CVE-2024-49971: Linux AMD DRM memory safety fix to prevent out-of-bounds write

A small but consequential flaw in the Linux AMD DRM display stack — tracked as CVE‑2024‑49971 — can cause an out‑of‑bounds write when the driver reads the third element of a two‑element array; the upstream remedy is a simple, surgical change to increase the dummy_boolean array size from 2 to 3, but the operational implications for shared and embedded systems demand immediate attention.

Background / Overview​

The vulnerability lives in the Linux kernel's AMD Direct Rendering Manager (DRM) display code path — specifically the drm/amd/display subsystem. The problematic code uses a dummy_boolean array that was declared with two elements while downstream code accesses a third element (hw_debug5 = &s->dummy_boolean[2]). That mismatch creates an out‑of‑bounds write whenever the third element is referenced and subsequently modified. The upstream fix is to increase dummy_boolean to three elements to match the code's expectations and remove the overrun. This is a classic memory‑safety issue in privileged kernel code. Unlike many userland bugs, an overrun in kernel drivers runs with kernel privileges and can result in immediate availability loss (kernel OOPS, driver crash, compositor failure or even host instability) — which is why distributions and vendors treated the issue with moderate to medium priority and shipped backported patches. Multiple distributor trackers, including Debian, Ubuntu, SUSE and Red Hat, have catalogued the CVE and integrated the upstream commit into their kernel trees.

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Technical anatomy — what exactly went wrong​

The bug in plain terms​

At the heart of CVE‑2024‑49971 is a mismatched array bound. The driver declared dummy_boolean with a size of 2, but later code paths in dml2_core_shared_mode_support and dml_core_mode_support take a pointer to the third element (dummy_boolean[2]) and assign it to hw_debug5. Any write to hw_debug5 therefore writes past the end of the allocated buffer, creating an out‑of‑bounds memory write. The change applied upstream simply makes the array one element larger to accommodate the access.

Why this matters in kernel code​

Kernel drivers have direct access to privileged memory. Even small overruns can lead to unpredictable results: corrupted kernel structures, driver state corruption, oopses, and in some layouts, a full kernel panic. For a graphics driver like AMD's amdgpu, the user‑visible consequences are immediate: compositor crashes, black screens, session terminations, or host instability requiring reboot. Because the driver executes in kernel context, operators must treat this class of bug as an availability risk even if confidentiality and integrity impacts are not directly apparent.

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Affected systems and exposure model​

• Component: Linux kernel — drm/amd/display (AMD DRM / amdgpu) code.
• Typical exposure: Desktop and workstation systems using AMD GPUs where amdgpu is loaded and DRM device nodes (/dev/dri/*) are accessible to unprivileged users.
• High‑priority exposures: Multi‑tenant hosts and CI runners that expose GPUs to untrusted tenants, VDI and shared workstation fleets, and embedded or OEM devices whose kernels lag upstream. These environments let an unprivileged local process more easily exercise DRM ioctls or compositor operations that reach the vulnerable code path.

Distributions that tracked and patched the CVE include Debian (fixed in multiple series), Ubuntu (published advisory with priority mapping), SUSE, Red Hat and vendor feeds such as Amazon’s ALAS. In practice the fix was merged upstream and cherry‑picked into stable kernel branches and vendor patches; operators should consult their distribution's security tracker for the exact package versions that contain the remediation.

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Severity, CVSS and practical impact​

Multiple trackers converge on a Medium severity classification. Canonical numeric mappings reported by distribution and vendor pages place the CVSS v3 base around 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H), reflecting a local attack vector, low complexity, and an availability‑only impact. SUSE’s advisory shows comparable scoring and flags the concern for availability. Oracle, Debian and Red Hat advisories echo the medium rating and recommend patching. Operationally, the key consequence is denial‑of‑service (DoS) of the graphics stack or entire host. An attacker with local access (or an untrusted tenant) that can reach DRM operations can provoke an overrun and trigger crashes repeatedly. Although DoS is the primary and immediate impact, kernel overruns are a dangerous primitive and should not be considered harmless — they may, in specific memory layouts and with additional bugs, be part of an escalation chain. No authoritative public exploit demonstrating reliable privilege escalation related to this CVE was documented at disclosure.

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How to detect whether you are exposed​

Quick triage checklist for system administrators:

• Check the running kernel version: uname -r. Compare it with your distribution’s advisory mapping to see whether the patch is present.
• Confirm amdgpu (or the relevant AMD DRM modules) is loaded: lsmod | grep amdgpu. If it is not loaded, kernel exposure is unlikely unless the module can be loaded later.
• List DRM device nodes and permissions: ls -l /dev/dri/* — if /dev/dri is readable/writable by unprivileged users or containers, exposure is higher.
• Search kernel logs for symptoms: dmesg or journalctl -k may show amdgpu oops, pageflip timeouts, or other display sequencing failure traces. Preserve logs immediately if an incident occurs.

Operational indicators that the vulnerable code was hit include compositor crashes, repeated driver resets, pageflip timed‑out messages, and kernel backtraces referencing drivers/gpu/drm/amd/display code paths. These are the practical fingerprints in environments that do not run sanitizer‑instrumented kernels.

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Remediation and mitigations​

The definitive remediation is to install a kernel package from your vendor/distribution that contains the upstream fix and then reboot into the patched kernel. Because the change is in kernel code, a reboot is mandatory to activate the fix. Vendors have backported the small network of upstream commits into their stable trees. Recommended remediation steps:

• Inventory and prioritize hosts by exposure: multi‑tenant GPU hosts, VDI servers, CI runners, and developer machines that run untrusted workloads should be patched first.
• Obtain and install vendor/distribution kernel updates that list CVE‑2024‑49971 or include the upstream commit ID. Confirm the package changelog or security advisory for the commit mapping before wide deployment.
• Reboot into the patched kernel. Validate: run representative display operations (hot‑plug, MST hubs, docking, fullscreen media) in a staging window (24–72 hours recommended for catching intermittent regressions).

Temporary compensations if immediate patching is impossible:

• Restrict access to DRM device nodes: change udev rules and group ownership so /dev/dri/* is accessible only by trusted groups. This reduces the ability for unprivileged processes to exercise DRM ioctls.
• Avoid exposing GPUs to untrusted containers or VMs via passthrough, --device=/dev/dri, or vfio unless absolutely required.
• Increase monitoring and SIEM alerts for kernel oops messages that reference amdgpu, pageflip timeouts, or repeated driver resets. Preserve logs for post‑mortem.

Because the upstream change is minimal, it is straightforward to cherry‑pick into custom or vendor kernels. That makes backporting practical for embedded vendors who maintain their own kernel trees; however, many embedded/OEM vendors historically lag upstream and create a long‑tail risk that must be managed by device owners.

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Verification steps and test guidance​

If you maintain custom kernels or build your own device images, verify the presence of the fix in the source tree:

• Search the AMD DRM display sources (e.g., under drivers/gpu/drm/amd/display) for dummy_boolean and confirm the array is sized to include at least three elements. The upstream change increases the array dimension to 3 so hw_debug5 has a valid backing slot.

Operational verification after installing vendor updates:

• Reboot into the new kernel and check uname -r and your kernel package changelog for the remedial commit.
• Run a representative set of display operations for 24–72 hours (hot‑plug cycles, MST hub tests, fullscreen media) and monitor dmesg / journalctl -k for any residual amdgpu oopses.

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Vendor mappings and patch status​

Distribution advisories list the CVE and map fixed package versions for their releases. Debian’s tracker shows fixed package versions across multiple releases and kernel series, Ubuntu documents the advisory and assigns a Medium priority, SUSE and Red Hat reflect remediation and include vendor errata references, and cloud vendors have flagged package status where relevant. Always consult your distro’s security tracker for the exact package name/version before declaring systems remediated.

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Critical analysis — strengths of the fix and residual risks​

Notable strengths​

• The upstream remedy is small and surgical: increasing the dummy_boolean array size is an obvious, low‑risk correction that aligns actual usage with the declared storage. Small patches minimize regression risk and ease distribution backports.
• Multiple independent vendors and trackers (Debian, Ubuntu, SUSE, Red Hat, Oracle) converged quickly on the issue and integrated the fix into stable kernels, which makes coordinated remediation straightforward for mainstream distributions.

Residual risks and caveats​

• Long‑tail exposure: embedded devices, vendor kernel forks, and appliances often lag upstream updates; these fleets can remain vulnerable long after mainstream distributions ship patches. Device owners should press vendors for timelines or build and test cherry‑picked backports.
• Detection gaps: production kernels usually lack sanitizers like UBSAN and therefore do not show neat runtime traces. Operators must rely on indirect symptoms (pageflip timeouts, driver resets, compositor crashes) which are noisier and can be misattributed. Robust log collection is essential.
• Contextual severity: numeric CVSS values varied slightly among trackers early in the lifecycle, a reminder that scoring differences reflect varied assumptions about privileges and environment. Prioritize remediation by exposure and service criticality rather than relying only on the CVSS score.

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Practical playbook — prioritized steps for operators​

• Inventory: Run uname -r, lsmod | grep amdgpu, and ls -l /dev/dri/* across your fleet to identify exposed hosts.
• Map: Check your distribution/vendor security tracker for CVE‑2024‑49971 package mappings. Confirm the remediation commit presence in changelogs before rollout.
• Patch: Apply vendor kernel updates that include the fix; schedule reboots into patched kernels.
• Mitigate: Until patching, reduce exposure by tightening /dev/dri permissions, removing device exposure from untrusted containers, and disabling passthrough where possible.
• Monitor and verify: After patching, run representative display tests and monitor dmesg/journalctl -k for at least 24–72 hours. Preserve logs for any anomalies.

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Where things stand on exploitation and public proof‑of‑concepts​

At disclosure and in subsequent vendor advisories, analysts treated CVE‑2024‑49971 as an availability/DoS issue. Public trackers indicate only a low to negligible EPSS probability for near‑term exploitation, and there was no authoritative public PoC demonstrating reliable remote or privilege escalation stemming from this precise array overrun at the time of vendor advisories. That said, kernel memory corruption remains a high‑value primitive for attackers in some contexts, so conservative handling is warranted.

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Final assessment and recommendations​

CVE‑2024‑49971 is a textbook example of how a small coding mismatch — an array declared with too few elements — can translate into meaningful operational risk when it occurs in kernel drivers. The good news: the upstream fix is tiny, easy to audit, and easy to backport, and major distributions have already integrated the change into stable kernel packages. The bad news: the practical exposure model includes many shared, multi‑tenant and embedded environments where unprivileged actors can trigger the problematic code path, and vendor/OEM kernels can remain vulnerable for months if not patched.
Actionable closing guidance for system owners:

• Treat the CVE as an operational availability risk for any host that loads amdgpu and exposes /dev/dri to untrusted code. Patch promptly.
• If you cannot patch immediately, restrict device access, avoid GPU passthrough, and increase log monitoring.
• For embedded and OEM devices, demand a vendor timeline for the patched image or plan to cherry‑pick and backport the upstream commit into your own kernel builds and run hardware smoke tests across your topology.

CVE‑2024‑49971 underlines a perennial lesson: kernel driver correctness matters. Even trivial fixes matter greatly when they close crash‑prone paths in the privileged display stack. Operators should verify patched package mappings, update kernels promptly, and limit local attack surface until their fleets have booted into remediated images.

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Conclusion: the code change that fixes CVE‑2024‑49971 is straightforward — increase dummy_boolean to three elements — but the operational response must be decisive: inventory exposed hosts, apply vendor kernel updates and reboot, and enforce compensating controls on any host that must remain unpatched. The risk is availability‑focused and actionable; addressing it quickly reduces the chance that a local DoS primitive becomes a tactical problem for shared infrastructure.

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Source: MSRC Security Update Guide - Microsoft Security Response Center