CVE-2024-25742: Linux Kernel Privilege Escalation Flaw

CVE-2024-25742 Overview

In the Linux kernel before version 6.9, an untrusted hypervisor can inject virtual interrupt 29 (#VC) at any point in time and trigger its handler. This vulnerability affects AMD SEV-SNP (Secure Encrypted Virtualization-Secure Nested Paging) and AMD SEV-ES (Secure Encrypted Virtualization-Encrypted State) implementations, potentially allowing a malicious hypervisor to compromise the confidentiality and integrity guarantees these security features are designed to provide.

Critical Impact

A malicious or compromised hypervisor can bypass AMD SEV security boundaries by injecting arbitrary #VC exceptions, potentially leading to guest VM compromise in confidential computing environments.

Affected Products

• Linux kernel versions before 6.9
• Systems using AMD SEV-SNP (Secure Nested Paging)
• Systems using AMD SEV-ES (Encrypted State)

Discovery Timeline

• 2024-05-17 - CVE CVE-2024-25742 published to NVD
• 2025-03-27 - Last updated in NVD database

Technical Details for CVE-2024-25742

Vulnerability Analysis

This vulnerability exists in the Linux kernel's handling of AMD SEV (Secure Encrypted Virtualization) virtual interrupt exceptions. AMD SEV-SNP and SEV-ES are security technologies designed to protect virtual machines from potentially malicious hypervisors by encrypting VM memory and register state. The #VC (VMM Communication Exception, interrupt 29) is a critical mechanism used for communication between the guest VM and the hypervisor in SEV environments.

The flaw allows an untrusted hypervisor to inject #VC exceptions at arbitrary points during guest VM execution. This breaks the security model of AMD SEV, which assumes the hypervisor cannot arbitrarily influence guest execution in security-sensitive ways. By triggering the #VC handler at unexpected times, an attacker controlling the hypervisor could potentially manipulate guest VM state, leak sensitive information, or cause the guest to perform unintended actions.

The vulnerability is classified under CWE-828 (Signal Handler with Functionality that is not Asynchronous-Safe), indicating the core issue relates to improper handling of asynchronous signals/interrupts.

Root Cause

The root cause lies in insufficient validation of #VC exception conditions in the Linux kernel's SEV exception handler. The kernel did not properly verify that a #VC exception was legitimately triggered by an actual VM exit condition before processing it. This allowed a malicious hypervisor to inject spurious #VC interrupts that the kernel would process as legitimate, bypassing the trust boundaries SEV is designed to enforce.

Attack Vector

The attack requires local access through a malicious or compromised hypervisor. The attacker must control the hypervisor layer to inject virtual interrupt 29 (#VC) into a guest VM running on AMD SEV-SNP or SEV-ES. This is a sophisticated attack targeting confidential computing environments where the hypervisor is considered untrusted. The attack could:

1. Inject #VC exceptions during sensitive guest operations
2. Trigger instruction emulation at unintended times
3. Manipulate the guest's perception of hardware state
4. Potentially extract encrypted guest data or corrupt guest execution

The following patch was applied to harden the #VC instruction emulation:

 	if (result != ES_OK)
 		goto finish;

+	result = vc_check_opcode_bytes(&ctxt, exit_code);
+	if (result != ES_OK)
+		goto finish;
+
 	switch (exit_code) {
 	case SVM_EXIT_RDTSC:
 	case SVM_EXIT_RDTSCP:

Source: GitHub Linux Commit e3ef461

Additional logging capabilities were added to help detect and diagnose potential attacks:

 */

 #ifndef __BOOT_COMPRESSED
-#define error(v)	pr_err(v)
-#define has_cpuflag(f)	boot_cpu_has(f)
+#define error(v)			pr_err(v)
+#define has_cpuflag(f)			boot_cpu_has(f)
+#define sev_printk(fmt, ...)		printk(fmt, ##__VA_ARGS__)
+#define sev_printk_rtl(fmt, ...)	printk_ratelimited(fmt, ##__VA_ARGS__)
 #else
 #undef WARN
 #define WARN(condition, format...) (!!(condition))
+#define sev_printk(fmt, ...)
+#define sev_printk_rtl(fmt, ...)
 #endif

 /* I/O parameters for CPUID-related helpers */

Source: GitHub Linux Commit e3ef461

Detection Methods for CVE-2024-25742

Indicators of Compromise

• Unexpected or high-frequency #VC exceptions logged in guest VM kernel logs
• Anomalous behavior in SEV-protected guest VMs that may indicate instruction emulation manipulation
• Kernel messages indicating failed opcode validation in SEV exception handling code

Detection Strategies

• Monitor kernel logs for SEV-related error messages, particularly those from the hardened vc_check_opcode_bytes() function
• Implement runtime integrity monitoring for guest VMs in confidential computing environments
• Deploy kernel-level telemetry to track #VC exception frequency and context

Monitoring Recommendations

• Enable verbose SEV logging in Linux kernel boot parameters for enhanced visibility
• Monitor for unusual patterns in VM exit reasons reported by AMD SEV attestation mechanisms
• Implement baseline monitoring for #VC exception rates in production SEV environments
• Review hypervisor audit logs for suspicious interrupt injection patterns

How to Mitigate CVE-2024-25742

Immediate Actions Required

• Upgrade the Linux kernel to version 6.9 or later which contains the security fix
• For systems that cannot be immediately updated, evaluate the risk exposure based on hypervisor trust levels
• Review and validate the integrity of hypervisor infrastructure in confidential computing deployments

Patch Information

The vulnerability has been addressed in Linux kernel version 6.9. The fix introduces additional validation through the vc_check_opcode_bytes() function that verifies the legitimacy of #VC exceptions before processing them. This ensures that only valid VM exits trigger instruction emulation.

Relevant security resources:

• Linux Kernel ChangeLog 6.9
• Linux Kernel Commit e3ef461
• AMD Security Bulletin SB-3008

Workarounds

• If kernel updates are not immediately possible, consider temporarily migrating sensitive workloads away from SEV-protected VMs until patching can be completed
• Ensure hypervisor infrastructure is from trusted sources and properly hardened
• Implement additional monitoring and alerting for SEV-related kernel messages
• Consider using hardware attestation to verify guest VM integrity in high-security environments

# Check current kernel version
uname -r

# Verify SEV status on AMD systems
dmesg | grep -i sev

# Update to patched kernel (example for Debian/Ubuntu)
apt update && apt install linux-image-6.9.0-generic

# Verify the patch is applied by checking for vc_check_opcode_bytes symbol
grep vc_check_opcode_bytes /proc/kallsyms