Re: [RFC][PATCH 13/24] x86/pti: Extend PTI user mappings

[Alexandre Chartre]

On 11/11/20 12:39 AM, Andy Lutomirski wrote:
> > On 11/9/20 6:28 PM, Andy Lutomirski wrote:
> > > On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
> > > <alexandre.chartre@xxxxxxxxxx> wrote:
> > > > Extend PTI user mappings so that more kernel entry code can be executed
> > > > with the user page-table. To do so, we need to map syscall and interrupt
> > > > entry code,
> > >
> > > Probably fine.
> > >
> > > > per cpu offsets (__per_cpu_offset, which is used some in
> > > > entry code),
> > >
> > > This likely already leaks due to vulnerable CPUs leaking address space
> > > layout info.
> >
> > I forgot to update the comment, I am not mapping __per_cpu_offset anymore.
> >
> > However, if we do map __per_cpu_offset then we don't need to enforce the
> > ordering in paranoid_entry to switch CR3 before GS.
>
> I'm okay with mapping __per_cpu_offset.

Good. That way I can move the GS update back to assembly code (paranoid_entry/exit
will be mostly reduce to updating GS), and probably I won't need to disable
stack-protector.


> > > > the stack canary,
> > >
> > > That's going to be a very tough sell.
> >
> > I can get rid of this, but this will require to disable stack-protector for
> > any function that we can call while using the user page-table, like already
> > done in patch 21 (x86/entry: Disable stack-protector for IST entry C handlers).
>
> You could probably get away with using a different stack protector
> canary before and after the CR3 switch as long as you are careful to
> have the canary restored when you return from whatever function is
> involved.

I was thinking about doing that. I will give it a try.

Thanks,

alex.