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

[Yu-cheng Yu]

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.

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