CVE-2026-89806 Overview
CVE-2026-89806 is an integer overflow vulnerability in the Linux kernel's drm/sysfb subsystem, specifically within the Open Firmware Direct Rendering Manager (ofdrm) driver. The flaw resides in the framebuffer size calculation where fb_size = linebytes * height can overflow when both operands are large integers. Because linebytes and height are both signed int values, the multiplication executes as int * int, producing undefined behavior on overflow. For example, 46341 * 46341 exceeds INT_MAX. The kernel maintainers resolved the issue by adopting check_mul_overflow(), aligning ofdrm with the safer patterns already used in simpledrm.c and corebootdrm.c.
Critical Impact
A local attacker able to influence framebuffer dimensions can trigger undefined behavior in the kernel, leading to memory corruption, denial of service, or potential privilege escalation.
Affected Products
- Linux kernel drm/sysfb subsystem — ofdrm (Open Firmware DRM) driver
- Linux kernel versions prior to the commits c6f48e59ece0, d9daf9a6e7a6, and ded6ad826fe0
- Distributions shipping vulnerable stable kernel branches without the backported fix
Discovery Timeline
- 2026-09-16 - CVE-2026-89806 published to NVD
- 2026-09-16 - Last updated in NVD database
Technical Details for CVE-2026-89806
Vulnerability Analysis
The ofdrm driver computes the framebuffer memory footprint by multiplying linebytes (bytes per scanline) with height (number of scanlines). Both values are declared as signed int. When the product exceeds INT_MAX (2,147,483,647), the multiplication wraps or produces undefined behavior under the C standard. The resulting fb_size value is smaller than the true buffer requirement, which causes subsequent allocation and mapping logic to operate on an undersized region while the driver treats it as fully sized.
This mismatch creates the conditions for out-of-bounds writes when the framebuffer is populated, since downstream code assumes the allocation matches the requested geometry. The upstream fix replaces the raw multiplication with check_mul_overflow(), which returns an error when the operation would overflow. This mirrors the hardened approach already present in simpledrm.c and corebootdrm.c.
Root Cause
The root cause is missing overflow validation on an arithmetic operation involving attacker-influenceable geometry values. The signed integer multiplication in the framebuffer sizing path lacked any guard against exceeding INT_MAX, violating safe integer arithmetic practices in kernel code [CWE-190].
Attack Vector
Exploitation requires local access to a system running a vulnerable kernel on hardware or firmware paths that expose ofdrm. An attacker with the ability to supply or influence framebuffer parameters through Open Firmware device tree properties can craft linebytes and height values whose product overflows a signed 32-bit integer. This produces an undersized allocation while later code writes data as if the full framebuffer had been reserved, corrupting adjacent kernel memory.
No verified proof-of-concept code is published for this issue. Refer to the upstream commits at kernel.org commit c6f48e59ece0, kernel.org commit d9daf9a6e7a6, and kernel.org commit ded6ad826fe0 for the exact code changes.
Detection Methods for CVE-2026-89806
Indicators of Compromise
- Unexpected kernel oops or panic messages referencing ofdrm, drm_sysfb, or framebuffer allocation paths
- Kernel logs showing memory corruption, slab corruption, or KASAN reports near DRM subsystem functions
- Anomalous local process activity attempting to load or interact with the ofdrm driver on PowerPC or Open Firmware-based systems
Detection Strategies
- Inventory running kernels and compare build versions against the fixed commits c6f48e59ece0, d9daf9a6e7a6, and ded6ad826fe0
- Enable KASAN and kernel lockdown telemetry on test systems to surface out-of-bounds writes triggered by crafted framebuffer geometries
- Correlate DRM subsystem error messages with local user session activity in centralized logging
Monitoring Recommendations
- Forward dmesg and journald kernel logs to a centralized SIEM for DRM and framebuffer error patterns
- Alert on repeated kernel crashes tied to ofdrm module load or unload events
- Track privileged local process behavior on systems exposing Open Firmware DRM paths
How to Mitigate CVE-2026-89806
Immediate Actions Required
- Apply the stable kernel updates containing commits c6f48e59ece0, d9daf9a6e7a6, and ded6ad826fe0
- Restrict local access to systems running affected kernels until patches are deployed
- Audit which endpoints load the ofdrm driver and prioritize patching those first
Patch Information
The fix replaces the unchecked multiplication with check_mul_overflow() in the ofdrm framebuffer size calculation. The change is available in the upstream stable tree via the commits referenced in kernel.org commit c6f48e59ece0, kernel.org commit d9daf9a6e7a6, and kernel.org commit ded6ad826fe0. Consume the fix through your distribution's kernel package once the backport is released.
Workarounds
- Blacklist the ofdrm module on systems that do not require Open Firmware framebuffer support using modprobe.d configuration
- Enforce least-privilege on local accounts and disable untrusted user shell access on affected hosts
- Where feasible, switch to a DRM driver path already using check_mul_overflow(), such as simpledrm
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.