CVE-2017-8797 in Linux
Summary
by MITRE
The NFSv4 server in the Linux kernel before 4.11.3 does not properly validate the layout type when processing the NFSv4 pNFS GETDEVICEINFO or LAYOUTGET operand in a UDP packet from a remote attacker. This type value is uninitialized upon encountering certain error conditions. This value is used as an array index for dereferencing, which leads to an OOPS and eventually a DoS of knfsd and a soft-lockup of the whole system.
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Analysis
by VulDB Data Team • 12/09/2022
The vulnerability described in CVE-2017-8797 represents a critical flaw in the Linux kernel's implementation of the Network File System version 4 protocol specifically affecting NFSv4 servers. This issue manifests in kernel versions prior to 4.11.3 where the server fails to properly validate layout type information during processing of pNFS GETDEVICEINFO or LAYOUTGET operations. The vulnerability occurs exclusively when handling UDP packets from remote attackers, making it particularly dangerous in networked environments where untrusted clients can potentially exploit this weakness.
The technical root cause of this vulnerability stems from improper initialization of a type value within the kernel's NFSv4 server implementation. When certain error conditions are encountered during packet processing, the layout type variable remains uninitialized rather than being properly set to a default or invalid value. This uninitialized variable subsequently gets used as an array index during memory dereferencing operations, creating a classic out-of-bounds access condition. The vulnerability is classified under CWE-457 as "Use of Uninitialized Variable" and represents a memory safety issue that can lead to system instability.
The operational impact of this vulnerability is severe and encompasses complete system denial of service through soft-lockup conditions. When exploited, the uninitialized type value causes the kernel to attempt memory access at an invalid array index location, resulting in an OOPS (kernel panic) condition that terminates the knfsd process responsible for NFS services. More critically, this condition triggers a soft-lockup of the entire system, rendering it unresponsive and effectively denying all services. The attack requires only a single malicious UDP packet from a remote attacker, making it particularly dangerous in networked environments where NFS servers are exposed to untrusted networks.
This vulnerability aligns with ATT&CK technique T1499.004 which covers "Endpoint Denial of Service" and specifically targets the kernel-level denial of service vector. The exploitability is enhanced by the fact that NFSv4 servers are commonly exposed in enterprise environments, making this attack surface particularly valuable to threat actors. The vulnerability demonstrates poor input validation and memory management practices that are typical of kernel-level buffer overflow conditions, though it manifests as a use-after-free scenario due to the uninitialized variable dereference.
Mitigation strategies for CVE-2017-8797 primarily involve immediate kernel updates to versions 4.11.3 or later where the vulnerability has been patched. System administrators should also implement network segmentation to limit exposure of NFS servers to untrusted networks, disable unnecessary NFS services, and consider implementing firewall rules that restrict UDP traffic to NFS ports. Additionally, monitoring systems should be deployed to detect unusual NFS traffic patterns that might indicate exploitation attempts. The patch addresses the uninitialized variable issue by ensuring proper initialization of the layout type value before it is used as an array index, preventing the out-of-bounds memory access that leads to system instability.