Inverted memory-permission check in the RTIO syscall verifier allows a user-mode thread to read and write arbitrary kernel memory

Summary

CVE	CVE-2026-87041
State	PUBLISHED
Assigner	zephyr
Source Priority	CVE Program / NVD first with legacy fallback
Published	2026-11-04 06:17:06 UTC
Updated	2026-11-04 06:17:07 UTC
Description	The RTIO submission-queue verifier rtio_vrfy_sqe() in subsys/rtio/rtio_syscalls.c (subsys/rtio/rtio_handlers.c before v4.3.0) accumulated the results of K_SYSCALL_MEMORY() into its valid_sqe flag with &=. That macro returns nonzero on failure (it is built on K_SYSCALL_VERIFY_MSG(), which yields !(expr)), so every buffer permission check contributed the opposite of its result. A submission whose tx.buf, rx.buf or txrx.tx_buf/txrx.rx_buf pointer was not accessible to the calling thread was accepted, while a legitimate, thread-owned buffer caused the calling thread to be killed with a kernel oops. The verifier runs on behalf of the rtio_sqe_copy_in_get_handles() system call (also reached through the inline rtio_sqe_copy_in() wrapper), so it is driven directly by untrusted user-mode input on any image built with CONFIG_USERSPACE. A user thread that has been granted an RTIO context and an rtio_iodev object — the intended usage of RTIO userspace support, established via rtio_access_grant() — can submit an SQE whose buffer pointer and length name arbitrary kernel memory; the object check on sqe->iodev is unaffected and still passes, and the crafted SQE is forwarded to the bus driver unchanged. Downstream consumers dereference the pointer with supervisor privilege. The generic I2C and I3C RTIO fallback handlers (drivers/i2c/i2c_rtio_default.c, drivers/i3c/i3c_rtio_default.c) copy sqe.rx.buf/sqe.tx.buf into struct i2c_msg and call i2c_transfer(), and the vendor SPI/I2C RTIO drivers behave equivalently. An RTIO_OP_RX submission therefore has peripheral data written into kernel memory of the attacker's choosing (arbitrary kernel write), an RTIO_OP_TX submission clocks kernel memory out over the bus (kernel memory disclosure), and a pointer to unmapped memory faults the kernel. This is a full escape from the user-mode isolation boundary that CONFIG_USERSPACE exists to enforce. The defect was masked for years because the in-tree user-mode RTIO tests only exercise RTIO_OP_NOP and mempool-backed reads, neither of which reaches the affected checks. The fix negates each K_SYSCALL_MEMORY() result and additionally validates the TXRX transmit buffer read-only, matching the SPI, I2C and I3C handlers.

Risk And Classification

Primary CVSS: v3.1 7.8 HIGH from [email protected]

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Version	Source	Type	Score	Severity	Vector
3.1	[email protected]	Secondary	7.8	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`
3.1	CNA	CVSS	7.8	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`

CVSS v3.1 Breakdown

Attack Vector

Local

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Vendor Declared Affected Products

Source	Vendor	Product	Version	Platforms
CNA	Zephyrproject	Zephyr	affected 3.3.0 4.3.2 semver	Not specified
CNA	Zephyrproject	Zephyr	affected 4.4.0 4.5.0 semver	Not specified

References

Reference	Source	Link	Tags
github.com/zephyrproject-rtos/zephyr/commit/454a848006595ce2d77fc2c9e458...	[email protected]	github.com
github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-mvxr-jx9h-...	[email protected]	github.com
CVE Program record	CVE.ORG	www.cve.org	canonical
NVD vulnerability detail	NVD	nvd.nist.gov	canonical, analysis

No vendor comments have been submitted for this CVE.

There are currently no legacy QID mappings associated with this CVE.