From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-98748][MODERATE 7.0] serial: 8250_bcm7271: fix use-after-free in brcmuart_remove() Date: Mon, 19 Oct 2026 06:05:21 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-98748 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e List-Id: CVE: CVE-2026-98748 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: serial: 8250_bcm7271: fix use-after-free in brcmuart_remove() Commit: f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98748 Analysis date: Mon, 19 Oct 2026 06:05:21 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A teardown race can rearm a Broadcom UART timer before IRQ shutdown and cause its callback to dereference freed driver state, producing a host-admin-triggered kernel crash without a demonstrated privilege-escalation primitive. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98748 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98748 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 7.0 Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-98748 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98748 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A race in brcmuart_remove can rearm the UART high resolution timer after it has been cancelled but before the serial port and its IRQ are unregistered. The timer callback can then run after devm has freed brcmuart_priv and dereference freed kernel memory, resulting in a use after free. For the CVSS the PR:H is selected because reliable triggering normally requires administrative control over device or driver removal while a receive timeout interrupt reaches the UART. AC:H applies because the interrupt and removal paths must overlap within a limited race window. The issue is not directly network reachable and normally requires a system with the affected Broadcom STB UART driver and hardware. Impact is at least a local denial of service through a kernel crash. Because the confirmed primitive is a kernel use after free, reuse of the freed allocation could theoretically affect confidentiality or integrity, although reliable privilege escalation is not demonstrated and requires manual review. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 6.4 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation 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 LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.4 and ActionableScore 0 with primitive class CLASS_B and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) MAYBE PRINTF-TTY UAF TIMER HARDWARE DEBUGFS LINUS LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_B RealUAF=YES HostAdminRequired=YES NamespaceScopedPrivilege=NO ImportantCandidate=NO ManualReview=RECOMMENDED AutoDowngradeAllowed=YES RequiresHostUID0=UNKNOWN AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO ## 1. ActionableScore * Raw conservative score: 0 * Conservative score: 0 * Raw paranoid score: 0 * Paranoid score: 0 * No floor or cap applied. * Final recommended bucket: **Ordinary Moderate**:: ## 2. Signal breakdown Positive signals: * +2 Real UAF. `brcmuart_handle_irq()` can rearm `priv->hrt` after cancellation, and `brcmuart_hrtimer_func()` later dereferences `priv` after devm has freed it. * +1 Reliable kernel crash or strong DoS. A timer callback dereferencing reclaimed driver-private memory can realistically cause an Oops or kernel panic. Negative signals: * -2 Host-admin-equivalent privilege. An attacker-invocable complete trigger requires removal or unbinding of the Broadcom platform driver or device, which is a host device-management operation. * -1 Hard one-shot teardown race. A receive-timeout interrupt must occur after the initial `hrtimer_cancel()` but before `serial8250_unregister_port()` disables the port and frees its IRQ. Arithmetic: `+2 +1 -2 -1 = 0`. Important non-awarded signals: * Controlled reclaim or attacker-shaped replacement contents are not demonstrated. * No arbitrary or directed kernel write is established. * No callback, ops pointer, credential, refcount, or ownership field is consumed from attacker-controlled reclaimed memory. * No concrete repeated release or refcount mutation exists. * Structured stale-object leverage is not awarded because the established stale operation is a timer callback dereference during device teardown. * Weak or strong LPE leverage is not awarded merely from the UAF class. * No additional availability point is awarded for the same possible kernel crash. * Confidentiality and integrity points are not awarded based only on theoretical allocator reuse. * Remote or ordinary network reachability is not established. * No rare-hardware penalty is applied because the patch establishes hardware specificity but does not provide enough deployment evidence to classify the hardware as rare under the scoring rule. * NEW52 B5-R, NEW53 B3-PENDING, V32, V33, and V34 do not apply. There is no accumulated OOB write, shared-backing write, or checksum-metadata corruption. * NEW54 B4-PENDING does not apply because no authorization decision or mandatory security guard is involved. ## 3. Reachability analysis The vulnerable sequence occurs in `brcmuart_remove()`. Reliable attacker initiation therefore requires control over driver unbinding, module or device removal, or an equivalent host teardown operation. These are host device-management operations rather than namespace-scoped interfaces, so `HostAdminRequired=YES` and `NamespaceScopedPrivilege=NO`. The race additionally requires a receive-timeout IRQ while hardware flow control is enabled. The IRQ must arrive after the first timer cancellation and before serial-port unregistration disables the port and frees the IRQ. This is a narrow, one-shot teardown window, and the supplied evidence does not show that an ordinary user can repeatedly drive both sides of it. The affected Broadcom STB UART driver and corresponding hardware must be present. The issue is not remotely network-triggerable. External serial input might influence IRQ timing, but it does not independently provide the required device-removal authority. `RequiresHostUID0=UNKNOWN` because effective host-administrator device-management power is established, but the supplied evidence does not prove that numeric UID 0 is the only possible principal. ## 4. Primitive and severity interpretation The exact primitive is an asynchronous post-lifetime dereference of `struct brcmuart_priv`. The object is structured driver-private state, but the established stale use is constrained to a timer callback during device teardown. It is therefore a real CLASS_B lifetime violation rather than a simple invalid-state CLASS_C bug. This is closer to a constrained cleanup or unregister UAF than an active attacker-controlled object-reuse primitive. There is no earlier release followed by a second release or refcount mutation on the same reference. The repeated-release exception and UAF floor 5 therefore do not apply. The demonstrated consequence is a possible host kernel crash. A stronger consequence would require reclaimed memory to be reused and relevant callback-consumed fields to provide useful attacker influence. That mechanism is not established. `ImportantCandidate=NO` because the complete trigger is host-admin-gated, timing-sensitive, hardware-specific, and lacks meaningful demonstrated leverage beyond a stale callback dereference. `AutoDowngradeAllowed=YES` because no unresolved controlled corruption, repeated lifetime mutation, authorization bypass, or trust-boundary primitive remains. Likely operational severity is Low to Moderate, with patching recommended for systems using the affected Broadcom UART driver. ## 5. One-sentence report phrase A teardown race can rearm a Broadcom UART timer before IRQ shutdown and cause its callback to dereference freed driver state, producing a host-admin-triggered kernel crash without a demonstrated privilege-escalation primitive. ## 6. Manual review recommendation MANUAL CHECK RECOMMENDED The UAF is concrete, but the supplied evidence supports a constrained host-admin-gated teardown crash rather than an Important-grade exploitation primitive. ## 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: serial: 8250_bcm7271: fix use-after-free in brcmuart_remove() Commit: f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e Commit description: brcmuart_handle_irq() starts priv->hrt to work around a receive-timeout quirk of the 8250 core when hardware flow control is enabled; the timer callback brcmuart_hrtimer_func() dereferences priv. brcmuart_remove() cancels the hrtimer before it unregisters the port, so a receive-timeout interrupt that arrives in between re-arms the timer. After devm frees priv, the callback runs on freed memory. Unregister the port before cancelling the hrtimer. serial8250_unregister_port() shuts the port down and frees its IRQ, so brcmuart_handle_irq() can no longer restart the timer, and the final hrtimer_cancel() drains any timer that was armed before or during the unregister. This issue was found by an in-house static analysis tool. Fixes: 41a4694 ("serial: 8250: Add new 8250-core based Broadcom STB driver") Cc: stable@kernel.org Assisted-by: Codex:gpt-5.6 Co-developed-by: Song Li Signed-off-by: Song Li Signed-off-by: Fan Wu Reviewed-by: Florian Fainelli Link: https://patch.msgid.link/20260909084002.686911-1-fanwu01@zju.edu.cn Signed-off-by: Greg Kroah-Hartman Changed files: drivers/tty/serial/8250/8250_bcm7271.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=f5ce06e648d775cc1ccf66f4f594bb8b559a7d8e ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-98748 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98748 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:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 6.4 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: 0 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: YES Published priority for this report (same as in Subject): MODERATE 7.0 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).