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

[Alexandre Chartre]

On 7/12/19 3:51 PM, Dave Hansen wrote:
> On 7/12/19 1:09 AM, Alexandre Chartre wrote:
> > On 7/12/19 12:38 AM, Dave Hansen wrote:
> > > 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):
>
> No fair!  I had per-cpu variables just for PTI at some point and had to
> give them up! ;)
>
> > +ÂÂÂ /*
> > +ÂÂÂÂ * Map the percpu ASI sessions. This is used by interrupt handlers
> > +ÂÂÂÂ * to figure out if we have entered isolation and switch back to
> > +ÂÂÂÂ * the kernel address space.
> > +ÂÂÂÂ */
> > +ÂÂÂ err = ASI_MAP_CPUVAR(asi, cpu_asi_session);
> > +ÂÂÂ if (err)
> > +ÂÂÂÂÂÂÂ 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.
>
> OK, that makes sense.  You only need to call C code when interrupted
> from something in the kernel (deeper than the entry code), and those
> were already running kernel C code anyway.

Exactly.

> If this continues to live in the entry code, I think you have a good
> clue where to start commenting.

Yeah, lot of writing to do... :-)
 
> BTW, the PTI CR3 writes are not *strictly* about the interrupt coming
> from user vs. kernel.  It's tricky because there's a window both in the
> entry and exit code where you are in the kernel but have a userspace CR3
> value.  You end up needing a CR3 write when you have a userspace CR3
> value when the interrupt occurred, not only when you interrupt userspace
> itself.

Right. ASI is simpler because it comes from the kernel and return to the
kernel. There's just a small window (on entry) where we have the ASI CR3
but we quickly switch to the full kernel CR3.

alex.