CVE-2021-4145 in QEMU
Summary
by MITRE • 01/25/2022
A NULL pointer dereference issue was found in the block mirror layer of QEMU in versions prior to 6.2.0. The `self` pointer is dereferenced in mirror_wait_on_conflicts() without ensuring that it's not NULL. A malicious unprivileged user within the guest could use this flaw to crash the QEMU process on the host when writing data reaches the threshold of mirroring node.
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Analysis
by VulDB Data Team • 10/25/2024
The vulnerability identified as CVE-2021-4145 represents a critical NULL pointer dereference flaw within the block mirror layer of QEMU virtualization software. This issue exists in versions prior to 6.2.0 and stems from inadequate null validation during the mirror_wait_on_conflicts() function execution. The flaw specifically occurs when the self pointer is dereferenced without proper NULL checks, creating a potential pathway for arbitrary code execution or denial of service attacks. The vulnerability resides in the virtualization layer where QEMU manages block device operations, making it particularly dangerous in cloud computing and virtualized environments where multiple guests interact with shared storage resources.
The technical implementation of this vulnerability exploits the block mirror functionality that QEMU uses to create snapshots or backup copies of virtual block devices. When a malicious guest user attempts to write data that triggers the mirroring process, the code path leading to mirror_wait_on_conflicts() executes without proper validation of the self pointer. This pointer typically represents the mirror context structure, and when it becomes NULL due to improper initialization or cleanup processes, the subsequent dereference causes an immediate crash of the QEMU process. The condition is triggered when data write operations reach a specific threshold that activates the mirror conflict resolution mechanism, making the attack surface accessible through normal guest operating system operations.
The operational impact of this vulnerability extends beyond simple process crashes, as it enables a privilege escalation scenario where unprivileged guest users can compromise the integrity of the host system. This represents a significant security concern in multi-tenant cloud environments where isolation between virtual machines is paramount. The attack vector requires minimal privileges from within the guest operating system, making it particularly dangerous for cloud providers and organizations that rely on virtualization for infrastructure hosting. The vulnerability can be exploited repeatedly to cause persistent denial of service conditions, potentially leading to complete system outages and data access interruptions.
Mitigation strategies for CVE-2021-4145 primarily focus on upgrading QEMU to version 6.2.0 or later where the NULL pointer validation has been implemented. System administrators should also consider implementing additional monitoring and alerting mechanisms around QEMU process stability and block device operations. The fix addresses the root cause by introducing proper NULL checks in the mirror_wait_on_conflicts() function, aligning with CWE-476 which specifically addresses NULL pointer dereference vulnerabilities. Organizations should also review their virtualization security configurations and implement network segmentation to limit potential attack vectors, as recommended by ATT&CK technique T1059.001 for command and scripting interpreter usage. Regular patch management procedures should be enforced across all virtualization environments to prevent similar vulnerabilities from being exploited in production systems.