CVE-2024-0775: ext4 remount use-after-free explained

A subtle memory-management bug deep inside the ext4 remount path—tracked as CVE-2024-0775—can turn routine mount option changes into a kernel-level use-after-free, enabling a local attacker to crash systems or leak kernel memory if left unpatched.

Background​

ext4 is the default filesystem for many Linux distributions and is widely deployed across servers, desktops, and embedded devices. Its remount logic (the code path that handles mount option changes without a full unmount/mount cycle) is intended to be robust, but it must also carefully manage transient state when options change. CVE-2024-0775 was discovered in that remount logic: the vulnerable code lives in __ext4_remount within fs/ext4/super.c and involves freeing quota-related resources at the wrong time during an option-change failure path.
In practical terms, the kernel sometimes needs to roll back to the old mount options if an attempted remount fails. During that rollback, the ext4 code was freeing the names for old quota files before the code finished restoring other state, resulting in a use-after-free: subsequent code could still dereference those now-freed pointers. The immediate operational outcomes reported by vendors and maintainers are availability impact (kernel oops/crash) and potential information disclosure (reading freed kernel memory).
Multiple distributions and vendors issued fixes and advisories after the upstream kernel patch landed; operators who manage Linux systems should treat this as an availability-first issue that also carries confidentiality risk in some exploit scenarios.

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What exactly went wrong: technical overview​

The remount sequence and why it’s fragile​

When user space asks the kernel to change mount options via remount (for example, switching between read-only and read-write, toggling quota options, or changing data= modes), the filesystem must:

• Parse and apply new mount options,
• Initialize any resources required by the new configuration (e.g., quota structures, DAX state, system-zone metadata),
• Validate feature compatibility (inodes, group descriptors, checksums),
• On failure, reliably restore the previous, working configuration.

That last requirement—restoring the previous state—creates nested lifetimes and rollback responsibilities. If rollback code frees resources too early or fails to re-establish references, the remainder of the remount/rollback can end up dereferencing pointers into freed memory.

The vulnerable pattern in __ext4_remount​

CVE-2024-0775 manifests when the kernel frees old quota file name buffers while a later step in the rollback path can still reference them. Key elements of the failure mode:

• The code path is executed during remounts that change options affecting quotas or read-write state.
• On an error (for example, failing to switch back to read-write because of a group descriptor checksum verification failure, or an explicit option-incompatibility), ext4 walks a series of cleanup labels to undo partial work.
• One of those cleanup steps frees quota-related name buffers or otherwise releases quota-managed structures before the rollback completes.
• Later rollback steps can still call into quota-handling routines (dquot or dquot_resume variants) that expect those names to remain valid, creating a use-after-free.

This combination is classic kernel lifecycle mismanagement: resource deallocation occurring while pointers to that resource remain accessible on an error-recovery path.

Consequences of a use-after-free in the kernel​

A use-after-free inside the kernel is dangerous because it allows:

• Deterministic or probabilistic system crashes (kernel oops, panic) — an availability impact that can be sustained if triggered repeatedly.
• Potential disclosure of kernel memory, because freed kernel memory may contain sensitive data and can be read back out if the dangling pointer is used to copy data to user space or to other kernel buffers in an exploitable way.
• In some memory-corruption cases (not necessarily reported for this CVE), arbitrary code execution paths can be constructed, although ext4 remount logic here has been characterized as an information-leak/DoS vector rather than a direct remote code execution path.

Importantly, exploitation requires local access to the vulnerable system. The typical exploitation model is a local user or process that can trigger a remount operation (or otherwise cause the kernel to change mount options), so attackers with local privileges or guest VM users on shared hosts are the primary risk profiles.

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Who and what is affected​

• The vulnerability occurs in the ext4 filesystem implementation within the Linux kernel. In upstream terms, kernels before the patch release (fixed in the 6.4 development cycle) are affected; vendors backported fixes into their stable kernel trees and released distro patches.
• Major Linux vendors and distributions listed affected builds and issued updates; historically, ext4 bugs like this are present across a wide range of kernels because ext4 is in the mainline and stable trees and because distributions maintain various kernel branches by backporting patches.
• Several enterprise distributions released advisories and kernel updates to remediate the issue; some vendor advisories noted the vulnerability by its CVE and described the patched kernel packages and live-patch updates.

Because ext4 is so widely used, administrators should assume servers, cloud instances, virtual machines, and embedded devices running older or unpatched kernels could be impacted. The vulnerability is local in scope: attack requires the ability to trigger a remount or similar filesystem operation on that host.

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Severity and scoring — a note about differing metrics​

Public vulnerability listings show some variability in scoring and phrasing:

• Several vulnerability databases reported the base CVSS v3.1 score in the mid-to-high range (commonly 6.7 to 7.1), reflecting a High/Medium impact classification driven by confidentiality and availability effects.
• The NVD entry and vendor advisories provide more conservative language: the primary operational consequences are denial-of-service (kernel crash) and information leakage; no authoritative public reports at disclosure time proved remote or unauthenticated exploitation beyond the local context.

Two practical takeaways from these variations:

• Treat CVE-2024-0775 as an availability-first risk with realistic confidentiality implications in certain local-privilege scenarios.
• Prioritize patching and mitigations in environments where local privilege is available to many users (multi-tenant hosts, shared servers, developer workstations) or where high-availability is essential.

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Vendor responses and patches​

Kernel maintainers fixed the regression in the upstream tree; the fix was propagated to stable kernel trees and then packaged by major Linux distributions.
What operators should expect and do:

• Upstream: the kernel fix was merged into the appropriate branch in the 6.4 development cycle. Distribution vendors backported the fix into their stable kernels and released security updates.
• Distributions: enterprise vendors and popular distributions issued advisories and listed patched kernel package versions and live-patch updates. Where live-patching was supported, vendors provided live-patch bundles to remediate running systems without immediate reboot in supported configurations.
• Cloud and appliance vendors: some public cloud images and appliance kernels were updated; customers should check provider-specific security bulletins or patch feeds for CVE-2024-0775 coverage.

Administrators should obtain and apply the vendor-supplied kernel updates for their distribution and kernel ABI (packages or live-patches), and validate that the running kernel version now contains the remediation changes.

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Practical mitigation and remediation guidance​

If you manage Linux systems, follow this plan. The steps are ordered and pragmatic for production environments.

1. Inventory — Quickly identify vulnerable hosts
   • Enumerate kernel versions across your fleet. Any kernel built from upstream trees older than the patched commit(s) is potentially vulnerable unless the distribution backported a fix.
   • Prioritize systems running ext4 as the root or large data filesystem and machines with multiple untrusted local users.
2. Patch — Apply vendor updates immediately (recommended)
   • Install vendor security updates for the kernel and apply live-patches where offered and supported.
   • For systems that cannot be rebooted immediately, apply live-patch updates if your distribution provides them and they cover the relevant kernel branch.
3. Test — Validate before broad rollouts
   • In a staging environment, boot patched kernels and run workload tests that stress mount/remount operations, quota manipulations, and remounts under error conditions, to ensure compatibility.
   • Confirm that services relying on the filesystem function normally and that no regression was introduced by the patch.
4. Short-term controls where patching is delayed
   • Limit local account issuance and shell access: prevent untrusted users from obtaining accounts that could perform remounts or manipulate mount options.
   • Harden container / virtualization environments: restrict guest capabilities that allow remount syscalls (for example, disallow CAP_SYS_ADMIN in containers unless strictly required).
   • Use access controls (sudoers, SELinux, AppArmor) to prevent unprivileged escalation that might allow triggering remount operations.
5. Monitoring and detection
   • Monitor kernel logs for ext4 warnings and oops messages tied to remount failures or quota operations.
   • Use endpoint detection to look for repeated remount attempts or unusual quota file manipulations.
   • Investigate any unexplained kernel crashes on ext4 systems occurring near mount option changes; treat them as potentially exploit attempts if unpatched.
6. Audit and harden operations
   • Review automated processes that perform remounts (configuration management, auto-scaling, system orchestration) to ensure they use updated kernels or have mitigations applied.
   • Where possible, prefer full unmount/remount sequences for risky option changes during maintenance windows, rather than in-place remounts from untrusted automation.

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Detection and forensics​

Because the vulnerability is local and often exercised via standard kernel operations (mount/remount), detecting exploitation requires a combination of reactive and proactive signals.

• Kernel logs: look for 'ext4' messages, mount failure logs, and kernel oops/panic traces showing call stacks pointing to fs/ext4/super.c or __ext4_remount. Remount failures followed by oops are a strong indicator of attempted exploitation or an unhandled error path that could be CVE-related.
• Process traces: identify processes invoking remounts or calling mount(2) with MS_REMOUNT flags. Automated services or scripts that perform remounts without explicit operator action are higher risk.
• Auditd and syscall auditing: enable auditing of mount-related syscalls and privileges to catch suspicious local activity that attempts to change filesystem mount options.
• Memory forensic artifacts: a use-after-free exploit sometimes leaves artifacts such as repeated kernel oops pointing to similar addresses or re-used freed-slots; correlate crash dumps across systems to look for patterns.

If you encounter a suspected exploitation event, collect a kernel crash dump (kdump) or capture the oops log immediately, preserve the system state, and escalate to security engineering with the crash artifacts.

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Risk analysis: who should worry most?​

• High-risk: multi-tenant hosts, shared developer or CI machines, and cloud VM hosts where untrusted users or less-isolated guests can request remounts or manipulate file system settings.
• Medium-risk: production database and application servers where ext4 is used and local user access is tightly controlled but where accidental or automated remounts could occur.
• Lower-risk (but not safe): isolated single-user desktops without exposed local accounts; however, operators should not rely on isolation and should still patch, because availability can be critical and kernel vulnerabilities can be used as pivot points.

The most compelling operational risk is sustained denial-of-service: repeated or automated triggers of the use-after-free can make systems unstable until patched or rebooted. The confidentiality risk—reading freed kernel memory—is less certain but non-negligible; attack scenarios where freed quota name buffers are exfiltrated to user space remain plausible under certain conditions.

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Testing and validation checklist for operators​

• Confirm the patched kernel ID or vendor package is installed; distribution advisories list package names and fixed versions—validate the exact package set.
• Reproduce a safe remount workflow on a patched staging host; ensure remounts that previously failed now rollback cleanly without oops.
• Run filesystem stress tests that exercise quota toggling, remounts between RO/RW, and option changes that previously exercised failing code paths.
• Validate automated orchestration agents (Ansible, Chef, Puppet) do not temporarily enable unsafe mount operations during rolling updates.

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Developer and maintainer perspectives​

This class of bug is instructive for kernel developers and maintainers: rollback and error-handling paths must be designed with the same rigor as success paths. Resource lifetimes should be clearly bounded and protected across all possible exit points—especially when option parsing, quota handling, or system-zone initialization can allocate temporary structures that are referenced elsewhere during cleanup.
Common hardening approaches include:

• Using reference counting or explicit ownership models to ensure resources are only freed when no dangling references exist.
• Prefer RCU or synchronized accessors where read-mostly structures can be accessed concurrently with remount operations, and ensure frees happen after a grace period if reads can occur concurrently.
• More defensive error-handling: rollback should re-establish conservative defaults first before releasing any resources that later code could touch.

Maintainers also weigh the cost of backporting fixes into long-term stable kernels: ext4 changes must be conservative to avoid regressions, so vendor backports often accompany extensive testing before being shipped in production kernels.

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Closing analysis: strengths, weaknesses, and operational advice​

CVE-2024-0775 is not a glamorous remote RCE, but it is a practical, real-world kernel memory-safety bug with clear consequences:

• Strengths of the response: maintainers responded with an upstream fix and major distributions backported patches and released updates — the normal kernel-security lifecycle worked as intended.
• Ongoing weaknesses: the vulnerability underlines the persistent class of lifecycle bugs in kernel subsystems that only show up under error-recovery or rare remount conditions. These code paths are legitimately complex and easy to exercise via automated testing tools, yet they continue to produce high-impact findings when they fail.
• Operational advice: treat this CVE as a patch-now issue for systems where local privilege is available to untrusted users or where availability is critical. For other systems, schedule updates into the next maintenance window but adopt the short-term mitigations described above.

In short: do not underestimate remount logic. Apply vendor kernel updates promptly, monitor logs for ext4 remount warnings or crashes, and harden local access controls to reduce the attack surface until every system in your environment is running a patched kernel.

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Source: MSRC Security Update Guide - Microsoft Security Response Center