GHSA-mvxr-jx9h-jxh4: Inverted memory-permission check in the RTIO syscall verifier allows a user-mode thread to read and write arbitrary kernel memory

The RTIO submission-queue verifier rtiovrfysqe() in subsys/rtio/rtiosyscalls.c (subsys/rtio/rtiohandlers.c before v4.3.0) accumulated the results of KSYSCALLMEMORY() into its validsqe flag with &=. That macro returns nonzero on failure (it is built on KSYSCALLVERIFYMSG(), which yields !(expr)), so every buffer permission check contributed the opposite of its result. A submission whose tx.buf, rx.buf or txrx.txbuf/txrx.rxbuf 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 rtiosqecopyingethandles() system call (also reached through the inline rtiosqecopyin() wrapper), so it is driven directly by untrusted user-mode input on any image built with CONFIGUSERSPACE. A user thread that has been granted an RTIO context and an rtioiodev object — the intended usage of RTIO userspace support, established via rtioaccessgrant() — 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/i2crtiodefault.c, drivers/i3c/i3crtiodefault.c) copy sqe.rx.buf/sqe.tx.buf into struct i2cmsg and call i2ctransfer(), and the vendor SPI/I2C RTIO drivers behave equivalently. An RTIOOPRX submission therefore has peripheral data written into kernel memory of the attacker's choosing (arbitrary kernel write), an RTIOOPTX 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 CONFIGUSERSPACE exists to enforce.

The defect was masked for years because the in-tree user-mode RTIO tests only exercise RTIOOPNOP and mempool-backed reads, neither of which reaches the affected checks. The fix negates each KSYSCALLMEMORY() result and additionally validates the TXRX transmit buffer read-only, matching the SPI, I2C and I3C handlers.