CVE-2026-87041

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.