CVE-2022-50720

Vulnerability

Overview

In the Linux kernel, CVE-2022-50720 addresses a flaw in the x86 APIC subsystem where the kernel could attempt to disable x2APIC mode or switch back to legacy xAPIC mode even when the system firmware (BIOS) has locked the APIC in x2APIC mode. This locking is performed by setting the LEGACY_XAPIC_DISABLED bit in the IA32_XAPIC_DISABLE_STATUS MSR, typically on platforms where Intel SGX or TDX are enabled [1].

Attack

Vector and Exploitation

If the kernel disregards the locked state and tries to revert to legacy APIC mode, a general protection fault (GP fault) would occur, potentially crashing the system or disrupting the secure execution environments like SGX enclaves. The MMIO-based xAPIC interface is known to have security issues, notably the APIC leak vulnerability [2], which can allow an attacker to extract sensitive data from SGX enclaves. By forcing the APIC into legacy mode, an attacker could exploit this interface to bypass protections enforced by SGX or TDX.

Potential

Impact

An attacker with local access or the ability to trigger a kernel operation that disables x2APIC could cause a denial of service (system crash) or, in scenarios where x2APIC is not properly locked, could degrade the security of memory encryption technologies like SGX and TDX. The vulnerability underscores the importance of ensuring the kernel respects BIOS-level locks on APIC mode to maintain the security guarantees of these hardware features.

Mitigation

Status

The fix introduces checks that prevent the kernel from disabling x2APIC or reverting to legacy mode when the BIOS has locked it. This is achieved by reading the IA32_XAPIC_DISABLE_STATUS MSR and avoiding operations that would cause a GP fault. Users should apply the kernel patch referenced in [1] to ensure their systems correctly handle locked x2APIC configurations.