The Linux kernel received a targeted robustness fix for a device‑mapper ioctl bug tracked as CVE‑2024‑23851: a missing check in copy_params (drivers/md/dm-ioctl.c) could let an ioctl request lead the kernel to try to allocate more than INT_MAX bytes and crash, producing a local denial‑of‑service on affected kernels through 6.7.1.
Background / Overview
Device‑mapper (dm) is the kernel subsystem that implements logical block devices used by LVM, encrypted volumes (dm‑crypt), thin provisioning, multipath, snapshots and many distribution‑level storage facilities. Userspace tools communicate with the device‑mapper control interface — typically /dev/mapper/control — via the dm_ctl ioctl paths. Those ioctls are handled in kernel source under drivers/md/dm-ioctl.c and funnel through a central dispatcher called ctl_ioctl, which copies and validates user parameters, then invokes the requested control function.
CVE‑2024‑23851 concerns the copy_params helper that copies dm ioctl parameters from userspace into kernel memory. A missing validation of param_kernel->data_size in certain code paths could allow a crafted ioctl to request an enormous parameter buffer — large enough that the subsequent allocation attempt overflows or exceeds INT_MAX, producing an incorrect allocation size or a failed allocation that causes a kernel panic/oops. The weakness is local in nature (the ioctl is exposed to processes that can talk to the device mapper control device), and mitigations / fixes were applied upstream and backported by multiple distributions.
What exactly went wrong (technical deep dive)
Where the bug lives: ctl_ioctl → copy_params
- The userland-to-kernel control flow starts in dm_ctl_ioctl / ctl_ioctl, which first verifies the caller’s privileges and requested command, then calls copy_params to grab the ioctl input buffer into kernel space for validation and processing. If copy_params returns success, the kernel then validates the parameters and calls the requested handler.
- The vulnerable function, copy_params, computes how much kernel memory is required to hold the structure and any appended parameter area (param_kernel->data_size). That value comes from userspace via the dm_ioctl structure, which means a malicious or buggy userspace component can control it. If the code does not properly validate param_kernel->data_size before using it to size a kvmalloc/kcalloc call, it can attempt to allocate extremely large memory (more than INT_MAX bytes). On 32‑bit arithmetic paths, or where size calculations overflow, this can collapse to a small allocation and subsequent writes will cause memory corruption or a kernel oops; on 64‑bit builds it can still cause a failed allocation that the surrounding code does not handle gracefully, producing a crash.
The numerical problem: INT_MAX and allocation math
- The kernel API and many internal data types assume sizes fit in signed 32‑bit ranges for certain operations. Attempting to allocate more than INT_MAX can either overflow arithmetic used to compute the allocation size or simply exceed allocator expectations, returning NULL. If the code later dereferences the assumedly‑valid buffer, kernel memory is corrupted or the kernel panics. The patch inserted explicit checks that limit the allowed target/parameter counts and the parameter area size before any kvmalloc, thereby eliminating the unchecked path. The upstream device‑mapper tree logged the change as “limit the number of targets and parameter size area.”
Exploitability and impact — who’s at risk?
Local, privileged attacker model
- The device‑mapper control ioctls are explicitly gated by a capability check: callers must be capable(CAP_SYS_ADMIN). In other words, an unprivileged local user without any administrative capabilities cannot reach these ioctls directly. This reduces the practical remote/anonymous exploitation risk considerably for standard, non‑privileged hosts.
- However, the threat model becomes meaningful in environments where non‑root actors are given CAP_SYS_ADMIN (for example, some container configurations, build systems, or privileged helper processes), or where a process with limited privileges can trick a privileged process into issuing a malformed dm ioctl. In multi‑tenant or containerized cloud hosts, these misconfigurations matter.
Availability impact: crash and denial of service
- The vulnerability is an availability issue only — the code path does not expose confidentiality or integrity impacts in the published summaries. A successful trigger will cause a kernel allocation failure or memory corruption that leads to a kernel panic / oops and host crash, producing either a transient (sustained while attack continues) or persistent (host requires manual recovery) loss of availability. Upstream and distribution assessments consistently scored the issue as Medium (CVSS 3.x 5.5) with an Availability‑only impact vector.
Current exploitation evidence
- Public exploitation telemetry and scoring systems show a low likelihood of active exploitation for this CVE. EPSS and exploit trackers report near‑zero exploitation probability and there are no widely reported weaponized exploits in the wild tied to this issue as of the last public records; nevertheless, that does not remove the need for patching in security‑sensitive contexts. OpenCVE and other tracking sources note the attack surface as local and the exploitability as low-to-moderate.
What vendors and distributions did (patching status)
- The upstream kernel device‑mapper tree contains the corrective changes that limit the allowed numbers/sizes and added the missing check before kvmalloc. The commit series landed in the dm tree and was folded into stable kernel releases shortly thereafter; device‑mapper maintainers described the change as limiting target counts and parameter area size.
- Several mainstream distributions published fixes and backports:
- Ubuntu produced stable updates and SRU patches for LTS and interim kernels. The kernel‑team SRU message documents backport details and test notes.
- Amazon Linux issued ALAS advisories with package updates for Amazon Linux 2 and other channels that included the fix. Administrators are instructed to update their kernels via the vendor package manager.
- Debian, Fedora and other downstream vendors also issued updates (Debian security advisories and Fedora bugzilla entries referenced the upstream commit or equivalent backports). These vendors emphasized backporting into their stable trees rather than forcing a full upstream kernel upgrade in every environment.
- If you run older, distribution‑specific long‑term kernels, check your vendor advisories and apply the supplied kernel update or backport. Many vendors released patches as kernel updates to existing supported kernels, so the immediate operational pathway is usually: apply kernel/security updates from your distribution rather than pulling a vanilla upstream tree.
Detection, triage and response guidance
Immediate checklist (quick triage)
- Determine if your kernel is vulnerable:
- Run uname -r to get the running kernel version. Versions at or below 6.7.1 in upstream numbering are listed as in‑scope; vendor‑specific kernels may differ. If you run a vendor kernel, check your vendor advisory.
- Inspect kernel logs for recent panics or oops entries that reference dm or dm‑ioctl.
- Audit which processes have the CAP_SYS_ADMIN capability or are running as root inside containers; focus on containers that grant CAP_SYS_ADMIN, privileged containers, or host helpers that expose device control to less-trusted code.
How to tell whether a crash was caused by this CVE
- Look for kernel oops backtraces touching drivers/md/dm-ioctl.c or descriptions of an allocation failure inside copy_params/get_result_buffer paths. Distribution patch releases commonly include brief changelog text such as “dm ioctl: log an error if the ioctl structure is corrupted” or “limit the number of targets and parameter size area” which can be cross‑referenced with the kernel changelog.
Mitigation steps before patching
- If you cannot immediately update the kernel, reduce the attack surface:
- Ensure only trusted administrators have CAP_SYS_ADMIN. Remove CAP_SYS_ADMIN from containers and processes that do not need it; prefer dropping capabilities and using secure helpers rather than giving full administrative capability to untrusted code.
- Restrict access to /dev/mapper/control and the device‑mapper control interface at the host level; typically the device node is root‑owned, but misconfigured permission or helper daemons can expose it. Confirm device node permissions and that userspace tooling does not inadvertently expose access to untrusted actors.
- Avoid privileged/--privileged containers and drop unnecessary capabilities in container run profiles; use seccomp and stricter profiles to prevent accidental access to the device‑mapper control ioctls.
- These mitigations are defensive; the long‑term fix is to update the kernel.
How to remediate: a step‑by‑step playbook
- Inventory: identify all hosts that run kernels with version numbers at-or-below upstream 6.7.1 or vendor kernels that have not yet applied the dm fixes. Use your OS package manager’s security advisory feed to detect packaged kernel updates.
- Schedule updates: apply vendor kernel updates in a maintenance window. Where vendors backported the fix, the package changelog will list CVE‑2024‑23851 or a description such as “dm ioctl: log an error if the ioctl structure is corrupted.”
- For cloud images or appliances where kernel updates are not immediately available, implement the mitigation checklist above: remove CAP_SYS_ADMIN from containers, restrict privileged containers, and harden access to device nodes.
- Validate: after patching, verify the kernel contains the upstream fix by matching the commit message lines or by confirming the vendor changelog entry. Rebooting into the patched kernel and validating functionality is required for a complete remediation.
- Monitor: keep watch for unusual dm‑related oopses in kernel logs and for vendor advisories that reclassify or further annotate the issue. Continuous monitoring helps in case follow‑on fixes or regressions are identified.
Why this matters operationally — risk analysis
Not all kernel bugs are equal; this one is availability‑first
- CVE‑2024‑23851 is a robustness bug: it is not a remote code execution nor an information leakage issue. Its main operational impact is denial of service via kernel crash. That makes it a high‑impact concern for hosts that must remain continuously available (database servers, hypervisors, multi‑tenant hosts). The CVSS assessment (5.5) and the small EPSS score reflect a lower exploitability profile but a meaningful availability consequence if exploited.
The “privileged local user” caveat
- The capability gating (CAP_SYS_ADMIN) significantly lowers the exposed risk on standard single‑tenant servers. However, cloud and container platforms frequently run many different workloads where untrusted code may find ways to gain or be given elevated capabilities; privileged containers and misconfigured orchestration are recurring root causes for issues that would otherwise be benign on a single‑user server. In short: the presence of CAP_SYS_ADMIN in any less‑trusted process greatly expands the blast radius.
Patchability is a strength... but inertia matters
- The bug was fixed upstream and vendors have backported patches into stable kernels. That makes remediation straightforward in theory. In practice, long‑lived embedded devices, appliances, or custom kernels can lag behind vendor trees; operators of such systems must prioritize those images for kernel updates or else accept residual availability risk. Vendor attestations and advisory processes (for example, Microsoft's Azure Linux attestations or vendor CSAs) are helpful but administrators must verify their own images rather than rely solely on product‑level statements.
Practical recommendations (concise)
- Patch promptly: apply vendor kernel updates that include the dm fix.
- Harden containers: drop CAP_SYS_ADMIN and avoid privileged containers; restrict capabilities to the minimum necessary.
- Limit access: confirm /dev/mapper/control is not exposed to non‑trusted users or containers.
- Monitor: watch kernel logs for dm‑ioctl related oops/backtrace events and proactively scan inventory for vulnerable kernels.
- Verify vendor statements: don't assume a vendor‑level attestation (for example, “Product X includes this upstream component”) means every image you run is safe — check your exact build and kernel package.
Critical appraisal — strengths and residual risks
Strengths
- The fault is in a constrained ioctl path and is gated by a strong kernel capability check (CAP_SYS_ADMIN), limiting the typical exploit surface for unprivileged users.
- The upstream fix is narrow and localized; device‑mapper maintainers applied size limits and added explicit checks, which are low‑risk changes when backported correctly. Upstream device‑mapper logs and distribution SRUs show that the patch was accepted and shipped to users.
- Vendors quickly issued patches/backports for major distributions, meaning remediation is available through standard update channels.
Residual and systemic risks
- Multi‑tenant and containerized platforms are the main remaining concern. If a container or untrusted process possesses CAP_SYS_ADMIN (or can trick a privileged component into performing the ioctl), the bug becomes exploitable and can produce a host‑level outage.
- Some long‑lived appliances or vendors that do not backport fixes promptly may leave installations exposed. Administrators of embedded devices, network appliances, or self‑managed hypervisor hosts should treat kernel updates as high priority.
- While public exploit telemetry is low, the failure mode (host crash) is straightforward and attractive for attackers looking to take down services in targeted local contexts; a determined attacker with local capabilities or privileged container access can reliably cause availability disruption.
Final verdict
CVE‑2024‑23851 is a classic kernel robustness bug with a clear fix: validate user‑supplied parameter sizes before allocating kernel memory. For most single‑tenant servers with conservative capability models, the practical risk is limited because the control path requires CAP_SYS_ADMIN. For cloud, container, appliance, and multi‑tenant environments, the vulnerability is an operational priority: patch promptly, remove unnecessary CAP_SYS_ADMIN privileges from containers and user processes, and monitor for related kernel oops events.
Treat this as an availability‑first remediation: apply vendor kernel updates as soon as feasible and, where immediate patching is not possible, apply the mitigations above to shrink the attack surface and reduce the chance that an untrusted actor can reach the vulnerable ioctl path.
Source: MSRC
Security Update Guide - Microsoft Security Response Center