CVE-2011-1013 in OpenBSD
Summary
by MITRE
Integer signedness error in the drm_modeset_ctl function in (1) drivers/gpu/drm/drm_irq.c in the Direct Rendering Manager (DRM) subsystem in the Linux kernel before 2.6.38 and (2) sys/dev/pci/drm/drm_irq.c in the kernel in OpenBSD before 4.9 allows local users to trigger out-of-bounds write operations, and consequently cause a denial of service (system crash) or possibly have unspecified other impact, via a crafted num_crtcs (aka vb_num) structure member in an ioctl argument.
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Analysis
by VulDB Data Team • 11/06/2021
The vulnerability described in CVE-2011-1013 represents a critical integer signedness error within the Direct Rendering Manager subsystem of the Linux kernel and OpenBSD operating systems. This flaw exists in the drm_modeset_ctl function located in the drm_irq.c source files of both kernel versions before 2.6.38 and OpenBSD versions before 4.9. The vulnerability stems from improper handling of signed and unsigned integer values during the processing of DRM ioctl commands, specifically when dealing with the num_crtcs member of the vb_num structure. The issue manifests when local users can craft malicious ioctl arguments that exploit this signedness mismatch to manipulate memory access patterns.
The technical implementation of this vulnerability exploits a fundamental flaw in integer type handling where a signed integer variable is incorrectly compared against or used in calculations with an unsigned value. When the num_crtcs parameter is processed, the kernel fails to properly validate the range of this value, allowing an attacker to specify a negative value that gets interpreted as a large positive unsigned integer. This misinterpretation leads to out-of-bounds memory write operations that can corrupt kernel memory structures, ultimately resulting in system crashes or potential privilege escalation. The vulnerability operates at the kernel level within the graphics subsystem, making it particularly dangerous as it can be triggered by any local user with access to the DRM device interfaces.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially enable more severe security consequences. System crashes occur when the out-of-bounds writes corrupt critical kernel data structures or memory management components, leading to immediate system instability and reboot cycles. The unspecified other impacts mentioned in the CVE description suggest potential for privilege escalation or information disclosure, though the primary vector remains system crash. Attackers can leverage this vulnerability to repeatedly crash systems, creating persistent denial of service conditions that severely impact system availability and reliability. The vulnerability affects graphics-intensive systems and servers where the DRM subsystem is actively utilized, making it particularly concerning for enterprise environments.
Mitigation strategies for this vulnerability require immediate kernel updates to versions 2.6.38 or later for Linux systems and OpenBSD 4.9 or later for OpenBSD systems. System administrators should prioritize patching affected systems and monitor for any signs of exploitation attempts. Additionally, implementing proper input validation and bounds checking in kernel modules can prevent similar issues from occurring in the future. The vulnerability aligns with CWE-190, which describes integer overflow and signedness errors, and represents a classic example of how improper type handling can lead to memory corruption. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques through kernel exploitation and can be classified under the T1068, which covers exploitation of remote services, though in this case the exploitation occurs locally. Organizations should also consider implementing monitoring for unusual DRM ioctl activity and system crash patterns that might indicate exploitation attempts.