CVE-2022-49635: Linux i915 Selftests Subtraction Overflow Fix

The Linux kernel fix for a subtraction overflow in the Intel i915 DRM selftests—tracked as CVE-2022-49635—addresses a small but real arithmetic bug that could trigger kernel instability and denial-of-service in specific test or runtime scenarios; distribution advisories classify it as a medium-severity, availability-focused vulnerability and recommend applying updated kernel packages that include the upstream commit.

Background​

The vulnerability arises in the i915 graphics subsystem’s selftests code, where an arithmetic subtraction used to compute test loop bounds can underflow on some hardware or in particular mock/test setups. The upstream change replaces a fragile loop condition with an explicit check that prevents subtraction from wrapping when values like hole_end are small or when expressions such as (addr + 2 min_alignment)* might overflow under mocked test conditions. The fix was introduced as a small, surgical commit and cherry-picked into stable trees.
This is not a user-space parsing bug: the defect is internal to a kernel driver’s selftest code path. The practical symptom is availability impact—kernel oops or crashes—rather than an obvious confidentiality or integrity escalation. Multiple vendors (Ubuntu, SUSE, Amazon Linux advisories and other trackers) list the issue with a medium severity and identify the primary impact as availability (host or guest crash).

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What exactly is CVE-2022-49635?​

The technical summary​

• A subtraction overflow (wraparound) occurred in code under drivers/gpu/drm/i915/selftests/, notably in the function that iterates ‘holes’ or gaps while exercising the GEM/GTT machinery for Intel graphics in test harnesses.
• Two separate arithmetic risks were identified:
• hole_end could be small enough that subtracting other values produced an underflow.
• Expressions like (addr + 2 min_alignment)* used in mock tests could overflow an address calculation.
• The upstream remedy changes the loop condition to guard explicitly against both cases (checking that hole_end > addr and hole_end - addr >= 2 * min_alignment), avoiding arithmetic wrap and the subsequent out-of-range iteration.

Where the bug lived​

This was in the DRM/i915 selftests component—code executed when running kernel-integrated driver self-tests. Selftests are typically not enabled in production kernels by default, but the same code paths or similar logic can be exercised in some environments or during device teardown and driver testing. The patch was recorded in upstream kernel commits and then propagated to distribution kernels.

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Scope and affected systems​

Who is potentially affected​

• Systems that run kernels containing the vulnerable commit range in upstream/stable trees prior to the cherry-picked fix may be affected.
• In practice, this maps to systems running affected Linux kernel versions used by distributions that had not yet incorporated the patch into their packaged kernels at the time of advisory publication. Major distributions published CVE entries and package statuses (some marked fixed, some pending backport).

Likely real-world exposure​

• The most realistic exposure is limited: the code is in the selftests area and is most likely to be exercised in test or developer environments, QA rigs, or when mock/test harnesses run on otherwise unusual kernels.
• Embedded devices or vendor-kernel forks that include selftest code or altered driver configurations could expose the vulnerability in production hardware more easily than standard, distribution-packaged kernels.
• Trackers show low EPSS and limited public exploit activity to date, indicating a low probability of widespread exploitation; nevertheless, because kernel faults can crash a host, administrators should triage systems that run vulnerable kernels, especially where uptime and multi-tenant stability are critical.

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Impact: Why availability matters here​

Kernel-space arithmetic bugs are dangerous not because they always lead to code execution, but because a miscomputed address or loop bound in kernel space frequently leads to invalid memory access, oops, and host- or driver-level crashes. For graphics subsystems this can manifest as:

• A kernel oops that takes down the graphics driver or entire host.
• Loss of display or audio until a reboot or driver reload.
• For multi-tenant or shared systems, a single crash may impact many users or guests.

Security advisories consistently treat this class of bug as an availability (DoS) issue rather than a direct privilege-escalation vector; however, availability incidents at the kernel level are operationally severe for production or virtualized environments.
File-level analysis from kernel operators emphasizes that even narrowly scoped defensive fixes—like swapping an ordering of operations or tightening loop conditions—can eliminate an immediate source of host instability without functional regressions, which is why maintainers applied a surgical patch rather than redesigning the driver behavior.

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The patch and developer notes​

• The upstream change was small and targeted. The essence of the fix is to refuse to run the loop iteration when the arithmetic would underflow or when the address arithmetic could wrap, thereby preventing a negative or wrapped size from being used as a loop bound.
• The commit included in stable kernels was cherry-picked and referenced in multiple distribution advisories and kernel stable branches.
• Because the patch avoids behavioral changes to correct code paths, it is considered easy to backport and low-risk in terms of introducing regressions.

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Verification and cross-checks​

The following independent vendor and vulnerability sources corroborate the technical description and remediation path for CVE-2022-49635:

• Ubuntu’s CVE entry explicitly states the fix text—“drm/i915/selftests: fix subtraction overflow bug”—and tracks package status and priority.
• NVD (NIST) includes the CVE record and references the upstream kernel commit identifiers for verification.
• SUSE’s advisory lists the issue as resolved and gives the CVSS breakdown and availability impact classification.
• Amazon Linux (ALAS) and other trackers show CVSS scoring and package statuses across distributions, indicating where fixes are pending or not planned.
• Independent vulnerability databases (Rapid7, CVEdetails, Wiz) summarize the same root cause and link back to kernel.org commits for the exact code change.

Where vendor trackers differ is mostly in the precise CVSS numeric value or in whether a particular distribution has a backport available; those are packaging and policy differences rather than contradictions about the bug’s technical root cause.

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Exploitability, proof-of-concept, and risk assessment​

• Public exploit code for CVE-2022-49635 has not been broadly reported; EPSS and public tracker indicators suggest a low likelihood of active exploitation. The vulnerability’s attack vector is local and its primary consequence is availability.
• Turning this arithmetic bug into a reliable privilege escalation would likely require additional memory-corruption primitives beyond the selftests subtraction error; public advisories do not show evidence of a direct RCE chain stemming from this specific issue. Treat claims of RCE/privilege escalation via this CVE as unverified unless a credible, technical write-up or PoC appears.
• For defenders the practical triage priority should be: (1) confirm which hosts run affected kernels; (2) prioritize patches on systems where availability is critical (multi-tenant hosts, VDI, production appliances); (3) treat embedded or vendor-kernel devices as high-risk if they expose i915 paths or selftest-like code.

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Detection and validation guidance​

Administrators should adopt non-invasive checks and telemetry-driven detection rather than attempting to reproduce faults with exploit code.

• Inventory:
• Run uname -r to identify kernel version strings and cross-check them against distribution security advisories for CVE-2022-49635.
• Use package management tools (apt, dnf, zypper) to query whether the installed kernel package changelog mentions the CVE or the upstream commit ID.
• Log and crash monitoring:
• Watch kernel logs (dmesg and journalctl) for DRM or i915-related oops messages, stack traces mentioning i915 selftest components, or repeated driver reloads.
• Aggregate and alert on new or surge patterns of kernel oops/panics referencing GPU/DRM symbols.
• Post-patch verification:
• After installing the patched kernel package, confirm the kernel package changelog contains the CVE or commit and validate that the host no longer exhibits the earlier test-induced crash patterns in lab runs.

Do not attempt to run or share PoC exploit code. If a vendor-supplied detection script exists, prefer that over ad-hoc tests.

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Remediation and mitigation​

The definitive remediation is to install a kernel package that contains the upstream fix and reboot the host. Practical steps:

• Map CVE → package → KB:
• For distribution-managed systems (Ubuntu, Debian, SUSE, Amazon Linux), use the distribution security tracker to find the fixed kernel package version for your release and apply it through standard patching channels.
• Patch workflow:
• Identify high-priority hosts (multi-tenant, production appliances, VDI hosts).
• Stage the kernel upgrade in a pilot ring and validate critical workloads.
• Roll out staged updates and collect post-patch telemetry.
• Reboot hosts to activate the updated kernel.
• Compensating controls if immediate patching is impossible:
• Restrict local unprivileged access and reduce the attack surface by limiting who can run tests or load device drivers.
• Isolate management and test networks from production user networks.
• For embedded devices or vendor-kernel images, contact the vendor and escalate for a backport or replacement image.

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Operational considerations and caveats​

• Backport timelines vary: some distributions will mark packages “pending fix” while others will include the commit promptly; embedded vendors often lag and may require direct vendor engagement or manual kernel rebuilds. Verify package changelogs and upstream commit IDs before declaring systems patched.
• This type of fix is low-risk and surgical, but kernel updates can carry their own regressions. Use a staged rollout and maintain rollback plans (snapshots, known-good image restore points).
• The CVE primarily addresses availability and robustness; do not conflate it with a generic privilege-escalation or remote-execution class—those would need additional supporting evidence. Flag any claims of RCE via this CVE as unverified until proven.

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Developer takeaways: defensive coding lessons​

• Kernel selftest code is code too: arithmetic overflows and boundary checks must be treated defensively even in test harnesses because they can be exercised under unusual conditions in the field.
• Small, clearly scoped defensive fixes are often the safest path: swap order, add explicit bounds checks, and avoid implicit arithmetic assumptions that depend on upstream invariants.
• Maintainers’ approach—cherry-pick a concise commit into stable trees rather than large behavioral rewrites—minimizes regression risk while closing the immediate correctness window.

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Quick operational checklist​

• Inventory hosts that run Intel i915 driver stacks or custom kernels: uname -r; check package changelogs for CVE-2022-49635.
• Patch and reboot hosts where the kernel package contains the upstream fix.
• Prioritize multi-tenant and production display/audio appliances for immediate remediation.
• For embedded/vendor-kernel devices, file support tickets and track vendor advisories.
• Monitor kernel logs and set alerts for DRM/i915 oops patterns, then validate fixes in a pilot ring prior to broad deployment.

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Final analysis — strengths, risks, and closing assessment​

CVE-2022-49635 is a textbook example of a micro-fix that materially improves kernel robustness with minimal functional change. The technical fix is surgical, the risk profile—while real—is constrained, and distribution advisories reflect a medium-priority remediation largely centered on availability impact. Multiple independent trackers, distribution advisories, and the upstream kernel commits corroborate the cause and the remedy, so operators can confidently rely on vendor-supplied kernel packages to remediate the issue.
Potential residual risks to watch for:

• Unpatched vendor or embedded kernels where backporting is slow or absent represent the primary operational blind spot.
• Kernel updates, while necessary, may introduce regressions; use staged rollouts and maintain monitoring.
• Claims that this CVE enables privilege escalation or RCE are not supported by the public advisories and should be treated as unverified until a credible technical exploit or write-up appears.

In sum: apply the distribution-supplied kernel updates, prioritize hosts with high availability impact or multi-tenancy, and validate fixes with targeted, non-destructive tests and log monitoring. The underlying code change is straightforward, the operational remedy is standard (patch + reboot), and the benefit—removing an arithmetic underflow that can cause kernel crashes—is immediate and worthwhile.

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Source: MSRC Security Update Guide - Microsoft Security Response Center