---
id: "CVE-2026-98764"
kind: "vulnerability"
aliases: ["CVE-2026-98764"]
title: "Linux kernel: FunctionFS ioctl crashes on missing endpoint descriptor"
severity: "medium"
cvss: null
epss: null
exploited: false
status: "open"
category: "other"
products: []
ecosystems: []
tags: ["embedded", "linux-kernel", "other", "usb-gadget"]
not_affected_if: ["default-config", "no-usb-gadget-functionfs"]
published_at: "2026-10-18T16:17:30+00:00"
first_seen_at: "2026-10-23T21:57:16+00:00"
updated_at: "2026-10-25T05:16:44+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98764", "first_seen_at": "2026-10-23T21:57:16+00:00"}]
---

# Linux kernel: FunctionFS ioctl crashes on missing endpoint descriptor

A FunctionFS USB gadget set up with only full-speed descriptors can be made to dereference a NULL endpoint descriptor when user space issues the FUNCTIONFS_ENDPOINT_DESC ioctl at high or super speed. The result is a kernel NULL pointer dereference and panic, which takes the machine down.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- Update the Linux kernel to a release that includes the upstream f_fs fix for FUNCTIONFS_ENDPOINT_DESC (the fix falls back through lower-speed descriptors and returns -EINVAL when none exist); the exact fixed version is unknown from the sources.

## Timeline

- 2026-10-23T21:57:16+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98764

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: f_fs: Fix NULL pointer dereference in FUNCTIONFS_ENDPOINT_DESC

When user space sets up FunctionFS with only full-speed descriptors
(e.g., flags = FUNCTIONFS_HAS_FS_DESC) and the gadget operates at a
higher speed (such as USB_SPEED_HIGH or USB_SPEED_SUPER),
config_ep_by_speed() detects that the function lacks descriptors for
the current speed, logs a warning, and falls back to full-speed
descriptors. The endpoint is then successfully enabled via
usb_ep_enable(), and epfile->ep becomes valid.

However, when user space subsequently issues the FUNCTIONFS_ENDPOINT_DESC
ioctl to query the endpoint descriptor, ffs_epfile_ioctl() selects the
descriptor index solely based on gadget->speed:

	switch (epfile->ffs->gadget->speed) {
	case USB_SPEED_SUPER:
	case USB_SPEED_SUPER_PLUS:
		desc_idx = 2;
		break;
	case USB_SPEED_HIGH:
		desc_idx = 1;
		break;
	default:
		desc_idx = 0;
	}
	desc = epfile->ep->descs[desc_idx];
	memcpy(&desc1, desc, desc->bLength);

Because user space only supplied full-speed descriptors,
epfile->ep->descs[1] is NULL. Dereferencing desc->bLength triggers an
immediate kernel NULL pointer dereference panic.

Fix this by falling back through lower speed descriptors if the current
speed descriptor was not supplied, matching the fallback logic in
config_ep_by_speed() and historical f_fs behavior. If still missing,
fall back to epfile->ep->ep->desc (the active descriptor), and return
-EINVAL safely if no valid descriptor is found. In addition, mark desc
as const to match struct usb_ep::desc and avoid discarding qualifiers.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_