A subtle memory leak in the Linux kernel’s udmabuf driver — tracked as CVE-2024-56712 — has been closed by a small, surgical change to the export path that prevents orphaned dma_buf objects when file-descriptor assignment fails; the bug is low in severity but meaningful for systems that repeatedly hit FD limits or that expose udmabuf to untrusted local code.
Background / Overview
udmabuf is a lightweight kernel driver that lets user-space allocate and export memory as a DMA buffer (dma_buf) so devices — GPUs, FPGAs, and other DMA-capable hardware — can access user memory directly. The driver exposes an export path used when user-space asks the kernel to hand out a file-descriptor that represents the dma_buf. That FD creation step uses dma_buf_fd, which in turn interacts with the process’s FD table. The CVE describes a narrow error-path bug: when export_udmabuf successfully creates the dma_buf object but dma_buf_fd fails (for example, because the process’s file-descriptor table is full), cleanup code in udmabuf_create tear-downs the udmabuf but
does not release the already-created dma_buf. The result is a dma_buf instance left in kernel memory holding a dangling pointer; the practical outcome reported by multiple trackers is a memory leak rather than corruption or privilege escalation. Vendors and trackers characterize the impact as availability/resource-exhaustion (local Denial-of-Service) rather than confidentiality or integrity compromise: repeated triggering can accumulate kernel memory pressure in long-lived or aggressively looped scenarios. The issue was fixed upstream by reordering the FD-assignment logic so that creation and FD assignment can be reliably cleaned up together.
What the bug actually did (technical anatomy)
The vulnerable control flow, in plain language
- The driver creates a dma_buf wrapper for the udmabuf.
- It then attempts to give that dma_buf a file descriptor using dma_buf_fd, which returns an FD number or a negative error.
- If dma_buf_fd fails after dma_buf exists, the current udmabuf error path would free udmabuf resources but not drop the dma_buf reference.
- The dma_buf remains allocated in kernel space even though the user-space operation ultimately failed — leaking kernel memory.
This is not a use-after-free or memory-corruption bug; it’s a lifecycle/cleanup omission. The leak exists only on the last error path where the FD assignment fails after dma_buf creation, which makes the exploit/trigger window narrow but real under specific conditions (e.g., artificially filling FD tables or racey loops calling the export IOCTL).
The fix (what changed in the code)
Upstream maintainers moved the dma_buf_fd call out of the part of the code that performs udmabuf teardown. By separating object creation from FD allocation, they made it possible to treat the two operations with distinct cleanup semantics: if dma_buf_fd fails, the code now calls dma_buf_put to release the dma_buf immediately instead of leaving it behind. The change is intentionally small and defensive — the kind of minimal rework kernel maintainers prefer for stable backports. A representative before/after pseudocode clarifies the intent:
- Before:
- create dma_buf
- fd = dma_buf_fd(dma_buf)
- if fd < 0 -> goto err (error path that tears down udmabuf but not dma_buf)
- After:
- create dma_buf
- fd = dma_buf_fd(dma_buf)
- if fd < 0 -> dma_buf_put(dma_buf); goto err
This ensures that the dma_buf reference count is decremented when the FD assignment fails, preventing orphaned kernel allocations.
Impact assessment: who should care and why
A narrow but operationally relevant defect
- Attack vector: Local processes that can call the udmabuf export ioctl or otherwise reach the udmabuf interface. The vector is not remote.
- Privileges required: Typically low to local privileges depending on device permissions; on many systems /dev/udmabuf access is restricted to privileged users, but that is a configuration point.
- Primary impact: Availability via kernel memory leak. Repeated or automated triggering can elevate slab and kernel memory usage until performance degrades or OOM behavior appears.
- Escalation / data risk: There is no authoritative public evidence that this leak enables privilege escalation, information disclosure, or arbitrary code execution. Multiple vulnerability trackers explicitly describe the outcome as a leak/availability issue.
Environments at elevated risk
- Long-running servers with aggressive automation or untrusted local workloads (CI runners, multi-tenant build hosts).
- Embedded appliances and vendor kernels where udmabuf might be exposed and whose kernels receive slow or irregular backports.
- Test farms or device labs that repeatedly create and destroy udmabufs as part of automated test harnesses.
These environments are more likely to see repeated error conditions or richer attack surfaces that produce measurable accumulation of leaked kernel objects. Community analysis around similar kernel cleanup leaks underscores how a small leak can accumulate into an operational issue when triggered repeatedly.
Affected versions and vendor response
Public vulnerability trackers map the issue to upstream Linux kernel commits and list affected kernel ranges prior to the stable commit that merged the fix. Distribution security trackers (Debian, Ubuntu, SUSE) and centralized repositories recorded the CVE and listed which packaged kernels are patched versus still vulnerable, with backports provided into stable trees where maintainers considered it safe to do so. Debian’s tracker, for example, identifies fixed versions across its stable/unstable branches and marks later kernels as containing the backported fix. A few operational notes from the vendor landscape:
- Upstream: small fix merged to stable kernel branches; the patch is small and low-risk.
- Distributions: most major distros ingested the upstream change and listed fixed kernel packages; check your distro’s specific advisory for exact package names and versions.
- Embedded/OEM kernels: these are the long tail. Many embedded or OEM providers do not backport promptly, so devices may remain exposed longer even after upstream fixes are published. Community analysis cautions operators to track vendor notices for long-tail devices.
Practical mitigation and remediation playbook
Immediate priorities are straightforward: patch, limit exposure, and monitor.
1. Patch (highest priority)
- Identify systems running kernels that include the vulnerable udmabuf code path. Use uname -r and your distribution’s security tracker to map package versions to the fix.
- Apply vendor kernel updates or install an upstream kernel that includes the fix. For production fleets, use your normal staged rollout process: pilot → validate → full rollout.
- For embedded devices, contact OEMs for vendor-signed images or apply in-house backports if you maintain device kernels.
Why patch? The fix is small and safe; it eliminates the leak and has minimal regression risk when backported.
2. Short-term compensations (if you cannot immediately patch)
- Restrict access to /dev/udmabuf or any device node tied to the udmabuf interface. On most systems device access is the critical gate.
- Apply FD limits to untrusted processes: tune /etc/security/limits.conf or systemd unit directives to reduce how many FDs a process may open, making it harder to hit the FD-table-full condition that triggers the path.
- Harden container/tenant configurations so unprivileged workloads do not gain direct access to host device nodes.
These steps reduce the likelihood of hitting the narrow error path that produces the leak.
3. Detection and monitoring
- Monitor kernel slab usage and kmemleak reports on test systems where you can safely enable kmemleak or kernel memory diagnostics. A steady, unexplained rise in dma_buf allocations correlated with udmabuf ioctl activity is a signal.
- Add log alerts for repeated export/ioctl failures from processes interacting with the udmabuf device.
- On systems with eBPF-based telemetry, instrument kernel allocation counters or fd-related syscall patterns to detect suspicious loops that repeatedly attempt to export udmabufs. Community guidance on leak detection emphasizes slab trends and repeated error traces as high-signal indicators.
4. Audit & inventory (operational hygiene)
- Inventory which hosts have udmabuf enabled or expose /dev/udmabuf; many kernels build udmabuf as a module and it may not be present on all systems.
- For devices where udmabuf is required for legitimate workflows, plan for a scheduled kernel update window and test flows that exercise device DMA paths to confirm no regressions.
Risk analysis — strengths and residual risks
Strengths of the fix
- The change is small and defensive: moving dma_buf_fd to a place where failures can be handled with an immediate dma_buf_put is low-risk and easy to backport to stable kernel branches. That makes it practical for distribution maintainers and OEM integrators to apply quickly.
- The vulnerability class (cleanup omission) is well understood and does not indicate a systemic design flaw in DMA buffering semantics — it’s a lifecycle bug fix rather than a protocol redesign.
Residual risks and caveats
- Distribution/backport lag: not all vendors or OEMs will push the patch at the same cadence. Embedded vendors and custom kernel forks are the highest residual risk. Operators must verify package changelogs rather than assume every kernel version in the wild is patched.
- Detection difficulty: leaks can be stealthy in noisy environments. A single leaked dma_buf is small; it’s repeated, automated triggers that create operational pain. Monitoring must therefore focus on trend detection rather than single-event telemetry.
- Limited attack surface reduction via FD limits: while limiting FD counts can make the specific failure path harder to reach, it is not a substitute for upstream patching and should only be treated as a temporary mitigation.
Operational checklist (one-page runbook)
- Inventory: list hosts with udmabuf present (modinfo udmabuf; ls /dev/udmabuf).
- Map: consult your distro security tracker for CVE-2024-56712 fixed package names and versions.
- Patch: upgrade kernel packages or install a patched upstream/stable kernel. Stage rollout using canary hosts that exercise DMA device workflows.
- Mitigate (if patching delayed): restrict device-node access, reduce FD limits for untrusted users, and sandbox processes that might call the export IOCTL.
- Detect: add alerts for (a) increase in dma_buf slab usage, (b) repeated export_udmabuf ioctl failures, and (c) kmemleak traces on test systems.
- Validate: after patch, test udmabuf export success/failure paths and confirm no lingering dma_buf allocations under a simulated FD-table-full failure scenario.
Final verdict and conclusion
CVE-2024-56712 is a classic kernel cleanup omission: small in scope, low in severity, but operationally relevant. The fix is straightforward and low-risk — a reordering so that dma_buf references are released when FD assignment fails — and is safe to backport into stable kernel trees. Administrators should treat this as a routine but necessary kernel update: prioritize patches on multi-tenant or long-running systems, confirm distribution backports for embedded or vendor devices, and add simple detection rules to catch repeated failed exports that could indicate leakage in practice. The lesson remains the same that kernel developers and operators have emphasized: even small, rare error-path mistakes can create persistent operational risk over time. Timely patching and sensible device-exposure controls eliminate both the leak and the need for any further workaround.
Source: MSRC
Security Update Guide - Microsoft Security Response Center