CVE-2008-7256 in Linux
Summary
by MITRE
mm/shmem.c in the Linux kernel before 2.6.28-rc8, when strict overcommit is enabled and CONFIG_SECURITY is disabled, does not properly handle the export of shmemfs objects by knfsd, which allows attackers to cause a denial of service (NULL pointer dereference and knfsd crash) or possibly have unspecified other impact via unknown vectors. NOTE: this vulnerability exists because of an incomplete fix for CVE-2010-1643.
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Analysis
by VulDB Data Team • 09/14/2021
The vulnerability described in CVE-2008-7256 represents a critical flaw in the Linux kernel's memory management subsystem, specifically within the shared memory filesystem implementation. This issue affects kernel versions prior to 2.6.28-rc8 and manifests when strict overcommit mode is enabled alongside a disabled security configuration. The vulnerability stems from improper handling of shmemfs object exports by the kernel's NFS server implementation known as knfsd, creating a pathway for malicious exploitation that can result in system instability and potential denial of service conditions.
The technical root cause of this vulnerability lies in the insufficient validation and handling of shared memory filesystem objects during the NFS export process. When strict overcommit is enabled, the kernel enforces stricter memory allocation policies that can interact poorly with the knfsd component when CONFIG_SECURITY is disabled. This configuration creates a scenario where shmemfs objects can be improperly exported without adequate null pointer checks, leading to a NULL pointer dereference condition. The vulnerability represents an incomplete remediation of a previously identified issue, specifically CVE-2010-1643, which demonstrates how partial fixes can leave systems exposed to similar attack vectors.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially enable more severe system compromise. A remote attacker can trigger the NULL pointer dereference by crafting specific NFS requests that exploit the improper shmemfs object handling, causing the knfsd process to crash and terminate. This crash can result in complete service disruption for NFS clients relying on the affected system, while also potentially exposing underlying system stability issues. The unspecified other impacts mentioned in the description suggest that this vulnerability might provide opportunities for privilege escalation or information disclosure, though these have not been fully characterized in the public record. The vulnerability's classification aligns with CWE-476 which addresses NULL pointer dereference conditions, and can be mapped to ATT&CK technique T1499.004 for denial of service attacks, while also potentially supporting techniques related to privilege escalation through kernel exploitation.
Mitigation strategies for this vulnerability require immediate kernel version updates to 2.6.28-rc8 or later, which contain the complete fix for this issue. System administrators should also consider disabling unnecessary NFS exports and implementing proper network segmentation to limit exposure. Additionally, monitoring for unusual NFS activity and system crashes can help detect exploitation attempts. The vulnerability highlights the importance of complete security fixes rather than partial solutions, as the incomplete remediation of CVE-2010-1643 left systems vulnerable to similar attack patterns. Organizations should also review their kernel configuration settings to ensure that security features are properly enabled and that unnecessary components are disabled to reduce the attack surface. This vulnerability serves as a reminder of the critical importance of thorough testing and validation of security patches before deployment in production environments.