From: Dave Voutila <dv@sisu.io>
Subject: Re: Kernel protection fault in fill_kproc()
To: Gerhard Roth <gerhard_roth@genua.de>
Cc: "tech@openbsd.org" <tech@openbsd.org>,  "mpi@openbsd.org" <mpi@openbsd.org>,  Carsten Beckmann <carsten_beckmann@genua.de>
Date: Mon, 11 Aug 2025 10:34:40 -0400

Gerhard Roth <gerhard_roth@genua.de> writes:

> About a year ago, the call to uvm_exit() was moved outside of the
> KERNEL_LOCK() in the reaper() by mpi@. Now we observed a kernel
> protection fault that results from this change.
>
> In fill_kproc() we read the vmspace pointer (vm) right at the very
> beginning of the function:
>
>         struct vmspace *vm = pr->ps_vmspace;
>
> Sometime later, we try to access it:
>
> 	/* fixups that can only be done in the kernel */
> 	if ((pr->ps_flags & PS_ZOMBIE) == 0) {
> 		if ((pr->ps_flags & PS_EMBRYO) == 0 && vm != NULL)
> 			ki->p_vm_rssize = vm_resident_count(vm);
>
>
> In the meantime the process might have exited and the reaper() can free
> the vmspace by calling uvm_exit(). After that, the 'vm' pointer in
> fill_kproc() points to stale memory. Accessing it will yield a kernel
> protection fault.
>
> BTW: only after freeing the vmspace of the process, the PS_ZOMBIE flag
> is set by the reaper().
>
> I propose to put the reaper()'s call to uvm_exit() back under the
> kernel lock to avoid the fault.

I don't think this is the correct approach.

I don't tend to work in this area, but this looks possibly related to
unlocking in sysctl given fill_kproc() is seeing the memory issues. A
lot has changed in kern_sysctl.c in the past few months.

>
> Gerhard
>
>
> Index: sys/kern/kern_exit.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_exit.c,v
> diff -u -p -u -p -r1.252 kern_exit.c
> --- sys/kern/kern_exit.c	10 Aug 2025 15:17:57 -0000	1.252
> +++ sys/kern/kern_exit.c	11 Aug 2025 10:30:57 -0000
> @@ -498,10 +498,15 @@ reaper(void *arg)
>  		} else {
>  			struct process *pr = p->p_p;
>
> -			/* Release the rest of the process's vmspace */
> +			/*
> +			 * Release the rest of the process's vmspace
> +			 * Use the kernel lock to avoid a race with fill_kproc()
> +			 * accessing the vmspace while the process isn't yet a
> +			 * zombie.
> +			 */
> +			KERNEL_LOCK();
>  			uvm_exit(pr);
>
> -			KERNEL_LOCK();
>  			if ((pr->ps_flags & PS_NOZOMBIE) == 0) {
>  				/* Process is now a true zombie. */
>  				atomic_setbits_int(&pr->ps_flags, PS_ZOMBIE);