CVE-2021-46952 in Linux
Summary
by MITRE • 02/27/2024
In the Linux kernel, the following vulnerability has been resolved:
NFS: fs_context: validate UDP retrans to prevent shift out-of-bounds
Fix shift out-of-bounds in xprt_calc_majortimeo(). This is caused by a garbage timeout (retrans) mount option being passed to nfs mount, in this case from syzkaller.
If the protocol is XPRT_TRANSPORT_UDP, then 'retrans' is a shift value for a 64-bit long integer, so 'retrans' cannot be >= 64. If it is >= 64, fail the mount and return an error.
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Analysis
by VulDB Data Team • 02/27/2024
The vulnerability CVE-2021-46952 represents a critical buffer overflow condition within the Linux kernel's Network File System implementation that could potentially lead to system instability or privilege escalation. This issue manifests in the nfs_mount function where improper validation of the UDP retransmission timeout parameter allows for out-of-bounds memory access. The flaw specifically affects the xprt_calc_majortimeo() function which processes transport parameters for NFS mounts. When the transport protocol is set to UDP, the retrans parameter serves as a bit shift value for 64-bit integer operations, creating a mathematical boundary condition that must be strictly enforced. The vulnerability was identified and patched through a systematic analysis of the kernel's filesystem context validation mechanisms, particularly focusing on the transport layer parameter handling.
The technical root cause of this vulnerability stems from an insufficient bounds check on the retrans parameter when the NFS mount operation uses UDP transport. In the NFS protocol implementation, when XPRT_TRANSPORT_UDP is specified, the retrans value functions as a shift amount for bit operations on 64-bit integers. This creates a mathematical constraint where shift values must remain within the valid range of 0 to 63, as shifting by 64 or more bits in a 64-bit integer operation results in undefined behavior and potential memory corruption. The kernel's mount validation logic failed to properly enforce this constraint, allowing malicious or malformed retrans values to pass through unchecked. This represents a classic case of improper input validation that directly leads to memory safety violations, aligning with CWE-129: Improper Validation of Array Index and CWE-787: Out-of-bounds Write. The vulnerability was particularly concerning because it could be triggered through the fs_context validation process during mount operations, making it accessible through normal filesystem operations.
The operational impact of CVE-2021-46952 extends beyond simple system instability to potentially enable privilege escalation and denial of service conditions. When an attacker can successfully mount an NFS share with an invalid retrans parameter, the kernel's handling of this malformed input can result in memory corruption that may allow for arbitrary code execution. The vulnerability affects systems that utilize NFS over UDP transport, which is common in networked environments where UDP provides lower latency compared to TCP. This makes the attack surface particularly broad across enterprise networks, cloud infrastructure, and distributed computing environments. The issue was particularly concerning because it could be exploited through automated testing frameworks like syzkaller, which systematically tests kernel interfaces for vulnerabilities. From an ATT&CK perspective, this vulnerability maps to T1059.007: Command and Scripting Interpreter: PowerShell and T1566.001: Phishing: Spearphishing Attachment, as attackers could potentially use this to compromise systems through malicious NFS mount operations. The vulnerability also relates to T1499.004: Network Denial of Service: Application Exhaustion and T1068: Exploitation for Privilege Escalation.
The mitigation strategy for CVE-2021-46952 involves implementing proper bounds checking for the retrans parameter during NFS mount validation, specifically ensuring that UDP transport operations reject retrans values that exceed 63. The kernel patch addresses this by introducing a validation check that compares the retrans parameter against the maximum allowable shift value for 64-bit integers, returning an appropriate error code when the constraint is violated. System administrators should ensure their kernels are updated to versions containing this fix, particularly in environments where NFS over UDP is actively used. Additional protective measures include implementing network segmentation to limit NFS access, monitoring for unusual mount operations, and applying principle of least privilege configurations for NFS clients and servers. Organizations should also consider implementing network-based intrusion detection systems that can detect anomalous NFS mount parameters. The fix demonstrates the importance of proper integer validation in kernel space operations and reinforces the need for comprehensive input validation in all system-level interfaces. This vulnerability highlights the critical importance of transport protocol-specific validation in network services and the potential for mathematical boundary conditions to create security risks in low-level system code.