GHSA-3rh4-hpgj-jrww: Unvalidated user pointers in RTIO syscall verifiers allow a user-mode thread to write kernel memory

Two RTIO syscall verifiers, in subsys/rtio/rtiosyscalls.c (subsys/rtio/rtiohandlers.c before v4.3.0), did not validate user-supplied pointers before the kernel wrote through them.

zvrfyrtiosqecancel() performed no validation at all. It passed the caller's struct rtiosqe pointer straight to zimplrtiosqecancel(), which converts it with CONTAINEROF to a struct rtioiodevsqe, ORs RTIOSQECANCELED (BIT(3)) into its 16-bit flags field in supervisor mode, then loads its next pointer and repeats until that pointer is NULL. The syscall takes no RTIO context argument, so it needs no object grant: any user-mode thread on a CONFIGUSERSPACE build can set that bit in a 16-bit word at an address of its choosing in kernel memory, and the kernel continues the write along whatever pointer chain it finds at the corresponding offset. Setting chosen bits in kernel data structures, such as object permission or thread state fields, can be used to escalate privilege from user mode.

zvrfyrtiosqecopyingethandles() validated its RTIO context and the input SQE array, but only NULL-checked the handle out-parameter before writing a kernel SQE pointer through it, with no KSYSCALLMEMORYWRITE check. A user thread that holds a granted RTIO context can therefore make the kernel store a pointer value at an arbitrary address.

The fixes validate the handle out-parameter as writable, bound the cancel handle to real submission-pool storage, and require permission on the RTIO context that owns the matching pool. Without that last check a bounded handle still let any user thread cancel submissions belonging to other contexts, silently aborting their I/O. Systems built without CONFIGUSERSPACE or without CONFIGRTIO are unaffected.