Machine Introspection (VMI): VMI tools can monitor VM memory for suspicious activity, including repeated access patterns that might indicate a Row Hammer attack.

3. Guest OS Configuration

• Kernel Updates: Keep your guest operating systems fully patched with the latest security updates. Kernel developers are constantly adding protections against new attacks.
• Disable Unnecessary Features: Reduce the attack surface by disabling any unnecessary features or services within the guest OS that might increase memory access opportunities.
• Memory Randomisation (ASLR): Ensure ASLR is enabled in your guest operating systems. This makes it harder for attackers to predict memory addresses.

4. Monitoring and Detection

• Performance Monitoring: Monitor VM performance for unusual CPU or memory activity. Sudden spikes in memory access could be a sign of an attack.
• Log Analysis: Regularly review hypervisor logs for suspicious events related to memory management.

Checking for Mitigation (Example – Linux)

You can check if certain mitigations are enabled on a Linux guest OS:

cat /proc/cpuinfo | grep srbds

If ‘srbds’ is present in the output, it indicates that Serialized Row Hammer Detection (SRBD) is likely enabled. Note this isn’t a guarantee of full protection but shows some mitigation is active.

Important Considerations

• Shared Folders/Devices: Be extremely cautious when using shared folders or devices between VMs, as these can create pathways for attackers to access memory.
• Regular Security Audits: Conduct regular security audits of your virtualized environment to identify and address potential vulnerabilities.