CVE-2015-7549 in QEMU
Summary
by MITRE
The MSI-X MMIO support in hw/pci/msix.c in QEMU (aka Quick Emulator) allows local guest OS privileged users to cause a denial of service (NULL pointer dereference and QEMU process crash) by leveraging failure to define the .write method.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 01/05/2023
The vulnerability identified as CVE-2015-7549 represents a critical flaw in the QEMU virtualization platform's implementation of MSI-X Message Signaled Interrupts Memory-Mapped I/O support. This issue resides within the hw/pci/msix.c source file and specifically targets the handling of MSI-X capabilities within virtual PCI devices. The vulnerability stems from inadequate input validation and error handling mechanisms that fail to properly initialize or validate the .write method callback function during MSI-X MMIO register operations. When a guest operating system with privileged access attempts to interact with MSI-X registers without proper method definition, the virtualization layer encounters a NULL pointer dereference condition that ultimately results in the complete crash of the QEMU process. This represents a significant security risk as it allows local privileged users within the guest operating system to execute a denial of service attack against the virtual machine itself, effectively compromising the availability of the emulated system. The flaw demonstrates a classic software defect pattern where the absence of proper error checking and validation leads to exploitable conditions in virtualized environments.
The technical implementation of this vulnerability exploits the fundamental architecture of QEMU's PCI MSI-X handling mechanism. When MSI-X capabilities are configured for virtual PCI devices, the system expects specific method callbacks to be properly defined for read and write operations on the MMIO registers. The absence of a defined .write method callback creates a scenario where subsequent write operations to MSI-X registers trigger a NULL pointer dereference within the QEMU kernel code. This condition occurs because the software assumes that all required method callbacks are properly initialized, but when they are not, the execution path leads to an invalid memory access that terminates the QEMU process. The vulnerability specifically affects the virtualization layer's ability to handle malformed or improperly configured MSI-X register access patterns, creating a direct pathway for privilege escalation and system instability within the emulated environment.
The operational impact of CVE-2015-7549 extends beyond simple denial of service to potentially compromise entire virtualized infrastructures. In multi-tenant virtualization environments, a malicious guest user could exploit this vulnerability to crash the QEMU process running other virtual machines, leading to cascading failures and service disruption across the entire virtualization platform. The vulnerability affects systems where QEMU is used for desktop virtualization, cloud computing services, or server virtualization, making it particularly dangerous in enterprise environments where virtualization is extensively deployed. From an attacker's perspective, this vulnerability represents a low-effort, high-impact method for causing system instability, as it requires only local privileged access within the guest operating system to execute the attack. The crash condition typically results in complete loss of virtual machine functionality until the QEMU process is manually restarted, potentially leading to data loss and service interruption. This vulnerability also demonstrates weaknesses in the virtualization security model where guest OS privileges can be leveraged to impact the host system's stability.
The remediation of CVE-2015-7549 requires implementing proper input validation and error handling mechanisms within QEMU's MSI-X MMIO support. The fix should ensure that all method callbacks are properly validated before execution, with appropriate error handling for undefined or NULL method pointers. This approach aligns with security best practices outlined in the CWE-476 weakness category, which addresses NULL pointer dereference vulnerabilities. Additionally, implementing proper initialization checks for MSI-X capabilities and ensuring that all required callback methods are defined before allowing guest access to MSI-X registers would prevent this class of vulnerability. System administrators should apply the relevant QEMU security patches that address this specific flaw, while also implementing monitoring solutions to detect potential exploitation attempts. Organizations using virtualization platforms should conduct comprehensive vulnerability assessments to identify other potential issues in their virtualization stacks and ensure proper isolation between guest environments to limit the impact of such vulnerabilities. The fix should also consider implementing runtime protections that can detect and prevent malicious access patterns that could trigger similar NULL pointer dereference conditions in other parts of the virtualization stack.