Linux udmabuf CVE-2025-37803: Kernel Buffer Size Overflow Fixed

A small, arithmetic oversight in the Linux kernel’s udmabuf driver has been assigned CVE‑2025‑37803 — a buffer‑size overflow discovered during udmabuf creation that lets a crafted local action cause kernel memory corruption and sustained denial of service unless systems are patched or the module is mitigated.

Background / Overview​

udmabuf (sometimes seen in userland projects as u‑dma‑buf) is a lightweight kernel facility that lets user space allocate and export physically contiguous memory as DMA buffers (dma_buf), exposing those buffers through device nodes such as /dev/udmabuf*. The driver exists to support user‑space drivers and I/O components that need contiguous memory regions for DMA without writing a full kernel driver. The codepath that creates these buffers enforces an upper size limit, expressed in megabytes, and convertage‑count cap as part of udmabuf_create().
What went wrong here was a classic integer‑width/cast mistake: a size limit value (size_limit_mb) was used in arithmetic that assumed 64‑bit precision in order to compute a page limit, but it could be treated as a narrower integer during the calculation. The upstream fix — and the basis of the CVE description — is to explicitly cast size_limit_mb to a 64‑bit unsigned value when computing pglimit so the arithmetic cannot overflow and the driver always enforces the intended page limit.
Multiple distro and vulnerability trackers recorded and published the issue shortly after the upstream patches were merged; the CVE was published on May 8, 2025 and quickly assigned a High score because the flaw can yield high confidentiality, integrity and availability impacts on affected kernels.

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Why this matters: technical risk and real‑world impact​

At a code level this is uncomplicated — a size arithmetic/overflow that leads to incorrect bounds checks — but the real risk comes from where udmabuf lives (kernel space), who can interact with it (local, typically low‑privileged processes), and what the consequences are when the kernel’s internal memory bookkeeping goes wrong.

• Kernel memory corruption is high‑value. Any path that allows user inputs to cause kernel memory to be read or written incorrectly opens paths to denial of service (kernel oops/panic, system hang) and, in some cases, privilege escalation or information disclosure when exploited creatively. The udmabuf bug was triaged as having the potential for memory corruption and system instability.
• Attack vector is local but low‑privileged. The consensus data from trackers shows the attack vector is local. That means either a local user or a containerized process with access to the device node could trigger it; the vulnerability requires only low privileges in many configurations, since udmabuf exists specifically to be accessed by user‑space code. Several distro advisories reflected that this is a local, low‑privilege risk.
• Availability impact is immediate and sustained. The kernel is unforgiving: a bad pointer, out‑of‑bounds write or page table corruption typically results in a kernel panic (crash) or unrecoverable state for affected resources. The vulnerability was therefore assigned high availability impact ratings by trackers; the practical exploit goal most likely is denial of service rather than remote takeover (because the bug is local).

In short: a tiny arithmetic oversight produced an exploitable condition with outsized operational consequences.

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What exactly was fixed (the technical change)​

Upstream maintainers narrowed the problem to the calculation that converts the configured size limit (measured in megabytes) into a page count used to cap allocation requests. When that arithmetic allowed a wraparound or truncation because size_limit_mb was treated as a 32‑bit quantity in an otherwise 64‑bit operation, the resulting page limit check could be bypassed and the driver could allocate more memory than intended — producing an overflow condition or out‑of‑bounds buffer. The accepted upstream fix is straightforward and defensive:

• Convert/cast the size_limit_mb variable to an unsigned 64‑bit integer (u64) before performing the calculation that computes pglimit.
• Recompute pglimit with the widened type so intermediate multiplication (MB → bytes → pages) cannot overflow on 32‑bit intermediates.

That explicit u64 cast forces the arithmetic into a safe width and ensures the check against pglimit actually bounds the allocation as the author intended. Multiple distro trackers and CVE databases summarized the change in that way when the patch was merged.
Note: the fix is a classic example of “preventative typing” — using the correct integer width to match the arithmetic range rather than trying to micro‑adjust the checks elsewhere.

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Affected kernels and distributions​

Different vulnerability databases and distributors mapped the commit and produced lists of impacted kernel releases; the attack surface is broad because udmabuf is included in many stable kernels and backported into enterprise distributions:

• Public trackers list multiple kernel series as affected across a wide version range — broadly kernels in the 5.x and early 6.x families received the fix in their stable trees. One popular CVE aggregation lists affected ranges covering numerous stable kernels (for example: kernel versions between early 5.x and some 6.x releases before fixed revisions).
• Major Linux vendors published advisories mapping the upstream fix into distro packages. Oracle, Red Hat and SUSE posted errata or security advisories that incorporated the upstream patch into their kernel builds; Debian and other distributions likewise listed the CVE and release‑level fixes. Administrators should consult their vendor advisories for the exact package and errata identifiers that apply to their deployments.

Because kernel packaging and backporting differ across distributions, the safest operational approach is to follow the vendor’s advisory and apply the vendor‑supplied kernel update for the exact kernel package in use.

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Exploitability: is there a public PoC?​

As of our review, there is no widely published public proof‑of‑concept that weaponizes CVE‑2025‑37803 into a reliable privilege escalation chain, and no credible claims of widespread active exploitation tied to this CVE were present in the major trackers at the time of writing. Most public writeups and trackers describe the impact as memory corruption leading to denial‑of‑service when triggered by local users. Because this is a local‑access kernel bug affecting a module intended to be accessible from user space, the practical risk model is: low difficulty to cause a crash for an attacker with local access, but higher difficulty to gain remote code execution without additional kernel flaws. Administrators should treat the absence of a public PoC cautiously: lack of public exploit code does not imply the issue is low risk.
(We flagged the absence of public exploit artifacts because many CVE entries explicitly note exploit status; here the record suggests denial‑of‑service potential with unknown public exploit availability. If your environment allows untrusted users to interact with udmabuf, assume exploitability for availability impact and act accordingly.)

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Detection and indicators of compromise (what to look for)​

Because the most likely outcome of exploitation is kernel instability or oopses, the primary detection indicators are operational and forensic rather than network signatures:

• Kernel logs and oops messages. Watch dmesg, /var/log/kern.log and systemd journal entries for kernel oopses, warnings from the udmabuf driver, or page‑faults occurring near the udmabuf codepath. A crash that correlates to user activity opening/creating udmabuf devices is a red flag.
• Abnormal allocation or page‑related warnings. Messages mentioning page allocation failures, under/overflows when creating udmabuf devices, or direct references to 'udmabuf' in the call trace are strong indicators.
• Unusual use of /dev/udmabuf* device nodes. Audit which processes are opening or mmap()ing udmabuf devices. Where possible, enable auditing (auditd) rules to log opens and mmap operations on udmabuf device nodes for high‑sensitivity hosts.
• Container/VM escape attempts that call into udmabuf. Because udmabuf is useful to user‑space drivers and emulator/lab tooling, containment boundaries that expose udmabuf to untrusted workloads increase risk. Look for job/service processes that should not interact with kernel DMA buffers opening these devices.

Create detection rules that correlate process names, user IDs, and device accesses to reduce false positives. If a kernel oops is reproducible by a benign workload that opens udmabuf, capture a crash dump and orphaned stack traces to aid triage.

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Mitigations and remediation (practical steps for admins)​

• Patch: the only complete remediation is to install the vendor kernel update that includes the upstream udmabuf fix. Vendors (Debian, Oracle, Red Hat, SUSE, Ubuntu and others) have integrated the upstream patch into their kernel errata; follow your vendor’s guidance and replace kernels with patched builds appropriate for your distribution and kernel flavor.
• Temporary mitigations if you cannot patch immediately:
• Unload and blacklist the udmabuf module. If your workload does not require udmabuf, remove it with:
• rmmod udmabuf (or rmmod u-dma-buf depending on your build)
• add a blacklist entry to modprobe.d to prevent reload
  This prevents user processes from accessing the udmabuf device until you can install a patched kernel. The tradeoff is loss of any functionality that relied on user‑space DMA buffers.
• Tighten device node permissions. Limit access to /dev/udmabuf* via udev rules so only trusted system services (and not arbitrary unprivileged users or container workloads) may open them.
• Use SELinux/AppArmor profiles to constrain programs that would otherwise open udmabuf device nodes.
• For containerized environments, ensure workload isolation does not give untrusted containers access to host device nodes providing udmabuf.
• Audit and access control:
• Identify hosts that expose udmabuf devices (look in /dev and /sys/class for udmabuf devices).
• Use ephemeral monitoring to log and alert on access to these nodes.
• If your environment uses automated orchestration, ensure node images are updated and orchestration overlays remove udmabuf access where unnecessary.
• Test patches before deployment. Kernel updates can have operational impact. Stage the vendor kernel updates in a test environment that mirrors production I/O and user workloads to validate stability, then schedule a rolling kernel upgrade policy.

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Developer guidance and code hygiene takeaways​

This CVE is a textbook demonstration of how integer width and casting issues remain dangerous in low‑level systems code:

• Use explicit wider types for arithmetic that multiplies or scales values (MB → bytes → pages). Prefer u64 for sizes and widths where the platform can reasonably allocate large memory ranges, and apply helper macros for conversions (e.g., MB_TO_BYTES()) that force the right width.
• Validate and assert bounds before allocation. Do not rely on implicit integer promotions — make the casts and checks explicit and unit tested.
• Add unit tests that run edge cases for size_limit_mb and related configuration values, including maximums and negative/zero values, and exercise the path that calculates pglimit so conversion mistakes are caught by CI rather than later in the field.
• When a change is made, add a short comment calling out why the cast is necessary; these comments prevent future regressions when maintenance touches the same area.
• Consider instrumenting selftests to exercise both the vmap/mmap and device creation paths for udmabuf so future contributors have a regression baseline.

Multiple contributors and maintainers discussed cleanup and adjustments to the udmabuf code in 2025 kernel trees; the upstream patch list includes multiple targeted commits to harden the creation path and remove unneeded code. When backporting fixes, maintainers often send smaller defensive patches (like the explicit cast) to stable branches; vendors then package those stable commits into distro kernels.

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Practical checklist for defenders​

• Inventory: scan your fleet for udmabuf presence (look for /dev/udmabuf* or /sys/class/udmabuf). Tag machines that expose the device nodes.
• Prioritize: prioritize hosts where untrusted local users, CI runners or container workloads run, because those are easiest to exploit.
• Patch: map each affected host to the vendor kernel advisory and schedule updates. If you use a supported enterprise kernel (RHEL/Oracle/SUSE/etc.), use the vendor errata identifiers in your vendor portal to pull the correct packages.
• Temporary mitigation: if immediate patching is impossible, unload/blacklist the module and restrict access to the device node. Monitor logs for udmabuf activity and kernel oopses.
• Post‑patch validation: after updating, exercise workloads that use udmabuf functionality and monitor for stability regressions and kernel warnings. Keep crash dumps for any crashes that occur post‑patch to help vendor triage.

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Broader context: why small kernel fixes are important​

Kernel drivers like udmabuf operate at the boundary between user space and hardware; they are intentionally accessible to user processes to reduce the complexity of full kernel driver development. That accessibility makes bugs uniquely consequential: even a trivial arithmetic error can change a bounds check into a bypass that yields memory corruption inside kernel space.
CVE‑2025‑37803 is not an argument for chasing exotic attack types — rather, it is a reminder of classic software engineering discipline: proper typing, explicit conversions and defensive checks in systems code matter. The fix itself was simple, but until applied the operational implications could be severe for any system that exposes udmabuf to untrusted processes. Multiple vendors have published advisories and integrated the fix into their kernel streams, and operators should treat this bug as a high‑priority patching item where udmabuf is present.

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Final assessment and recommendations​

CVE‑2025‑37803 is a high‑impact, locally triggerable kernel vulnerability rooted in integer width/cast arithmetic during udmabuf creation. The technical fix is simple — cast size_limit_mb to a 64‑bit width when computing pglimit — but the operational risk is substantial because the bug lives in kernel space and can lead to memory corruption and denial of service. Multiple trackers (NVD, Debian, SUSE and vendor advisories) catalog the issue and indicate that patched kernel builds are available from vendors. Administrators should:

• Treat this as a patch‑now vulnerability for any host that exposes udmabuf to untrusted users or workloads.
• If patching cannot be immediate, remove or blacklist the udmabuf module and restrict device node access as a stopgap.
• Audit for device exposure and set up short‑term detection (dmesg monitoring, auditd rules on /dev/udmabuf*) so attempts to trigger the condition are visible.
• For kernel developers: add explicit typed arithmetic checks and edge‑case tests to prevent similar cast/overflow issues in future.

This CVE underlines the fact that small code changes in the kernel can produce large consequences — and that rapid, conservative mitigation and patching are the right responses when a kernel‑level bounds check has been compromised.

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Conclusion: apply vendor kernel updates that include the upstream udmabuf fix, or — where that is not immediately possible — disable udmabuf and harden access to the device nodes; monitor kernel logs for related oopses and prioritize hosts with untrusted local workloads for remediation.

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Source: MSRC Security Update Guide - Microsoft Security Response Center