Linux brcmnand PM Resume Fix CVE-2025-37840 Uninitialized Operation

The Linux kernel received a narrowly scoped but operationally important fix for a warning that could surface during power‑management resume on systems using Broadcom's raw NAND controller driver — the issue is tracked as CVE‑2025‑37840 and resolves an uninitialized nand_operation used during PM resume that triggers kernel WARN_ON output and can destabilize affected hosts during suspend/resume cycles.

Background / Overview​

The Memory Technology Device (MTD) subsystem in the Linux kernel provides support for raw NAND flash controllers used in set‑top boxes, embedded devices, routers and many consumer electronics platforms. The brcmnand driver (drivers/mtd/nand/raw/brcmnand/brcmnand.c) implements Broadcom NAND controller support and includes power management (PM) hooks which are invoked on system suspend and resume. CVE‑2025‑37840 was reported and fixed after maintainers discovered that a struct nand_operation was used without proper initialization during a PM resume path, causing a WARN_ON check to fire when the code validated the chip‑select (cs) field.
At surface level this looks like a diagnostic failure — a kernel warning logged to dmesg — but in practice WARN_ON traces that occur inside device resume paths can cascade into broader availability concerns: they can interrupt normal resume sequencing, appear repeatedly during automated resume cycles, clutter logs to the point of masking other failures, or in some conditions trip kernel taint/health checks and force manual intervention. Multiple public trackers characterize the issue as an availability hazard and list backported fixes in stable kernel trees.

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Technical root cause — what went wrong​

The immediate symptom​

The kernel log excerpt published in the public advisories shows a WARN_ON firing in the NAND internals code path — specifically from nand_reset_op called by brcmnand_resume — with an on‑screen call trace and the characteristic "[ cut here ]" kernel warning frame. That call trace demonstrates the bug appears during the device resume sequence, when the PM framework invokes brcmnand_resume and that function ends up calling nand_reset_op with an operation structure whose cs field was never set.

Why that matters​

• The code path involved is part of device resume, which executes synchronously while the kernel brings hardware back from a low‑power state. Failures or warnings here can stall or corrupt resume ordering.
• WARN_ON is intended as an operator visible early‑warning: it signals an internal invariant violation. Although a WARN_ON might not immediately kill the kernel, when it fires in PM code its consequences are operationally meaningful (resume hangs, repeated warnings flooding logs, or triggering developer/ops workflows).

The specific logic error​

Maintainers traced the problem to use of nand_reset_op() with an uninitialized struct nand_operation, and the fix changes the resume path to use the higher‑level nand_reset(chip, chipnr) API with an explicit chip number (chipnr = 0). This aligns with the controller’s expectation (single‑die NAND), ensures the chip‑select validation is performed on a properly initialized operation, and prevents the WARN_ON from firing. The Linux CVE announcement and associated stable commits list show the change and note the issue was introduced as far back as the 4.16 timeframe and was fixed by a commit merged into the 6.15 rc lineage (commit ddc210cf8b8a...).

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Affected systems and attack surface​

Which kernels and products are in scope​

Public trackers and distribution advisories indicate the fix was backported to a wide range of stable kernels. The initial discovery traces the regression to a commit introduced around kernel 4.16, so a very broad range of kernels — especially older stable branches that include the brcmnand driver — could have been compiled with the vulnerable code until the fix was applied or backported. Advisories list affected kernel ranges and show stable backports; operators should consult their distro's security advisories to see if a backport landed for their kernel package.

Where the risk is real​

• Embedded devices and consumer electronics that use Broadcom SoCs with raw NAND, particularly single‑die NAND chips, are the most likely to exercise this code path during suspend/resume. Broadcom‑based STBs and embedded Linux images are primary candidates.
• Cloud and general‑purpose server environments are less likely to load brcmnand by default, but certain virtual appliance images or vendor kernels for specialized hardware can include the driver. Microsoft’s advisory model (product attestation statements) has historically focused on whether a vendor’s distribution includes the vulnerable upstream component; vendors may attest only for some products (for example Azure Linux) while other images may still carry the driver. Administrators should inventory kernels and drivers rather than assume absence of the problem.

Exploitability and privileges required​

This defect is local in nature: it is triggered by the kernel’s PM resume path and depends on the platform loading the brcmnand driver. Public CVSS vectors assembled by trackers suggest an attack vector of local access with low privileges in many analyses (AV:L/PR:L), meaning an attacker that can trigger suspend/resume cycle operations or invoke sysfs interfaces might cause the warning to occur. There is no evidence of a remote code‑execution vector; instead, the impact is on availability/stability through WARN_ON and potential resume disruption. That said, some aggregators score the impact higher than others — there is not universal agreement on the exact severity.

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Real‑world impact — how bad is the bug?​

Multiple reputable vulnerability trackers characterize CVE‑2025‑37840 as an availability or stability issue rather than a confidentiality or remote‑execution hole. The published kernel call trace and the underlying reason (uninitialized struct used in chip‑select validation) make clear this is a correctness bug that manifests at resume time. In the worst‑case operational scenarios reported by vendors and security trackers, the symptom is:

• Repeated kernel warnings flooding logs on resume;
• Potential resume failures or hangs on affected hardware during suspend/resume cycles;
• In embedded fleet scenarios, a persistent problem that causes devices to require manual intervention or repeated reboots to recover.

These outcomes translate directly into service disruption for devices that rely on automatic suspend/resume or that are managed at scale (e.g., remote STB fleets). Because the trigger lives in a privileged kernel path, an adversary with local access — or a buggy driver sequence — can provoke repeated WARN_ONs and force sustained availability loss. Organizations should treat the vulnerability as an availability hazard and prioritize remediation if they operate affected systems.
Caveat: severity ratings in public feeds differ — for example, one vendor advisory lists a CVSS v3 base score of 5.5 while the NVD entry shows a 7.8 score under a vector emphasizing availability impact. These differences typically reflect divergence in how any confidentiality and integrity impacts are judged or whether the reporter assumes a path from warning to full crash. Administrators should therefore prioritize practical risk to their fleet (presence of brcmnand, suspend/resume usage) over an isolated numeric score.

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Detection and immediate triage steps​

If you manage Linux hosts or embedded devices, take these practical steps to detect whether you're affected and to triage active incidents.

• Check whether your kernel contains the brcmnand driver:
• Inspect your kernel config or modules: look for drivers/mtd/nand/raw/brcmnand/brcmnand. If the module is built as loadable, list it with lsmod and modinfo.
• Search logs for the warning pattern:
• dmesg | grep -i -E "nand_reset_op|brcmnand_resume|WARN_ON|nand_reset"
• journalctl -k --since "24 hours ago" | grep -i brcmnand
• Observe whether suspend/resume cycles are used in production; systems that frequently hibernate, suspend to RAM, or run automated resume tests are higher risk.
• In lab/test fleets, perform a controlled suspend/resume and watch dmesg for the known call trace (the published advisories include the exact trace header and function names).
• If you encounter the trace, treat the device as operationally impacted and consider placing it into maintenance mode for patching or rollback.

For operators of large device fleets, build a simple aggregator rule (SIEM or log shipper) that flags the signature lines associated with nand_reset_op and brcmnand_resume so you can quickly find affected endpoints.

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Mitigation and remediation​

The definitive remediation path is to install kernel updates that include the upstream fix or apply vendor backports. The kernel CVE notice and downstream advisories are explicit: update to the latest stable kernel or apply the stable backported commits listed by upstream if you cannot move to a modern kernel release. Administrators should follow their distribution vendor’s security guidance for kernel updates and backports rather than attempting to cherry‑pick patches into production kernels unless they have a rigorous kernel maintenance process.
Practical remediation checklist:

• Inventory: identify machines with brcmnand loaded or that contain drivers/mtd/nand/raw/brcmnand in their kernel image.
• Test: if you have staging devices representing affected hardware, apply the vendor kernel or upstream backport and run repeated suspend/resume cycles under test to validate the fix.
• Patch: apply vendor kernel updates that contain the fix; if vendor packages are not available, plan a controlled kernel upgrade to a stable release that contains the fix or obtain the vendor‑supported backport.
• Monitor: deploy log detection rules to confirm absence of the WARN_ON trace after remediation.
• Workarounds: if immediate kernel updates are impossible, consider disabling the brcmnand module temporarily (where safe) or adjusting suspend/resume scheduling to limit resume invocations; be aware these are stopgap measures and may not be viable for devices that require NAND access.

The upstream kernel CVE announcement lists several stable commit IDs for backporting; use those commit identifiers when coordinating with vendors or when verifying that a distribution kernel contains the remediation.

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Why you should care (a practical risk analysis)​

• Embedded fleets and STBs: devices that reboot or suspend frequently are most at risk — a WARN_ON in the resume path is not just a log line on these platforms; it can lead to remote maintenance costs, mail‑in repairs, or in‑field reboots.
• Large‑scale management: in environments where thousands of remote devices run unattended, even a low‑frequency resume failure that requires manual intervention becomes a high‑cost event.
• Vendor patching lag: not all vendors push kernel fixes at the same speed. Upstream kernel fixes may land quickly, but distro backports and vendor OEM firmware updates can lag days to months.
• False sense of safety: because the observable symptom is a kernel warning rather than immediate code execution, some teams may deprioritize remediation. That’s risky: a warning inside PM paths is a legitimate operational hazard and should be treated as such.

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What maintainers changed and why it’s correct​

Kernel maintainers replaced a direct nand_reset_op() call that depended on a caller‑supplied nand_operation with a call to the well‑defined nand_reset(chip, chipnr) API using chip number 0. This change both eliminates the uninitialized field usage and aligns with the controller’s single‑die expectation. The fix is conceptually simple and respects the kernel’s API layering: prefer higher‑level, well‑initialized helper functions when available rather than duplicating lower‑level operation structures that are easy to misuse. The CVE announcement documents when the defect was introduced and the upstream commit that fixed it.

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Cross‑references and corroboration​

Multiple independent trackers and advisories recorded the issue on and after May 9, 2025. The Linux kernel CVE list entry describes the exact file and affected function, the NVD entry reproduces the kernel warning trace, and distribution‑level advisories (for example cloud vendor advisory feeds) present CVSS scoring and affected package guidance. These independent records corroborate the diagnosis (uninitialized nand_operation used during resume) and the remediation approach (use higher‑level nand_reset API / backport stable commits).
Note: distribution and vendor feeds may report different CVSS base scores and slightly different affected‑version ranges depending on what backports were applied and how the vendor assessed impact. Treat the operational facts (presence of brcmnand, resume warnings in logs, and availability implications) as the primary drivers for triage decisions rather than a single numeric score.

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Operational recommendations (short, actionable)​

• If you run Broadcom‑based embedded images or device fleets: prioritize patching. Treat this as a medium‑to‑high availability risk depending on suspend/resume usage.
• If you operate general‑purpose servers: verify whether brcmnand is present; if not, your exposure is likely minimal.
• For vendors and integrators: include the stable backports identified by upstream in your kernel packaging and communicate to customers whether their device images contain the fix.
• For security teams: add log rules to detect the NAND resume WARN signature and run a one‑time sweep of fleet logs to estimate exposure.
• For developers: prefer higher‑level, initialized kernel helpers to avoid operation‑structure initialization pitfalls when writing or auditing PM code paths.

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Critical analysis — strengths of the remediation and residual risks​

Strengths​

• The fix is small and surgical: switching to nand_reset(chip, 0) eliminates the use of an uninitialized struct without a large refactor and is straightforward to backport. That makes remediation realistic for vendors and distributors to adopt.
• The vulnerability is local and non‑remote‑executable: there’s no public proof that this is a remote RCE; the realistic impact is availability/stability, which narrows the immediate threat model.

Residual risks​

• Backport lag: many vendors and embedded vendors have slower patch cycles and may not backport the upstream fix promptly; devices already deployed may remain exposed for extended periods.
• Operational surprises: ths can expose deeper race conditions on specific hardware revisions; while the immediate fix addresses the uninitialized field, there is always a non‑zero chance of related timing bugs on unusual SoCs that will require additional triage.
• Inconsistent scoring: public sources disagree on CVSS numbers and impact vectors; that can create a false sense of either urgency or complacency. Use your fleet’s actual exposure profile to drive prioritization rather than a single aggregator’s score.

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How we validated the publicly available claims​

To ensure accuracy of the technical facts in this article we cross‑checked the Linux kernel CVE announcement (which includes affected files, the traceback, and commit guidance) against NVD and several distributor vulnerability feeds; these independent sources consistently report the same root cause (uninitialized nand_operation used on PM resume in brcmnand) and the same remediation (use nand_reset(chip, chipnr) or backport the stable commits). The kernel community also lists the specific stable commit IDs for backports, which operators can use when coordinating vendor patches.
For broader context on MTD and flash driver availability fixes, community threads and archives documenting similar MTD/Spinand fixes and backports were consulted to place this CVE into the pattern of prior fixes where small code‑level correctness issues produce meaningful availability headaches for embedded fleets.

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Final verdict — what admins should do now​

• Inventory your kernels and determine whether brcmnand is present or loadable.
• If brcmnand is present and your devices use suspend/resume, prioritize patching with the vendor’s kernel update or by applying an upstream backport supplied by your distributor.
• Add a temporary detection rule for the NAND resume WARN signature and sweep current logs to estimate fleet exposure.
• For embedded vendors and integrators, ship the backport widely and communicate to device operators the practical risk (resume disruption) and the timeline for firmware/kernel updates.

This CVE is not an elegant, remote exploitation story — it is a classic kernel correctness issue that behaves like a practical availability hazard in the field. The good news is the fix is small, well understood, and already shipped upstream; the remaining problem is organizational: identify which images and devices still run the vulnerable code and get them patched before those devices begin to experience recurring resume failures.

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Source: MSRC Security Update Guide - Microsoft Security Response Center