CVE-2011-1751 in qemu
Summary
by MITRE
The pciej_write function in hw/acpi_piix4.c in the PIIX4 Power Management emulation in qemu-kvm does not check if a device is hotpluggable before unplugging the PCI-ISA bridge, which allows privileged guest users to cause a denial of service (guest crash) and possibly execute arbitrary code by sending a crafted value to the 0xae08 (PCI_EJ_BASE) I/O port, which leads to a use-after-free related to "active qemu timers."
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Analysis
by VulDB Data Team • 12/04/2021
The vulnerability described in CVE-2011-1751 resides within the QEMU-KVM hypervisor's PIIX4 Power Management emulation component, specifically in the pciej_write function located in hw/acpi_piix4.c. This flaw represents a critical security issue that exploits improper device management during hotplug operations, creating a pathway for privilege escalation and system instability. The vulnerability manifests when a guest operating system sends a crafted value to the PCI_EJ_BASE I/O port at address 0xae08, which triggers an improper handling of PCI-ISA bridge unplug operations.
The technical root cause of this vulnerability stems from a missing validation check within the pciej_write function that fails to verify whether a device is hotpluggable before attempting to unplug the PCI-ISA bridge. This oversight creates a use-after-free condition that occurs when qemu timers remain active after device removal, allowing for memory corruption and potential code execution. The flaw specifically relates to CWE-415 which addresses double free errors and CWE-416 which covers use after free conditions, both of which are fundamental memory safety issues that can lead to arbitrary code execution or system crashes.
The operational impact of this vulnerability is severe as it enables privileged guest users to trigger a denial of service condition that results in complete guest system crashes. Beyond simple DoS, the use-after-free condition creates opportunities for arbitrary code execution within the hypervisor context, potentially allowing attackers to escalate privileges and compromise the entire virtualization environment. This vulnerability affects all systems running QEMU-KVM with PIIX4 Power Management enabled, making it particularly dangerous in multi-tenant cloud environments where guest isolation is critical.
Mitigation strategies for CVE-2011-1751 should focus on implementing proper device validation before hotplug operations, ensuring that all device removal procedures include checks for hotpluggability status. The most effective approach involves applying the vendor-provided patches that modify the pciej_write function to properly validate device status before attempting PCI-ISA bridge unplug operations. Additionally, system administrators should consider implementing hypervisor-level restrictions that limit guest access to critical I/O ports and consider disabling unnecessary ACPI features when not required. The vulnerability aligns with ATT&CK technique T1059 which covers command and scripting interpreter usage, as attackers could potentially leverage this vulnerability to execute malicious code within the hypervisor environment. Organizations should also implement monitoring for unusual I/O port access patterns and consider network segmentation to limit potential attack vectors.