Linux Kernel Patch Fixes KSM Madvise Flag Bug (CVE-2025-40040)

The Linux kernel patch addressing CVE-2025-40040 fixes a subtle but consequential flag-dropping bug in the KSM madvise path: under specific 64-bit builds a bitwise operation in ksm_madvise could inadvertently clear the upper 32 bits of a VMA’s vm_flags, removing userfaultfd (UFFD) flags and producing kernel oopses or warnings when userfaultfd state was later inspected. This is a correctness bug rather than a classic memory-corruption exploit, but it can cause host instability and must be patched promptly on systems that enable KSM and use the affected code paths.

Background / Overview​

Kernel Samepage Merging (KSM) exposes the madvise interface (MADV_MERGEABLE and MADV_UNMERGEABLE) so processes and system components can mark VM areas as candidates for page-content merging. The kernel represents many per-VMA and per-mm flags in a single vm_flags field. When userfaultfd (UFFD) is used in minor mode, UFFD state is stored in VMA-associated structures and indicated by flags in vm_flags. Syzkaller discovered a reproduction that led to a kernel BUG in userfaultfd release code: a VMA still had a non-NULL userfaultfd context pointer but lost the UFFD flags in vm_flags, causing an inconsistency that triggered the oops. The root cause: a single bitwise expression in ksm_madvise that attempted to clear the VM_MERGEABLE flag accidentally cleared the high half of vm_flags on 64-bit builds.
This vulnerability was assigned CVE-2025-40040 and documented in public vulnerability feeds and upstream patch threads. The fix is intentionally minimal and surgical: make VM_MERGEABLE a properly typed 64-bit flag (use BIT() / unsigned long) so that the complement and AND operation interact with vm_flags correctly and do not zero the upper half by mistake. The change is a classic example of a type-width/promotion pitfall in C when mixing 32-bit constants with 64-bit bitfields.

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Technical anatomy — what went wrong and why​

The C type-promotion trap​

On typical x86_64 kernels:

• vm_flags is an unsigned long (64-bit).
• Many VM_* constants are declared as 32-bit values (unsigned int or int).
• The code in ksm_madvise did this operation:
• vma->vm_flags &= ~VM_MERGEABLE;
• VM_MERGEABLE historically was defined as a 32-bit constant 0x80000000 (the high bit of a 32-bit value).
• Applying the bitwise NOT (~) to a 32-bit unsigned value yields a 32-bit result (0x7fffffff). When that 32-bit value is promoted to unsigned long for the &= operation, the upper 32 bits are zero-filled (0x000000007fffffff), so the AND clears the upper 32 bits of vm_flags instead of preserving them.
• Because UFFD uses flags stored in those upper bits (on some architectures/configurations), they were accidentally cleared — the VMA still had vm_userfaultfd_ctx pointer but no UFFD flags — leading to inconsistent state and kernel oops or warnings when userfaultfd code inspected the VMA.

Why this only hit VM_MERGEABLE​

The problem arose specifically for VM*MERGEABLE because of the particular numeric value and the original type used for that constant. Many other VM** flags are declared as (signed) int constants with leading one-bits after ~, which when promoted to unsigned long produce 0xffffffffffffffff-style upper bits that do not zero the high half of vm_flags. VM_MERGEABLE’s 0x80000000 value is the corner-case that made the complement produce an upper-zero promotion. The upstream patch author therefore chose a narrow, correct fix: make VM_MERGEABLE a BIT() (unsigned long) so the complement has the correct 64-bit form and the &= works as intended.

Observable symptom and exploitability​

In the publicly reported syzkaller trace the kernel reported a BUG at mm/userfaultfd.c:2067, and in later trees that check was converted from BUG to WARNING; the underlying inconsistency remained the same. The failure is reproducible locally by an environment that uses UFFD in MINOR mode, KSM interaction via MADV_UNMERGEABLE, and the precise sequence that triggers the flag-clear. The attack vector is local-only: a process that controls userfaultfd registration and calls madvise can reach the code path. There is no indication that this bug gives an immediate remote code-execution or privilege escalation primitive; the primary impact is availability (host oops or warning leading to instability).

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Upstream response and patch details​

The patch was submitted to the kernel mailing lists as a focused correction to ksm_madvise and to the VM flag definitions. The change set includes:

• Re-defining VM_MERGEABLE as a BIT() (unsigned long) so that complements and bitwise operations preserve the full 64-bit layout.
• A small, targeted change line in mm/ksm (ksm_madvise) to use the new flag definition safely.
• A companion patch suggesting redefinition of VM_* macros using BIT() where appropriate to avoid future surprises.

The discussion and patch series were reviewed on LKML and spinics and were included in the MM hotfix pull sent to the stable kernels — indicating maintainers considered the fix safe to backport to stable branches due to its minimal scope and low risk of regression. That pattern — surgical casting/type corrections for mixed-width bit ops — is typical for kernel stable backports.

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Impact assessment — who should care​

• Affected systems: Any Linux build that contains the vulnerable commit range in mm/ksm and compiles vm_flags as unsigned long (typical 64-bit kernels) can be affected where the KSM madvise path is exercised and userfaultfd MINOR mode is in use. That primarily includes:
• Systems with CONFIG_KSM enabled (desktop and server kernels where KSM is compiled in).
• Environments that use userfaultfd features in MINOR mode (e.g., some VM memory management tools, specialized user-space memory-tracking code, or research tools).
• Multi-tenant or cloud hypervisors that may expose UFFD or KSM functionality to guests or that handle untrusted workloads calling madvise and userfaultfd APIs.
• Attackability: Local-only. An attacker requires the ability to run code on the host (or inside a guest with access to the host’s relevant interfaces) to exercise userfaultfd registration and madvise sequences. This makes the bug an availability/DoS concern in shared hosting, CI, or developer workstations where untrusted processes can run. There is no authoritative public PoC weaponizing this into privilege escalation or arbitrary code execution at disclosure.

Operationally, this makes the bug important to operations teams that run high-assurance, multi-user, or cloud services: a single host crash or persistent kernel warning that correlates with the bug’s trigger conditions can disrupt workloads for many tenants. For most home or single-user desktops the risk is lower unless the user runs tooling that registers UFFD and uses MADV_UNMERGEABLE.

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Detection, telemetry and indicators of compromise​

The defect exhibits clear kernel log indicators when triggered. Administrators should watch for these signatures in host logs:

• Kernel oops/BUG or WARN traces mentioning:
• userfaultfd_release_all or userfaultfd_release in the stack trace.
• mm/userfaultfd.c at the line numbers reported by upstream (e.g., the original trace at the userfaultfd release site).
• Messages such as "WARNING: CPU: … at mm/userfaultfd.c:2067" or earlier BUG variants seen in syzkaller output.
• Sudden or reproducible kernel oops/panic correlated with processes that create userfaultfd registrations and call madvise(MADV_UNMERGEABLE).
• Unexplained UFFD-related inconsistencies in crash dumps: VMA entries that contain a vm_userfaultfd_ctx pointer but do not have the corresponding UFFD vm_flags. Preserving dmesg and crash dumps before rebooting is essential for forensic analysis.

Add SIEM alerts to flag repeated userfaultfd/ksm traces or the specific mm/userfaultfd messages. If you see these traces, prioritize applying the kernel patch and isolating the offending workload while you update and validate the fix.

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Remediation and operational playbook​

The authoritative remediation is to install a kernel release that contains the upstream patch (or vendor backport) and reboot the host to run the corrected kernel image. The fix is small, easy to backport, and was included in MM hotfixes for stable branches, so vendor packages from major distributions should include the backport in short order. Follow this prioritized checklist:

• Inventory and triage
• Identify hosts that enable KSM (check kernel config and runtime): zgrep CONFIG_KSM /boot/config-$(uname -r) or check /proc/config.gz if available.
• Identify hosts that use userfaultfd in MINOR mode (application or orchestration-level knowledge).
• Map those hosts to distribution kernel package versions and vendor advisories that reference CVE-2025-40040 or the upstream commit IDs.
• Acquire and staged patching
• Locate vendor/distribution kernel updates that include the MM hotfixes. Use your vendor security tracker or package manager to find kernel packages that reference the relevant stable commit or the CVE identifier.
• Stage updates to a pilot group that includes hosts exercising UFFD or KSM behavior.
• Deploy and validate
• Deploy patched kernels and reboot controlled hosts.
• Verify host stability under representative workloads that previously triggered the warnings, if available, or run safe test-cases that exercise UFFD registration and MADV_UNMERGEABLE in a lab.
• Monitor
• After rollout, monitor kernel logs for regressions or related oops/warnings.
• Keep an eye on vendor advisories for any follow-up microfixes.

If you cannot immediately patch:

• Limit the ability of untrusted code to call madvise on arbitrary VMAs or restrict who can register userfaultfd objects in sensitive hosts.
• Isolate workloads that require userfaultfd to dedicated hosts not running multi-tenant workloads until patched.

Because the fix is surgical and low-risk, standard staged kernel-rollout practices apply: pilot → staged rollouts → full production deployment. The small footprint makes vendor backporting straightforward, but do not skip pilot tests — any kernel change should be validated in your environment.

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Why the fix is small — and why that matters​

Surgical fixes that correct type-width or mixed-integer promotion errors are low-risk and tend to be accepted quickly by upstream maintainers because they preserve algorithmic semantics for valid inputs while eliminating corner-case behavior. In this case the change converts a single flag constant to the correct width (unsigned long) via the BIT() macro and adjusts a two-line change in ksm_madvise; the result preserves expected behavior and prevents accidental zeroing of the vm_flags upper half. That small footprint reduces regression risk and speeds stable backports.
However, there are operational caveats:

• Vendor packaging and embedded/OEM kernels may lag. Appliances, vendor Android kernels, and firmware images often have slower patch cycles and may remain vulnerable longer.
• Some environments have little or no KSM usage; for those the operational urgency is lower. Prioritize multi-tenant hosts, HPC systems using memory deduplication, and hosts running untrusted workloads.

These distribution and vendor propagation realities are familiar; past kernel fixes with tiny changes have still required careful mapping from upstream commits to vendor package versions to ensure accurate remediation.

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Detection and verification example commands​

Operational checks you can run while planning remediation:

• Confirm kernel version:
• uname -r
• Confirm whether KSM is configured or available:
• grep CONFIG_KSM /boot/config-$(uname -r) || zgrep CONFIG_KSM /proc/config.gz
• Search kernel logs for the userfaultfd trace pattern:
• journalctl -k | grep -E "userfaultfd_release|mm/userfaultfd.c|WARNING: CPU: .* userfaultfd"
• After installing a vendor kernel, verify the changelog or package notes reference the upstream mm hotfixes or CVE identifier.

Use staged tests for hosts that actually use UFFD: run the application or test-case that uses MINOR-mode UFFD in a lab environment, exercise MADV_UNMERGEABLE sequences, and verify the kernel no longer logs the earlier BUG/WARNING traces.

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Broader lessons for kernel maintainers and operators​

• Mixed-width constants in bitwise operations are a recurring source of subtle bugs. When constants are used in bitfields that may be wider on some platforms, prefer using BIT() / (1UL << n) or explicitly typed unsigned long constants to avoid promotion surprises.
• Small, surgical fixes that preserve existing semantics for valid inputs are preferable for stable backports. They minimize regression risk while removing corner-case failures that can be disruptive in production.
• Instrumenting kernel testing with syzkaller-style fuzzing is effective: this bug was found by syzkaller, demonstrating that fuzzing and robust coverage of VM ops remains essential to finding rare type/bitness issues.
• For operators: map CVE→upstream commit→vendor package rather than assuming CVE text alone maps to installed package versions. Distribution trackers and vendor advisories provide the final mapping for production rollouts.

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Final assessment and recommended timeline​

• Urgency: Medium for most environments; High for multi-tenant hosts and systems that use userfaultfd in MINOR mode combined with KSM or that permit untrusted local code execution.
• Short-term action (0–72 hours): Inventory affected hosts, identify kernels and vendor advisories, schedule pilot updates for high-risk hosts.
• Medium-term (days–weeks): Roll out vendor kernel updates via staged deployment and validate that UFFD traces no longer appear under representative workloads.
• If you cannot immediately patch: restrict untrusted code execution and isolate workloads that rely on userfaultfd until updates can be applied.

This bug illustrates how a tiny type-width mistake can produce a visible and disruptive kernel correctness failure. The fix is straightforward and low-risk; the operational work is packaging and deployment. Prioritize hosts by exposure and application of UFFD/KSM usage, and verify the fix via kernel logs and vendor changelogs after update.

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CVE-2025-40040 is a reminder that correctness bugs in kernel bitfields — particularly where user-facing APIs (madvise/userfaultfd) intersect with system-wide optimizations (KSM) — can have outsized operational impact. The remedy is available and safe to deploy; the sensible operational response is inventory, patch, validate, and monitor.

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Source: MSRC Security Update Guide - Microsoft Security Response Center