CVE-2026-74634: Linux Kernel Use-After-Free Vulnerability

CVE-2026-74634 Overview

CVE-2026-74634 is a use-after-free vulnerability in the Linux kernel's ring buffer subsystem. The flaw resides in ring_buffer_subbuf_order_set(), which frees buffer pages while resizing is disabled. A non-consuming reader can trigger a use-after-free condition through rb_advance_iter() when accessing freed pages. The fix returns -EBUSY on resize_disabled, aligning behavior with ring_buffer_resize(). Local, authenticated attackers with low privileges can exploit this issue to compromise confidentiality, integrity, and availability on affected systems.

Critical Impact
A local attacker with low privileges can trigger a use-after-free in the kernel ring buffer, potentially leading to memory corruption and privilege escalation.

Affected Products

Linux kernel (upstream stable trees receiving the referenced backports)
Distributions shipping vulnerable kernel builds with ring buffer tracing enabled
Systems where non-consuming ring buffer readers can be exercised by local users

Discovery Timeline

2026-08-22 - CVE-2026-74634 published to NVD
2026-08-25 - Last updated in NVD database

Technical Details for CVE-2026-74634

Vulnerability Analysis

The Linux kernel ring buffer subsystem supports adjusting the sub-buffer page order at runtime through ring_buffer_subbuf_order_set(). The function reallocates and frees buffer pages to change the underlying storage layout. When resizing is disabled through the resize_disabled flag, other operations still access these buffer pages.

A non-consuming iterator advanced via rb_advance_iter() can hold references to buffer pages that ring_buffer_subbuf_order_set() frees. The iterator then dereferences freed memory, producing a use-after-free condition. This can result in kernel memory corruption, information disclosure, or arbitrary kernel execution depending on how the freed slab memory is reallocated.

Root Cause

The root cause is missing state validation in ring_buffer_subbuf_order_set(). The function did not check resize_disabled before freeing buffer pages, unlike ring_buffer_resize() which returns -EBUSY under that condition. The inconsistency allowed page frees to race with active non-consuming readers holding pointers to those pages.

Attack Vector

Exploitation requires local access with the ability to open trace ring buffers and invoke the subbuf order interface. An attacker triggers a non-consuming reader on the ring buffer, then races with a subbuf order change to free pages the iterator still references. Follow-on rb_advance_iter() calls dereference freed memory, giving the attacker a primitive to corrupt kernel state.

No verified public exploit code is available. See the upstream fix commits Kernel Commit 62978cf, Kernel Commit 7568e9e, Kernel Commit b45b91d, and Kernel Commit bf98d7b for technical details.

Detection Methods for CVE-2026-74634

Indicators of Compromise

Kernel oops or panic messages referencing rb_advance_iter, ring_buffer_subbuf_order_set, or ring_buffer_iter.
KASAN reports flagging use-after-free access on ring buffer sub-buffer pages.
Unexpected crashes on hosts where local users interact with /sys/kernel/tracing/buffer_subbuf_size_kb or similar interfaces.

Detection Strategies

Enable KASAN in test kernels to surface use-after-free access on ring buffer pages during fuzzing or CI runs.
Audit kernel package versions across the fleet and compare against the fixed commits listed in the upstream references.
Correlate crash telemetry with process activity that opens trace files and writes to sub-buffer configuration nodes.

Monitoring Recommendations

Forward kernel.log, dmesg, and crash dumps to a centralized log platform to spot repeat oopses in the ring buffer path.
Monitor local access patterns to /sys/kernel/tracing/ and /sys/kernel/debug/tracing/ interfaces, especially from non-root workloads.
Track kernel upgrade compliance and alert on hosts that remain on pre-patch builds.

How to Mitigate CVE-2026-74634

Immediate Actions Required

Apply the vendor kernel update that incorporates the upstream fix commits to all affected hosts.
Restrict access to tracing interfaces so only trusted administrators can modify sub-buffer order at runtime.
Reboot systems after patch installation to load the fixed kernel image.

Patch Information

Upstream fixes are available in the following commits: Kernel Commit 62978cf, Kernel Commit 7568e9e, Kernel Commit b45b91d, and Kernel Commit bf98d7b. Track your distribution's security advisories for backported builds and install them through the standard package manager.

Workarounds

Remount tracing filesystems with restrictive permissions or unmount them where tracing is not required.
Use Linux capabilities and Mandatory Access Control policies to block untrusted users from writing to sub-buffer order interfaces.
Disable CONFIG_RING_BUFFER sub-buffer order tuning in custom kernel builds where the feature is not needed.

bash

# Restrict tracing interface access to root only
chmod 700 /sys/kernel/tracing
chmod 700 /sys/kernel/debug/tracing 2>/dev/null || true

# Verify installed kernel version after patching
uname -r