CVE-2026-89499 Overview
CVE-2026-89499 is a Linux kernel vulnerability in the tracing ring-buffer subsystem. The __rb_get_reader_page_from_remote() function mishandles failures returned by the remote swap_reader_page callback. When the callback returns -EBUSY during a writer/reader race, the kernel continues execution with an unchanged reader ID and rearranges the local page list as if the swap succeeded. This can splice the same page as both the previous and new reader, corrupting kernel data structures.
Critical Impact
A local attacker with low privileges can trigger the race by generating an event storm against a small ring buffer, leading to memory corruption with high confidentiality, integrity, and availability impact.
Affected Products
- Linux Kernel (mainline tracing ring-buffer subsystem)
- Linux Kernel stable branches containing the remote reader page swap logic
- Distributions shipping kernels prior to the fix commits 5eab74874d and a14a97048e
Discovery Timeline
- 2026-09-11 - CVE-2026-89499 published to NVD
- 2026-09-13 - Last updated in NVD database
Technical Details for CVE-2026-89499
Vulnerability Analysis
The Linux kernel ring-buffer subsystem supports a remote reader mechanism used by tracing consumers. The swap_reader_page callback exchanges the current reader page with a newly filled page from the writer. Under contention, when the writer advances the head before the remote reader catches up, the callback returns -EBUSY to signal a transient failure.
The function __rb_get_reader_page_from_remote() previously emitted only a WARN and continued as though the swap had completed. Because the reader ID was not updated, the caller path rearranged the local page list using stale state. The resulting linkage placed the same page as both the previous and new reader, producing memory corruption in kernel-space tracing data structures.
Root Cause
The root cause is missing error propagation on a recoverable callback failure. The code treated -EBUSY from swap_reader_page as a warning condition rather than a control-flow decision. The subsequent list splicing logic assumed a successful swap, so it operated on inconsistent reader-page state. This is a race condition combined with improper error handling in kernel memory-management code.
Attack Vector
Exploitation requires local access with low privileges to a system exposing the tracing ring-buffer to unprivileged consumers, or a workload capable of driving high-rate trace events on a small buffer. An attacker generates an event storm to increase the probability that the writer overtakes the remote reader, forcing swap_reader_page to return -EBUSY. Repeated failures corrupt the reader page list. No user interaction is required, and the attack is confined to the local kernel scope. See the upstream fix commits Linux Kernel Commit 5eab74874d and Linux Kernel Commit a14a97048e for the exact code path.
Detection Methods for CVE-2026-89499
Indicators of Compromise
- Kernel log entries containing WARN messages from __rb_get_reader_page_from_remote() on unpatched kernels.
- Repeated pr_warn_ratelimited() messages referencing swap_reader_page failures on patched builds indicate active contention that could be probed for exploitation.
- Unexpected kernel oops or list-corruption panics originating in the ring_buffer code paths.
Detection Strategies
- Inventory Linux hosts and correlate installed kernel versions against the fix commits to identify unpatched systems.
- Monitor dmesg and journald for ring-buffer warnings and list-debug splats tied to tracing consumers.
- Audit which local users or containers have access to tracefs, perf_event_open, or BPF ring-buffer interfaces.
Monitoring Recommendations
- Forward kernel logs to a centralized analytics tier and alert on ring-buffer warnings and list corruption messages.
- Track processes that generate sustained high-volume trace events, which is the precondition for the race.
- Baseline normal tracing subsystem activity per host and alert on statistical deviations.
How to Mitigate CVE-2026-89499
Immediate Actions Required
- Apply the upstream fixes from Linux Kernel Commit 5eab74874d and Linux Kernel Commit a14a97048e, or install vendor kernel updates that incorporate them.
- Restrict access to tracefs and BPF ring-buffer interfaces to trusted administrators only.
- Review container runtime capabilities so unprivileged workloads cannot open kernel tracing facilities.
Patch Information
The fix modifies __rb_get_reader_page_from_remote() to treat a failed swap_reader_page callback as a recoverable error. The function now reports the failure with pr_warn_ratelimited() and returns NULL, which existing callers already handle as a failed attempt. This prevents splicing the same page as both the previous and new reader and avoids log flooding under contention. Distribution vendors will ship the change in their next stable kernel updates.
Workarounds
- Unmount or restrict tracefs (mount -o remount,mode=700 /sys/kernel/tracing) where operational requirements allow.
- Set kernel.perf_event_paranoid=3 and kernel.unprivileged_bpf_disabled=1 to reduce unprivileged access to tracing interfaces.
- Increase ring-buffer size via echo <kb> > /sys/kernel/tracing/buffer_size_kb to lower the probability of writer/reader races during high-rate events.
# Configuration example: reduce unprivileged access to kernel tracing interfaces
sysctl -w kernel.perf_event_paranoid=3
sysctl -w kernel.unprivileged_bpf_disabled=1
chmod 700 /sys/kernel/tracing
echo 8192 > /sys/kernel/tracing/buffer_size_kb
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
