CVE-2026-53083: Linux Kernel BPF RCU Stall DoS Vulnerability

Technical summary

CVE-2026-53083 addresses a missing cond_resched() call in bpf_fd_array_map_clear() within the kernel's BPF array map implementation. The vulnerability manifests when clearing PROG_ARRAY maps containing many entries. The cleanup path invokes prog_array_map_poke_run() for each entry, an expensive operation that updates BPF program jump tables. Without scheduler preemption points, this loop can run uninterruptibly long enough to trigger RCU stalls—a kernel liveness condition where the RCU state machine becomes blocked, generating stack dumps and potentially causing system instability. The fix is a single cond_resched() insertion to allow the kernel to check for pending work and yield if necessary.

Business impact

This vulnerability poses a localized denial-of-service risk to systems running untrusted BPF programs or exposing BPF syscalls to unprivileged users. Organizations leveraging BPF for container runtimes, network packet filtering, or observability tooling may face unexpected system hangs during cleanup operations. The impact is limited to availability (no data corruption or privilege escalation path disclosed), but affected services could require forceful restart, disrupting dependent workloads.

Affected systems

The Linux kernel is affected, with the vulnerability present in the BPF subsystem's array map handling code. Impact scope depends on kernel configuration: systems with CONFIG_BPF enabled are susceptible if BPF syscalls are exposed to non-root users (common in containerized and modern userspace-driven networking scenarios). Embedded systems, data centers running Kubernetes, and edge platforms using eBPF-based monitoring or networking are typical deployment contexts.

Exploitability

Exploitation requires local access and the ability to invoke BPF syscalls—typically available to unprivileged users on modern distributions, though some hardened configurations restrict this. No remote attack vector exists. An attacker would need to create or manipulate a PROG_ARRAY map with a large number of entries and trigger its deletion, causing the kernel to stall. The complexity is low once BPF access is available, making this a practical concern in multi-tenant or hostile-user scenarios.

Remediation

Apply the upstream kernel patch that adds cond_resched() to the bpf_fd_array_map_clear() function in kernel/bpf/arraymap.c. This allows the scheduler to preempt the cleanup loop and prevents RCU stalls. Verify the patch against your vendor's advisory, as backport details may vary. Interim mitigation: restrict BPF syscall access via LSM policies or seccomp on systems where unprivileged BPF use is not required.

Patch guidance

Patch availability depends on your Linux distribution. Check your vendor's security advisory for targeted kernel updates. Upstream, the fix has been integrated into mainline after the initial report. For enterprise distributions (RHEL, Ubuntu, Debian, SUSE), security updates should be released through standard package channels. When patching, ensure your BPF programs are tested post-update, as this fix only adds a scheduling point and should not alter program behavior. Kernel reboot is required to activate the patched code.

Detection guidance

Monitor kernel logs for RCU stall warnings (rcu: CPU ... detected stall) coinciding with BPF workqueue activity, particularly prog_array_map_clear_deferred entries. Kernel perf or ftrace can capture bpf_fd_array_map_clear() execution time; sustained durations without preemption suggest vulnerability exploitation. On affected systems, instrument BPF syscall access and monitor for repeated BPF_MAP_CREATE and BPF_MAP_DELETE_ELEM operations on PROG_ARRAY types with abnormally large counts. SELinux or AppArmor audit logs can reveal unprivileged BPF syscall invocation.

Why prioritize this

This issue should be patched promptly in production environments, especially those relying on BPF for network or observability functions. The fix is surgical and low-risk, carrying minimal compatibility burden. However, it is not an immediate critical emergency given the localized impact and access requirements. Prioritize systems exposing BPF to untrusted users or those running long-lived BPF programs that manage many entries.

Risk score, explained

CVSS 5.5 (Medium) reflects the local-only attack requirement (AV:L), low attack complexity (AC:L), and the need for user-level privileges (PR:L). The impact is exclusively availability (A:H), with no confidentiality or integrity loss. The score appropriately calibrates this as a moderate risk requiring attention but not an emergency affecting systems with strict privilege boundaries.

Frequently asked questions