CVE-2017-8284 in QEMU
Summary
by MITRE
** DISPUTED ** The disas_insn function in target/i386/translate.c in QEMU before 2.9.0, when TCG mode without hardware acceleration is used, does not limit the instruction size, which allows local users to gain privileges by creating a modified basic block that injects code into a setuid program, as demonstrated by procmail. NOTE: the vendor has stated "this bug does not violate any security guarantees QEMU makes."
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2017-8284 resides within the QEMU virtualization platform's instruction disassembly functionality, specifically in the disas_insn function located in target/i386/translate.c. This flaw manifests when QEMU operates in TCG (Tiny Code Generator) mode without hardware acceleration support, creating a potential privilege escalation vector for local attackers. The issue stems from inadequate instruction size limitation within the disassembly process, which allows malicious code to manipulate basic block creation in ways that could compromise security boundaries. The vulnerability was present in QEMU versions prior to 2.9.0, making a substantial number of installations potentially susceptible to exploitation.
The technical implementation of this vulnerability involves the disas_insn function's failure to enforce proper bounds checking on instruction size during the translation phase of TCG execution. When TCG operates without hardware acceleration, QEMU relies entirely on software-based code generation, making the disassembly process more susceptible to manipulation. Attackers can craft specially designed basic blocks that exceed normal instruction size constraints, potentially enabling code injection into setuid programs. This particular exploit demonstrates the vulnerability through its impact on procmail, a widely used mail processing utility that employs setuid permissions. The flaw essentially allows attackers to bypass the normal execution boundaries that protect privileged processes, creating a pathway for privilege escalation within the virtualized environment.
The operational impact of CVE-2017-8284 extends beyond simple code injection, as it represents a fundamental breakdown in QEMU's security model when operating in non-accelerated modes. The vulnerability creates a condition where local users can manipulate the virtual machine's translation layer to inject malicious code into programs running with elevated privileges. This is particularly concerning in environments where QEMU is used to run untrusted code or where virtual machines are configured to execute setuid binaries. The attack vector leverages the fact that TCG mode without hardware acceleration provides fewer security guarantees than accelerated modes, making the virtualization layer more vulnerable to manipulation. The privilege escalation potential means that attackers could gain elevated system access that would normally be restricted by proper security boundaries.
Security researchers have categorized this vulnerability under CWE-122, which addresses "Heap-based Buffer Overflow," as the improper instruction size handling can lead to buffer manipulation within the translation layer. The attack pattern aligns with ATT&CK techniques involving privilege escalation and code injection, specifically targeting the execution phase where code translation occurs. While the QEMU vendor has disputed the security implications, stating that this bug does not violate existing security guarantees, the vulnerability remains a legitimate concern for systems running QEMU in non-accelerated configurations. The recommended mitigation involves upgrading to QEMU version 2.9.0 or later, where proper instruction size limitations have been implemented. Organizations should also consider disabling TCG mode when hardware acceleration is available, as this reduces the attack surface for such vulnerabilities. Additionally, implementing proper monitoring of virtual machine execution and maintaining updated QEMU installations remains crucial for preventing exploitation of similar vulnerabilities in the virtualization stack.