Linux Secretmem UAF Fix for CVE-2025-40272 in memfd_secret Fault Path

A recently assigned CVE, CVE-2025-40272, identifies a subtle but meaningful use-after-free race in the Linux kernel’s secret memory path (mm/secretmem) that was fixed upstream by reordering fault-handler cleanup so the direct map is restored before the allocated folio is freed. The flaw affects page-fault handling for memory regions created by memfd_secret(2) and can lead to a supervisor not‑present page fault (kernel oops) when two tasks fault the same page concurrently — a correctness and availability defect that is particularly sensitive because it touches kernel mechanisms designed to harden in‑memory secrets.

Background / Overview​

memfd_secret(2) is a Linux syscall that creates an anonymous, RAM-only file suitable for storing secret data: pages backing such mappings are kept out of the kernel direct map and are intended to be inaccessible via conventional kernel routines that walk the direct map. This syscall is used to raise the bar on in‑kernel or cross‑process exfiltration of secrets and is part of the kernel’s secretmem/mm infrastructure. The man page documents the call’s intent and behavior and notes that pages are allocated on demand as faults occur, and that access control and mapping semantics differ from ordinary anonymous memory. The vulnerability fixed by CVE-2025-40272 arises in the memfd_secret fault path. When a page fault for a memfd_secret mapping occurs the kernel allocates a folio (the kernel’s higher-level page abstraction), marks the underlying page as not-present in the direct map, and attempts to insert the folio into the file mapping. If two tasks fault the same page at almost the same time, both might allocate a folio and both might remove the PTE from the direct map. Only one of the tasks will ultimately succeed in inserting the folio into the mapping. The failing task then needs to roll back its partial work: it frees the folio it allocated and restores the PTE/direct-map entry. The bug was that the rollback sequence performed these two steps in the wrong order — freeing the folio first and restoring the direct map second — which temporarily made the physical page available to the allocator while the direct map still lacked a valid entry. If another CPU or thread allocated that same page during the window between the free and the direct map restore, subsequent kernel access via the direct map could trigger a supervisor not‑present page fault. The upstream remedy simply restores the direct map before freeing the folio, eliminating the brief window in which the page can be reallocated into an inconsistent state. This description aligns with the NVD/OSV advisories summarizing the fix.

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Why this matters: confidentiality, availability, and attack surface​

The special sensitivity of secretmem​

memfd_secret is explicitly designed to reduce in-kernel access to user secret pages by removing them from the direct map and disallowing traditional get_user_pages-style access. Because secretmem deliberately changes the kernel’s normal mapping invariants, bugs in its code paths can produce outsized risk: not only can they crash the kernel, they can also undermine the guarantees that make memfd_secret useful in the first place. Misordered cleanup in a fault handler that briefly re-exposes a page to the allocator threatens both stability and the logical integrity of the secretmem invariants.

Primary impact: availability (kernel oops, denial-of-service)​

The immediate, documented consequence of this defect is an availability problem: supervisor not‑present page faults and kernel warnings or oopses that can crash a process or the entire host. In infrastructure where many tenants or untrusted workloads run concurrently — cloud hypervisors, CI runners, multi-tenant containers — a triggered kernel oops can cause cascading failures and expensive recovery actions. Public vulnerability records frame this as a correctness problem whose observable result is instability rather than a clear, standalone remote code execution primitive.

Secondary concerns: exploitation chains and secrecy guarantees​

While the public disclosures do not document a working privilege-escalation or data-exfiltration exploit for this specific bug at disclosure time, kernel use-after-free and mapping inconsistencies are the sort of primitives that can be chained into stronger primitives by skilled attackers in favorable heap and timing conditions. In particular, anything that undermines memfd_secret’s protection model merits scrutiny because an attacker who already has local code execution could combine such a bug with other issues to undermine secrecy guarantees or improve an exploitation pathway. That said, the known and immediate risk remains availability; claims about in‑the‑wild exploitation should be treated as unverified until telemetry or vendor reports confirm otherwise.

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Technical anatomy — what went wrong and how the fix works​

How secretmem faults are supposed to behave​

When the kernel handles a page fault for a memfd_secret-backed mapping, it typically:

• allocate a folio to represent the physical memory for that page,
• mark the corresponding direct-map PTE as not-present so the page isn’t accidentally exposed via page_address / direct mapping helpers, and
• insert the folio into the file mapping so the faulting process can proceed.

This sequence preserves the invariant that while the page is active for secret use it is not accessible through the kernel’s direct map helpers.

The race and the wrong rollback order​

Under concurrent faults on the same page two different threads may each:

• allocate a folio,
• clear the direct-map entry (remove from direct mapping),
• race to insert their folio into the file mapping.

Only one thread successfully does the mapping insertion. The other thread must roll back: it frees the folio it allocated and restores the direct-map PTE. The original code performed the rollback in the order (a) free folio, (b) restore direct map. That left a brief window in which the folio had been freed (making the underlying page re-allocable) but the direct map had not yet been restored. Another thread might then allocate that same physical page and the kernel could later attempt to use the direct map entry that still points to the previous physical address — producing a supervisor not-present page fault. The fix moves the restore-of-direct-map before the folio free so that the physical page is never visible to the allocator while the direct mapping is inconsistent. The patch is intentionally small and focused on ordering rather than a broad subsystem overhaul — the classic conservative style for memory-management fixes in the kernel stable trees.

Patch scope and backports​

Upstream maintainers applied a minimal, surgical change to the fault handler to reorder cleanup. The NVD/OSV entries reference stable-kernel commit objects (the kernel stable git contains the precise diffs), indicating the patch landed in the upstream trees and is suitable for backporting to stable branches. Operators should watch vendor advisories and distribution package changelogs for the presence of the stable commit ID or an explicit backport. Where vendor packaging is used, the safest verification is to match the vendor kernel package changelog against the upstream stable commit hash(s) referenced by the CVE record.

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Who’s at risk and under what conditions​

• Systems that enable memfd_secret or ship with secretmem compiled in are the natural population to check. On many distributions memfd_secret has been enabled or made configurable in recent kernel series; consult your distro’s kernel config and changelogs.
• Multi‑tenant hosts and cloud hypervisors where untrusted local or guest code can deliberately exercise memfd_secret/ fault paths are higher risk than single-user desktop machines.
• Long‑tail appliance kernels and vendor-provided kernels (embedded builds, IoT, specialized appliances) are likely to lag upstream and therefore may remain vulnerable longer unless the vendor ships the backport.
• Desktop workstations are lower risk in typical single-user setups, but workloads that intentionally allocate secretmem pages (security tooling, specialized enclaves, sandboxed secrets stores) could hit the corner-case race.

In short: the attack vector is local or host-adjacent; exploitation requires concurrent faults on the same page and is not a trivial unauthenticated remote attack. The most pragmatic operational harm is denial-of-service or host instability.

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Detection, triage and immediate mitigations​

Signs to watch for​

• Kernel oops and WARN messages containing call stacks referencing mm/secretmem or the memfd_secret fault path.
• System instability or reproducible kernel crashes correlated with workloads that create memfd_secret mappings and cause concurrent faults.
• Package changelogs that list a stable-kernel commit or CVE-2025-40272 as the reason for a kernel update.

If you see memfd_secret-related backtraces in dmesg or persistently reproducible oopses while running workloads that use memfd_secret, prioritize patching. The vulnerability manifests clearly as a kernel correctness failure rather than as stealthy data leakage.

Short-term mitigations​

• Disable use of memfd_secret from untrusted code paths where possible: audit applications and container images for memfd_secret(2) use and avoid granting untrusted workloads the ability to use it.
• If memfd_secret support was enabled via kernel boot parameter or module options and is not needed, consider disabling it (note: how to disable depends on your kernel version and distribution packaging). Consult your distribution documentation — toggling kernel boot options or rebuilding with CONFIG_SECRET_MEM disabled are examples of system-wide mitigations.
• Isolate sensitive workloads: limit the ability of untrusted tenant code to create or interact with memfd_secret-backed mappings (namespace, capability or cgroup isolation).
• As an operational fallback for high-risk multi‑tenant infrastructure, prioritize kernel package updates and schedule reboots to load patched kernels.

Be cautious: changes that disable memfd_secret may affect applications that rely on its secrecy guarantees. Test changes in staging before broad deployment.

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Patch and remediation guidance​

• Inventory: identify kernels and hosts that build/enable secretmem. Use:
• uname -r and distribution package queries, and
• the kernel configuration exposed in /boot/config-$(uname -r) (look for CONFIG_SECRETMEM or related flags).
• Map vendor packages to upstream fixes: confirm whether your vendor’s kernel build contains the upstream stable commit(s) referenced in the CVE record.
• Apply vendor-supplied kernel updates that include the backport and reboot hosts. Kernel memory-management fixes require reboots to take effect.
• For appliances or vendor kernels that do not provide an update, engage vendor support or plan rebuilds with upstream stable commits applied.

NVD and other vulnerability indexes link to the upstream stable commits that implement the fix; use those commit identifiers to validate your vendor package or kernel build. If vendor advisories are not yet available for your platform, treat systems that enable memfd_secret and host untrusted workloads as higher priority for mitigation.

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Strengths of the upstream response — why the fix is reassuring​

• The patch is small and surgical: fixing operation ordering in a fault-handler rollback path is a narrowly scoped change that minimizes regression risk and is readily backportable to stable kernel branches.
• The fix preserves the original memfd_secret semantics and restores the intended invariant (the direct map is consistent during all windows of allocation), rather than disabling functionality or introducing heavy-handed workarounds.
• Upstream maintainers have recorded the change in the stable tree and CVE tracking systems (NVD/OSV reflect the fix and link to stable-kernel commits), which simplifies vendor backporting and distribution packaging. These are standard, appropriate practices for kernel-level correctness fixes.

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Risks, caveats and unanswered questions​

• Lack of public exploit telemetry: at disclosure time there is no authoritative public report of successful in‑the‑wild exploitation of CVE-2025-40272. That reduces immediate crisis-level concern but does not obviate the need for remediation because kernel use-after-free and mapping races are potentially valuable in chained exploit scenarios. Treat absence of public PoC as provisional and continue to monitor vendor/telemetry channels.
• Vendor backport lag: distribution and appliance vendors each have different backport and release cadences. Long‑tail kernels and embedded distributions are likely to lag and therefore represent a persistent exposure until vendors ship updates. Operators must map upstream commit IDs to vendor changelogs rather than rely solely on kernel version numbers.
• Operational trade-offs when disabling features: disabling memfd_secret system-wide may reduce exposure but also removes a protective mechanism used by applications that depend on it. Any such change should be evaluated for application compatibility and security trade-offs.
• Source verification caveats: the NVD entry cites kernel.org stable commit objects; some hosting front-ends throttle or require JS to view detailed pages. Administrators who need the exact patch diff should prefer fetching the upstream stable commit via the kernel.org git interface or by obtaining the vendor backport and matching the commit IDs.

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Practical checklist for administrators (prioritized)​

• Inventory hosts that use memfd_secret or have CONFIG_SECRETMEM/secretmem enabled; if unsure, search images and containers for uses of memfd_secret(2) or tools that call it.
• Check your distribution or vendor security tracker for CVE-2025-40272 and whether a kernel package update is available. If available, test and schedule rollouts quickly for multi‑tenant or cloud hosts.
• If no vendor package exists, consider temporary mitigations: disallow memfd_secret usage in untrusted workloads, isolate sensitive hosts, or engage vendor support for out-of-band backports.
• Preserve crash diagnostics: if you see memfd_secret-related kernel oopses, collect dmesg, vmcore, and relevant crash dumps before rebooting to aid vendor triage.
• Monitor upstream and vendor advisories for backports and CVE follow-ups; match package changelogs to upstream stable commit IDs to confirm the fix is present.

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Critical takeaways and editorial evaluation​

CVE-2025-40272 is a classic kernel correctness bug that sits at the intersection of memory-management complexity and a security feature (secretmem). The issue demonstrates how protective mechanisms that change low-level mapping invariants increase the complexity and the potential for subtle races. The upstream fix is appropriately small and targeted: reordering cleanup to restore the direct map prior to freeing the folio removes the inconsistent window without altering the broader memfd_secret design.
From an operational perspective, treat this CVE as a prioritized kernel update for hosts that:

• enable memfd_secret or have secretmem compiled in, and
• run untrusted or tenant-provided workloads that might deliberately or incidentally cause concurrent faults.

The immediate, observed risk is availability — kernel oops and host crashes — which in multi‑tenant environments is a high-priority operational issue. While no public exploit reports tie this CVE to privilege escalation or data leakage at disclosure, the fact that this bug touches secretmem semantics warrants attention: attackers with local access seek any weakening in secrecy guarantees, and kernel mapping races are the sort of thing that can be leveraged when combined with other primitives. Operators should remediate quickly where exposed, validate vendor backports by commit ID where possible, and treat the lack of public exploitation evidence as a temporary reassurance rather than a reason to delay patches.

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Conclusion​

CVE-2025-40272 is a narrowly scoped but important kernel bug: a use-after-free/race in mm/secretmem’s fault handler that could produce supervisor not‑present page faults when multiple tasks fault the same memfd_secret page concurrently. The upstream remedy is straightforward and low risk — reorder rollback operations so the direct map is restored before freeing the folio — and distributions should backport and ship the fix promptly. Administrators should inventory hosts with secretmem enabled, apply vendor kernel updates that include the stable commit, and temporarily restrict memfd_secret usage in untrusted contexts where immediate updates are not possible. The technical lesson is clear: kernel features that alter mapping invariants require careful lifecycle handling, and even small ordering errors can translate into serious availability problems in production environments.

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(General kernel-use-after-free patterns and remediation approaches are discussed in community analyses of recent CVEs for memory-management subsystems and can help inform patch validation and risk assessment.

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Source: MSRC Security Update Guide - Microsoft Security Response Center