Linux Kernel DRM XE Patch Fixes Arithmetic Overflow in Page Size Calculation

The Linux kernel patch credited to CVE‑2024‑42066 fixes a subtle but important arithmetic bug in the DRM XE driver: the code now explicitly casts tbo->page_alignment to a 64‑bit unsigned type (u64) before performing a bit shift used to compute min_page_size, removing a possible integer overflow that could otherwise yield a wrong page size and trigger kernel faults. This vulnerability is classified as a local, availability‑focused defect with a CVSSv3 base score commonly reported as 5.5 (Medium), and the corrected code has been merged into stable kernel releases and packaged by major distributions.

Background / overview​

The vulnerability stems from mixed‑width bit‑shift arithmetic inside the DRM XE code path where a page size (min_page_size) is computed from a tbo->page_alignment value. If that value is stored in a type narrower than the type used by the target variable, left‑shifting the narrow value can overflow and wrap before being assigned — producing an unexpectedly small or otherwise invalid page size. The upstream remediation is short and surgical: cast tbo->page_alignment to a 64‑bit unsigned integer (u64) so the subsequent left shift happens on a wide enough type and cannot overflow on typical builds. This change eliminates the arithmetic wrap and ensures the computed min_page_size is correct for large alignment values. This is a familiar class of kernel correctness bugs: mixed‑width arithmetic and left shifts are a recurring source of undefined or implementation‑dependent behavior. Kernel maintainers tend to prefer small, localized fixes — explicit casts or safe arithmetic helpers — to eliminate the corner case without changing intended semantics for valid inputs. Similar corrective patterns have been applied across several recent driver fixes where inserting a cast or a bounds check prevents truncation or arithmetic wrap.

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Why this matters: real-world impact and threat model​

• Primary impact: Availability (Denial of Service). Miscomputed page sizes or index offsets inside kernel graphics paths can lead to invalid memory accesses, kernel oopses, or driver crashes that destabilize the display stack or the entire host, especially when the kernel dereferences the resulting offset or relies on the computed size for memory mapping.
• Attack vector: Local. Triggering the vulnerable path requires local access to invoke the DRM XE driver code — typically by a process that can interact with kernel DRM device nodes (/dev/dri/*) or that exercises relevant device ioctls. In many desktop/embedded setups, unprivileged processes or sandboxed compositor helpers can reach these paths.
• Preconditions: the bug is not a trivial remote network attack. Instead it is a correctness/overflow condition that becomes meaningful when attacker‑controlled inputs or unusual hardware topologies produce large alignment values or when drivers process large buffers.
• Exploitability: the published disclosures and vendor advisories treat this as an availability issue rather than an immediate path to remote code execution. That said, kernel oops and reliably reproducible crash primitives are valuable to attackers building multi‑stage local exploit chains in some environments; absence of a public proof‑of‑concept (PoC) should be treated as provisional, not proof of no risk.

Multiple vulnerability trackers and distributors recorded the CVE and mapped it to fixed kernel versions; Debian/OSV entries show the fix was included in the stable kernel packages (fixed in linux 6.9.8 in Debian’s mapping), and Ubuntu’s security tracker classed the CVE as Medium (5.5) and published package advisories. Administrators should treat multi‑tenant hosts, CI runners, desktops with exposed DRM nodes, and embedded vendor kernels as higher priority for remediation.

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Technical analysis — what went wrong, precisely​

The arithmetic pitfall​

The core mistake is classic mixed‑width bit shifting:

• Code reads an alignment field (tbo->page_alignment) whose width, in some build configurations, can be narrower than the type used for the result (min_page_size).
• The code performs a left shift on that narrow type (e.g., shift by PAGE_SHIFT or similar) which is evaluated in the source type, allowing overflow/wrap at the narrower width.
• The wrapped result is then used (assigned to min_page_size or used to compute offsets), producing a wrong page size or index that later leads to out‑of‑range pointer arithmetic or mapping errors.

Left shifts are not safe when the operand can be large relative to its type width. The safe remedy is to perform the shift in a sufficiently wide type or validate the operand first.

The upstream fix​

The upstream change inserted an explicit cast: convert tbo->page_alignment to a u64 before performing the shift. This guarantees the shift executes on a 64‑bit unsigned integer, preventing wraparound on platforms where u64 is wide enough for expected alignments. The fix is intentionally small and preserves semantics for valid values while removing the arithmetic corner case that caused undefined or incorrect results.
This fix pattern — a narrow, deliberate type promotion — is a common kernel hardening technique for mixed‑width arithmetic. Comparable fixes in other DRM and driver subsystems use casts or safe arithmetic helpers to avoid left‑shift overflows and to satisfy sanitizers like UBSAN.

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Affected versions and vendor mapping​

Multiple sources agree on the broad scope and the corrected kernel versions:

• NVD records the CVE and summarizes the fix as "Explicitly cast tbo->page_alignment to u64 before bit‑shifting to prevent overflow when assigning to min_page_size." The NVD entry is a canonical aggregator for the CVE metadata.
• Debian/OSV show the vulnerability metadata and map the fix to kernel releases; in the Debian OSV entry the fix appears in linux 6.9.8‑1 for affected releases. Administrators using Debian‑based distributions should consult their distribution tracker to identify the exact fixed package for their release.
• Ubuntu’s security tracker published an advisory for CVE‑2024‑42066 with a CVSS 3.x score of 5.5, and lists the status for various HWE and release kernels; Ubuntu advisories provide packaged kernel updates and kernel image upgrade instructions.
• Amazon Linux, INCIBE and other national or vendor trackers list the CVE, severity, and relevant mitigations and point to upstream stable commits. These vendor entries align on the Medium classification and emphasize availability impact.

Important operational note: upstream commits are small and easy to backport, so mainstream distributions generally propagated the fix to their stable trees; however, vendor forks (Android OEM kernels, embedded vendors, appliance vendors) can lag, so inventorying custom or vendor kernels is essential.

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How to detect whether you are exposed​

Short checklist to triage exposure:

1. Identify kernels and drivers:
   • Check the running kernel version: uname -r.
   • Inspect loaded modules for DRM XE or related drivers: lsmod | grep -i drm; lsmod | grep -i xe (or inspect /sys/module for drm/xe components).
2. Map kernels to vendor advisories:
   • Consult your distribution’s security tracker (Ubuntu USNs, Debian tracker, downstream vendor advisories) and confirm whether the running kernel package includes the upstream fix or the stable‑series commit referenced in public advisories.
3. Review logs for symptoms:
   • Kernel oopses, dmesg entries, or repeated driver crashes tied to DRM routines or page‑size calculations signal that the miscomputed value may have been used and caused a fault.
4. Check for sanitizer outputs:
   • If you run kernels with UBSAN or other sanitizers in staging/test environments, sanitizer traces referencing shift/overflow in DRM XE math are direct indicators (developers reported similar traces in other DRM math fixes).

If you operate a fleet: prioritize systems that expose DRM device nodes to untrusted users or containers, multi‑tenant hosts, CI runners with GPU access, and vendor/embedded images that may not receive frequent backports.

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Recommended remediation and mitigations​

The definitive remediation is to install a kernel update that contains the upstream fix and reboot. The fix is in kernel code; it only takes effect after booting into the patched kernel.
Immediate remediation steps (ordered):

1. Inventory and prioritize:
   1. Run uname -r on all systems and list those running kernels prior to the fixed release line (distributions map varies — check Debian/Ubuntu trackers). 2. Identify hosts that load DRM XE or expose /dev/dri to untrusted processes.
2. Patch:
   1. Install vendor/distribution kernel updates that mention CVE‑2024‑42066 or that have the upstream stable commit.
   2. For custom kernels, cherry‑pick the upstream commit that performs the cast to u64 and rebuild the kernel. (Search your kernel tree for the small change to the page‑size assignment.
3. Reboot into the patched kernel to activate the fix.
4. Validate:
   1. After reboot, review dmesg/journal for absence of prior oops traces and run representative display workloads to ensure stability.
   2. If you maintain a test cluster, reproduce the workload that previously caused faults and confirm no crash occurs.

Compensating controls if patching must be delayed:

• Restrict access to DRM device nodes:
  • Use udev rules and group permissions to limit /dev/dri access to trusted users.
  • Remove /dev/dri device exposure from untrusted containers (no --device=/dev/dri unless necessary).
• Isolate high‑risk hosts:
  • Move multi‑tenant workloads off potentially vulnerable hosts to patched hosts where possible.
• Increase logging and alerts:
  • Add SIEM rules for kernel oops/panic traces that reference DRM, XE, or page‑size math, and set rapid alerts for any such signals.

A short, prioritized quick playbook for ops teams:

1. Patch high‑exposure hosts (multi‑tenant, VDI, CI runners) first.
2. Patch developer and test staging hosts that run GPU profiling tools.
3. Coordinate vendor OEM/embedded patching with vendors for long‑tail device fleets.

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Verification: what to look for in the patch and kernel tree​

If you build or review kernels yourself, verify the presence of the cast in the DRM XE source around the page size calculation:

• Search for code that assigns min_page_size and ensure any expression that shifts tbo->page_alignment is performed after casting that value to a 64‑bit unsigned type (u64) — for example (u64)tbo->page_alignment << some_shift.
• Confirm the commit message or changelog entry references the explicit cast or "Fix potential integer overflow in page size calculation".

Vendor advisories and stable‑tree commit messages typically contain the short fix description; use those to validate package provenance. Debian/OSV and Ubuntu advisory pages list the fixed package versions and provide checkpoints for validation.

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Cross‑checks and corroboration​

This account reconciles multiple independent sources:

• NVD’s CVE entry documenting the technical fix description and classification.
• Ubuntu and Debian/OSV advisories listing the CVE, the Medium severity (5.5), and mapping to fixed kernel packages. These distribution trackers give operators the actionable package names and release status they need to patch.
• National‑CERT and other vendor trackers (INCIBE, Amazon ALAS) echo the technical description and severity and point to the upstream commits for reference. These corroborating observations confirm the policy and remediation posture across different ecosystems.
• Kernel engineering practice: the chosen fix (explicit cast to a wider type) is consistent with prior kernel hardening patterns for mixed‑width arithmetic and shift operations; similar small defensive changes have been applied across other DRM and driver fixes. For context on that pattern and why a type promotion is chosen, see previous kernel writeups that describe the same corrective approach for left‑shift overflow conditions.

Where claims cannot be independently verified:

• There is no authoritative public evidence at the time of publishing that CVE‑2024‑42066 was weaponized in the wild; absence of a PoC or telemetry indication should not be taken as definitive proof that exploitation never occurred. Treat the lack of public exploitation reports as provisional and continue to consult vendor/security telemetry for indicators.

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Practical notes for specific audiences​

• Desktop users: If your distribution supplied kernel updates for your release (Ubuntu/Debian/RHEL family), apply the kernel image updates and reboot. On single‑user machines the risk is lower but still present if you run compositors or GPU‑accelerated apps that touch DRM device nodes.
• Cloud operators & multi‑tenant providers: Treat this as a priority for hosts that expose /dev/dri to tenants or allow unprivileged access to DRM ioctls. Migrate workloads off vulnerable hosts and push patched kernel images. Vendor cloud image providers often publish advisories mapping CVEs to specific image builds — rely on those mappings for rapid deployment.
• Embedded vendors & OEMs: Vendor kernels are the long‑tail exposure; consult upstream commits and backport fixes into your device trees. Devices that ship with older or heavily‑modified kernel trees should be inventoried and scheduled for firmware/kernel updates.

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Conclusion​

CVE‑2024‑42066 is a classic kernel arithmetic correctness defect: a mixed‑width left shift on a narrower field could overflow and produce an incorrect page size used inside the DRM XE driver. The fix is straightforward and low‑risk — cast tbo->page_alignment to u64 before shifting — and has been incorporated into upstream stable kernel releases and distributed through vendor security updates. The operational impact is primarily availability, and the recommended response is clear: install vendor/distribution kernel updates, reboot into the patched kernel, and apply compensating access restrictions for /dev/dri where immediate patching is impractical. Cross‑checks with distribution trackers (Ubuntu, Debian/OSV), NVD, and national vendor advisories confirm the technical specifics, affected package ranges, and the Medium severity rating operators should use to prioritize remediation.

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Key references used to verify facts and mappings (for operational lookup): NVD CVE‑2024‑42066, Ubuntu security advisory for CVE‑2024‑42066, Debian/OSV tracker entries, and vendor advisories that map the upstream commit and fixed kernel packages.

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Source: MSRC Security Update Guide - Microsoft Security Response Center