CVE-2026-68217: Linux Kernel Race Condition Vulnerability

CVE-2026-68217 Overview

CVE-2026-68217 is a resource management flaw in the Linux kernel pwc (Philips webcam) media driver. The vulnerability resides in pwc_isoc_init(), which submits isochronous URBs (USB Request Blocks) in a loop using usb_submit_urb(.., GFP_KERNEL). A race condition allows the completion handler pwc_isoc_handler() to execute on another CPU and populate pdev->fill_buf before subsequent URB submissions fail. The error path in start_streaming() only drains pdev->queued_bufs, leaking the buffer parked in pdev->fill_buf and triggering WARN_ON(owned_by_drv_count) in vb2_start_streaming().

Critical Impact
A buffer leak in the pwc driver error path leaves a videobuf2 buffer owned by the driver after streaming initialization fails, producing kernel warnings and potential resource exhaustion on affected systems.

Affected Products

Linux kernel media/pwc driver (Philips USB webcam subsystem)
Multiple stable Linux kernel branches receiving the backported fix
Systems using Philips-compatible USB webcams bound to the pwc driver

Discovery Timeline

2026-08-10 - CVE-2026-68217 published to NVD
2026-08-10 - Last updated in NVD database

Technical Details for CVE-2026-68217

Vulnerability Analysis

The defect is a race condition combined with incomplete error-path cleanup [CWE-459-class behavior]. pwc_isoc_init() submits multiple isochronous URBs sequentially. After the first URB is submitted with GFP_KERNEL, its completion handler pwc_isoc_handler() can run concurrently on another CPU. That handler invokes pwc_get_next_fill_buf(pdev), which detaches a buffer from pdev->queued_bufs and stores it in pdev->fill_buf.

If a subsequent usb_submit_urb() call in the same loop fails, pwc_isoc_init() returns the error. The caller start_streaming() then invokes pwc_isoc_cleanup(pdev) to kill URBs and calls pwc_cleanup_queued_bufs(pdev, VB2_BUF_STATE_QUEUED) to drain the queued buffer list. However, the buffer already detached into pdev->fill_buf is never returned to videobuf2. When vb2_start_streaming() inspects the driver-owned buffer count, WARN_ON(owned_by_drv_count) fires.

Root Cause

The root cause is asymmetric cleanup between the teardown and error paths. Commit 80b0963e1698 ("[media] pwc: fix WARN_ON") added fill_buf draining to stop_streaming() but did not mirror that handling in the start_streaming() error path. The URB completion handler's ability to run before submission of all URBs completes creates a window in which pdev->fill_buf can hold a live buffer at the moment of failure.

Attack Vector

The issue is triggered when isochronous URB submission fails after at least one URB has already completed a callback that populated pdev->fill_buf. Triggering the condition typically requires local access to a system with a pwc-compatible USB webcam and the ability to open the V4L2 device to initiate streaming. The primary observable impact is a kernel warning, leaked videobuf2 buffer ownership, and possible destabilization of the media subsystem across repeated failures. No remote attack vector has been identified. Technical details are available in the referenced Linux Kernel Commit 5d48126 and companion commits.

Detection Methods for CVE-2026-68217

Indicators of Compromise

Kernel log entries containing WARN_ON originating from vb2_start_streaming() referencing owned_by_drv_count.
Repeated usb_submit_urb failures logged near invocations of the pwc driver during V4L2 stream start.
Media device open/stream failures returning errors from VIDIOC_STREAMON on Philips-family USB webcams.

Detection Strategies

Monitor dmesg and journalctl -k for pwc-related warnings and stack traces referencing pwc_isoc_init or vb2_start_streaming.
Correlate USB device plug events with subsequent V4L2 streaming failures on the same device node.
Track kernel version and package inventory against the fix commits listed in the vendor advisory references.

Monitoring Recommendations

Aggregate kernel ring-buffer output from Linux endpoints into a centralized log store for warning-pattern search.
Alert on unexpected WARN_ON frequency spikes tied to media or USB subsystems.
Inventory endpoints exposing USB webcams to identify systems that must be prioritized for patching.

How to Mitigate CVE-2026-68217

Immediate Actions Required

Apply the upstream Linux kernel patches that mirror fill_buf draining in the start_streaming() error path.
Update to a stable kernel release that includes the referenced commits from kernel.org.
Where patching is delayed, restrict local user access to /dev/video* device nodes for untrusted accounts.

Patch Information

The fix drains pdev->fill_buf on start_streaming() failure so no buffer remains owned by the driver when the function returns an error. Patches are available in the following stable kernel commits: Linux Kernel Commit 5d48126, Linux Kernel Commit 906e410, Linux Kernel Commit 9afd605, Linux Kernel Commit a56e764, and Linux Kernel Commit acc789b.

Workarounds

Blacklist the pwc kernel module on systems that do not require Philips webcam support until patches are applied.
Limit V4L2 device permissions to trusted users to reduce the chance of triggering the error path.
Disable or physically disconnect affected USB webcams from production systems where streaming reliability issues have been observed.

bash

# Configuration example: blacklist the pwc module until patched
echo "blacklist pwc" | sudo tee /etc/modprobe.d/blacklist-pwc.conf
sudo modprobe -r pwc
sudo update-initramfs -u