CVE-2026-87041 · Runtime
NO FIX YET · SEE WHAT TO DO
The RTIO submission verifier in Zephyr's kernel inverts its buffer permission checks, so user-mode threads can submit buffer pointers they cannot access. The crafted requests reach bus drivers that read or write kernel memory, letting an attacker escape the user-mode isolation boundary.
first public Nov 4, 2026 · 1 source
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Applications built on Zephyr with CONFIG_USERSPACE enabled that use RTIO with user-mode threads granted an RTIO context and iodev. Images without userspace or without RTIO user access are not reached through this path. Sources do not describe a managed or hosted variant.
Not affected if kernel-only-threads, no-rtio, userspace-disabled
Who cares embedded, rtio, userspace, zephyr
CONFIG_USERSPACE enabled and uses RTIO (CONFIG_RTIO).subsys/rtio/rtio_handlers.c before v4.3.0.v4.3.0 or later (per the patch, the fix is in subsys/rtio/rtio_handlers.c from v4.3.0).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.
This page as .md · .json · in your agent: get_signal("CVE-2026-87041") · CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
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