Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

From: Yu-cheng Yu
Date: Fri Jun 07 2019 - 16:09:00 EST

On Fri, 2019-06-07 at 11:58 -0700, Dave Hansen wrote:
> On 6/7/19 11:29 AM, Andy Lutomirski wrote:
> ...
> > > I think this new MSR probably needs to get included in oops output when
> > > CET is enabled.
> >
> > This shouldnât be able to OOPS because it only happens at CPL 3,
> > right? We should put it into core dumps, though.
> Good point.
> Yu-cheng, can you just confirm that the bitmap can't be referenced in
> ring-0, no matter what? We should also make sure that no funny business
> happens if we put an address in the bitmap that faults, or is
> non-canonical. Do we have any self-tests for that?

Yes, the bitmap is user memory, but the kernel can still get to it (e.g.
copy_from_user()). We can do more check on the address.

> Let's say userspace gets a fault on this. Do they have the
> introspection capability to figure out why they faulted, say in their
> signal handler?

The bitmap address is kept by the application; the kernel won't provide it again
to user-space. In the signal handler, the app can find out from its own record.