Re: [RFC v2 00/27] Kernel Address Space Isolation

[Alexandre Chartre]

On 7/12/19 12:38 AM, Dave Hansen wrote:
> On 7/11/19 7:25 AM, Alexandre Chartre wrote:
> > - Kernel code mapped to the ASI page-table has been reduced to:
> >    . the entire kernel (I still need to test with only the kernel text)
> >    . the cpu entry area (because we need the GDT to be mapped)
> >    . the cpu ASI session (for managing ASI)
> >    . the current stack
> >
> > - Optionally, an ASI can request the following kernel mapping to be added:
> >    . the stack canary
> >    . the cpu offsets (this_cpu_off)
> >    . the current task
> >    . RCU data (rcu_data)
> >    . CPU HW events (cpu_hw_events).
>
> I don't see the per-cpu areas in here.  But, the ASI macros in
> entry_64.S (and asi_start_abort()) use per-cpu data.

We don't map all per-cpu areas, but only the per-cpu variables we need. ASI
code uses the per-cpu cpu_asi_session variable which is mapped when an ASI
is created (see patch 15/26):

+	/*
+	 * Map the percpu ASI sessions. This is used by interrupt handlers
+	 * to figure out if we have entered isolation and switch back to
+	 * the kernel address space.
+	 */
+	err = ASI_MAP_CPUVAR(asi, cpu_asi_session);
+	if (err)
+		return err;


> Also, this stuff seems to do naughty stuff (calling C code, touching
> per-cpu data) before the PTI CR3 writes have been done.  But, I don't
> see anything excluding PTI and this code from coexisting.

My understanding is that PTI CR3 writes only happens when switching to/from
userland. While ASI enter/exit/abort happens while we are already in the kernel,
so asi_start_abort() is not called when coming from userland and so not
interacting with PTI.

For example, if ASI in used during a syscall (e.g. with KVM), we have:

 -> syscall
    - PTI CR3 write (kernel CR3)
    - syscall handler:
      ...
      asi_enter()-> write ASI CR3
      .. code run with ASI ..
      asi_exit() or asi abort -> restore original CR3
      ...
    - PTI CR3 write (userland CR3)
 <- syscall


Thanks,

alex.