CVE-2021-3929 in QEMU
Summary
by MITRE • 08/26/2022
A DMA reentrancy issue was found in the NVM Express Controller (NVME) emulation in QEMU. This CVE is similar to CVE-2021-3750 and, just like it, when the reentrancy write triggers the reset function nvme_ctrl_reset(), data structs will be freed leading to a use-after-free issue. A malicious guest could use this flaw to crash the QEMU process on the host, resulting in a denial of service condition or, potentially, executing arbitrary code within the context of the QEMU process on the host.
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Analysis
by VulDB Data Team • 02/28/2025
The CVE-2021-3929 vulnerability represents a critical DMA reentrancy flaw within QEMU's NVM Express controller emulation that exposes systems to significant security risks. This vulnerability specifically affects the NVME controller implementation in QEMU's virtualization environment, where malicious guest operating systems can exploit a reentrancy condition during DMA operations to trigger a use-after-free scenario. The flaw demonstrates the complex nature of virtualization security where hypervisor components must maintain strict integrity controls to prevent guest-to-host privilege escalation.
The technical implementation of this vulnerability stems from improper handling of reentrant DMA write operations within the NVME controller emulation layer. When a malicious guest triggers a specific sequence of DMA writes that causes the nvme_ctrl_reset() function to execute, the underlying data structures become prematurely freed while still being referenced. This creates a classic use-after-free condition where subsequent memory accesses attempt to utilize already deallocated memory regions, leading to unpredictable behavior and potential code execution. The vulnerability closely parallels CVE-2021-3750, indicating a systemic issue within QEMU's NVME emulation architecture that requires comprehensive architectural review and remediation.
The operational impact of CVE-2021-3929 extends beyond simple denial of service conditions to encompass potential arbitrary code execution within the host environment. A malicious guest operating system can leverage this vulnerability to crash the QEMU process, effectively causing a denial of service that impacts all virtual machines hosted on that hypervisor. More critically, the use-after-free condition could potentially be exploited to execute arbitrary code with the privileges of the QEMU process, which typically runs with elevated permissions on the host system. This represents a severe privilege escalation vector that could compromise the entire host infrastructure and potentially lead to complete system compromise.
Mitigation strategies for CVE-2021-3929 should focus on immediate patch application from QEMU maintainers, as the vulnerability requires architectural changes to prevent reentrant DMA operations from triggering reset functions during active memory operations. System administrators should implement strict virtual machine access controls and monitor for unusual DMA activity patterns that might indicate exploitation attempts. The vulnerability aligns with ATT&CK technique T1059.001 for command and script interpreter usage and T1068 for exploit for privilege escalation, making it particularly concerning for environments where guest operating systems may not be fully trusted. Organizations should also consider implementing network segmentation and monitoring solutions to detect potential exploitation attempts, while adhering to CWE-416 for use-after-free vulnerabilities and CWE-362 for concurrent execution issues that enable race conditions in virtualization environments.