From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98521][MODERATE REGULAR] drm/i915: fix incorrect RCU teardown order Date: Sat, 24 Oct 2026 12:31:44 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98521 X-AL-KERNEL-Priority: MODERATE REGULAR X-AL-KERNEL-Severity: MODERATE REGULAR X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: 4d4ca84213327da1f84ef7af3946dea6b14c5d88 List-Id: CVE: CVE-2026-98521 Priority: MODERATE REGULAR AL-KERNEL base severity: MODERATE KPANIC flag: NO Patch: drm/i915: fix incorrect RCU teardown order Commit: 4d4ca84213327da1f84ef7af3946dea6b14c5d88 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4d4ca84213327da1f84ef7af3946dea6b14c5d88 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98521 Analysis date: Sat, 24 Oct 2026 12:31:44 -0400 ActionableScore: 3 ActionableScore lower bound: unknown Actionable bucket: Borderline / manual review recommended Manual review required: YES Summary: A local i915 GEM ioctl race can drop reservation-fence references before the RCU grace period and trap fence iteration in an endless restart loop. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98521 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98521 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: MODERATE REGULAR Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98521 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98521 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=1 LEGACY=MODERATE RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;*CWE-664;*CWE-835;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race in drm i915 allows i915_gem_busy_ioctl to iterate over reservation fences while the same GEM object is being destroyed. dma_resv_fini could drop the fence references before the RCU grace period ended, causing dma_resv_for_each_fence_unlocked to observe a destroyed fence and restart forever because no replacement fence list will be installed. For the CVSS the PR:L value is selected because reliable triggering requires a local process with access to the i915 DRM device and its GEM ioctls. AC:H reflects the required race between fence iteration and object teardown. The demonstrated primitive is an endless kernel loop with availability impact. There is no concrete evidence of a UAF dereference, OOB access, information disclosure, or writable memory corruption primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE OOB UAF HARDWARE MEMORY DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 3. * Conservative score: 3; no floor or cap applied. * Raw paranoid score: 3. * Paranoid score: 3; no additional exploit primitive is technically supported. * Final recommended bucket: **Borderline / manual review recommended**:: 2. Signal breakdown * +1 Concrete RCU referenced-object lifetime mismatch: `dma_resv_fini()` dropped fence references while the GEM object remained visible to RCU readers; moving it to the post-grace-period free callback restores the required lifetime ordering. * +1 Reliable strong DoS: once encountered, the iterator continually restarts because the destroyed fence list is never replaced, producing an endless kernel loop. * +1 Local below-host-admin trigger: supplied evidence identifies a local process with i915 GEM ioctl access. * No race penalty: concurrency is required, but the evidence does not establish a materially narrow, one-shot, or uncontrollable race beyond the CVSS AC:H label. * No UAF score: the shown iterator detects the destroyed fence and retries; no unsafe post-free dereference is demonstrated. * No OOB write, controlled corruption, disclosure, authorization bypass, or C/I impact is shown. * NEW52/B5-R and NEW53/B3-PENDING do not apply. 3. Reachability analysis A local process with permission to access the i915 DRM device and relevant GEM ioctls can race busy-fence iteration against object teardown. This supports below-host-admin reachability and does not require UID 0. The evidence does not require DRM master, compositor, display-server, or host-administrator authority. Namespace capability scope is not established. Intel GPU hardware is required, but no rare-hardware penalty is justified. 4. Primitive and severity reading The primitive is an RCU teardown-order violation causing repeated iterator restart and kernel livelock. It is not a demonstrated UAF, memory overwrite, or consumed corrupted replacement object. The shown impact is availability loss; stronger memory-corruption or privilege effects are unsupported. Therefore this is CLASS_C, not an Important candidate, and automatic Moderate downgrade remains permissible after review. 5. One-sentence report phrase A local i915 GEM ioctl race can drop reservation-fence references before the RCU grace period and trap fence iteration in an endless restart loop. 6. Manual review advice MANUAL CHECK RECOMMENDED Confirm device-node accessibility and whether the endless loop stalls one task, GPU activity, or the wider host. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: drm/i915: fix incorrect RCU teardown order Commit: 4d4ca84213327da1f84ef7af3946dea6b14c5d88 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4d4ca84213327da1f84ef7af3946dea6b14c5d88 Commit description: i915_gem_busy_ioctl uses dma_resv_for_each_fence_unlocked() to iterate over the fences in an GEM object without holding a reference but only the RCU read side lock. What can happen here is that the GEM object is destroyed concurrently while i915_gem_busy_ioctl is still running. This won't free the GEM objects memory, but still drops all the dma_fence references. Now when dma_resv_for_each_fence_unlocked() sees a destroyed dma_fence it assumes that a new fence list was installed and re-starts the loop. But in the case of a destroyed GEM object a new fence list is never installed, only the old one freed and therefore the iteration never finishes resulting in an endless loop. The solution is to drop the fence references only after the RCU grace period. The fixes tag is not necessary the patch introducing the problem, but the one making it so worse that we need to address it. This problem was pointed out by Sashiko-bot. Signed-off-by: Christian König Fixes: 912ff2e ("drm/i915: use the new iterator in i915_gem_busy_ioctl v2") CC: stable@vger.kernel.org Reviewed-by: Tvrtko Ursulin Signed-off-by: Tvrtko Ursulin Link: https://lore.kernel.org/r/20260903113621.54660-1-christian.koenig@amd.com (cherry picked from commit 5113479 ) Signed-off-by: Jani Nikula Signed-off-by: Greg Kroah-Hartman Changed files: drivers/gpu/drm/i915/gem/i915_gem_object.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4d4ca84213327da1f84ef7af3946dea6b14c5d88 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98521 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98521 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 3 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: MODERATE KPANIC detected for this report: NO Published priority for this report (same as in Subject): MODERATE REGULAR These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).