CVE-2026-74601 Overview
CVE-2026-74601 is a race condition in the Linux kernel's tracing ring buffer subsystem. The flaw resides in ring_buffer_swap_cpu(), which uses the per-CPU committing counter to detect whether a buffer is actively being written. A narrow window inside rb_move_tail allows this counter to transiently drop to zero while a write is still in progress, enabling an interrupt-driven swap to succeed against a busy buffer. The result is inconsistent buffer state and a triggered RB_WARN_ON inside rb_commit(). The upstream fix replaces the counter check with current_context state, which remains valid for the entire duration of ring_buffer_lock_reserve().
Critical Impact
A local low-privileged user interacting with tracing infrastructure can trigger kernel state corruption and denial of service through the ring buffer swap race.
Affected Products
- Linux kernel (upstream, mainline)
- Linux kernel stable branches receiving the referenced backports
- Distributions shipping the affected kernel/trace/ring_buffer.c code paths
Discovery Timeline
- 2026-08-22 - CVE-2026-74601 published to NVD
- 2026-08-25 - Last updated in NVD database
Technical Details for CVE-2026-74601
Vulnerability Analysis
The defect is a [CWE-362] concurrent execution race condition in the kernel tracing ring buffer. ring_buffer_swap_cpu() decides whether it is safe to swap the per-CPU buffer by reading cpu_buffer->committing. Under normal conditions, a non-zero value indicates an in-progress reservation and blocks the swap.
However, rb_reserve_next_event() can call into __rb_reserve_next() and then rb_move_tail(), which internally invokes rb_end_commit() and decrements committing back to zero before immediately re-incrementing it with local_inc(&cpu_buffer->committing). If an interrupt fires in that window and executes a swap, the reservation proceeds against cpu_buffer_a while the commit path later resolves against cpu_buffer_b. The mismatch triggers RB_WARN_ON(cpu_buffer, !local_read(&cpu_buffer->committing)) and leaves the ring buffer in an inconsistent state.
Root Cause
The root cause is treating the committing counter as a stable busy indicator when it is transiently released mid-write inside rb_move_tail(). The counter accurately reflects commit nesting but is not atomic with respect to the full lifecycle of a single reservation, so it cannot reliably gate a swap.
Attack Vector
Exploitation requires local access with permissions to interact with tracing facilities (typically the tracefs interface) and the ability to invoke per-CPU buffer swap operations while concurrent writers are active. An attacker with such access can race concurrent trace writes against buffer swaps to trigger the warning, corrupt ring buffer accounting, and destabilize the kernel. No verified public exploit is currently available for CVE-2026-74601, and the flaw is not listed in CISA KEV.
No verified proof-of-concept code is available. See the upstream commits in the references for the exact patched code paths.
Detection Methods for CVE-2026-74601
Indicators of Compromise
- Kernel log entries containing RB_WARN_ON warnings originating from rb_commit() or related ring_buffer.c functions.
- Unexpected WARNING: CPU: <n> PID: <pid> at kernel/trace/ring_buffer.c stack traces referencing rb_end_commit or ring_buffer_swap_cpu.
- Trace subsystem instability, dropped events, or tracefs consumers reporting corrupted event streams.
Detection Strategies
- Monitor dmesg and journald for ring buffer warnings and back-traces immediately following heavy tracing workloads.
- Audit which non-root users or containers hold write access to /sys/kernel/tracing and /sys/kernel/debug/tracing.
- Correlate kernel warning events with process activity that toggles trace instances or performs per-CPU buffer operations.
Monitoring Recommendations
- Ship kernel logs to a centralized analytics backend and alert on repeated ring buffer warnings from the same host.
- Track invocations of tracefs control files by unprivileged workloads and flag anomalous access patterns.
- Baseline expected tracing activity per host and alert on deviations that coincide with kernel warning bursts.
How to Mitigate CVE-2026-74601
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the advisory and rebuild or update to a fixed vendor kernel package.
- Restrict access to tracefs and debugfs so only trusted administrators can create trace instances or trigger per-CPU buffer swaps.
- Inventory hosts that expose tracing interfaces to containers or unprivileged users and prioritize them for patching.
Patch Information
The fix replaces the committing counter check in ring_buffer_swap_cpu() with a current_context check that is established at ring_buffer_lock_reserve() entry and remains valid throughout the write. The change is distributed across multiple stable branches. See Linux Kernel Commit 5b926fb0, Linux Kernel Commit 22709117, Linux Kernel Commit 26662bc8, Linux Kernel Commit 597f279b, Linux Kernel Commit 5e6e2a18, Linux Kernel Commit 6b524e6b, Linux Kernel Commit ad7e10c7, and Linux Kernel Commit f27bdc43.
Workarounds
- Unmount or restrict tracefs where tracing is not operationally required: mount -o remount,mode=0700 /sys/kernel/tracing.
- Remove tracing capabilities from container runtimes by dropping CAP_SYS_ADMIN and denying tracefs bind mounts.
- Disable per-CPU buffer swap workloads and snapshot features until the patched kernel is deployed.
# Configuration example: restrict tracefs to root only
mount -o remount,mode=0700,uid=0,gid=0 /sys/kernel/tracing
chmod 700 /sys/kernel/debug/tracing 2>/dev/null || true
# Verify running kernel version after patching
uname -r
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
