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

From: Dave Hansen
Date: Fri Jun 07 2019 - 14:03:57 EST

On 6/7/19 10:43 AM, Peter Zijlstra wrote:
> I've no idea what the kernel should do; since you failed to answer the
> question what happens when you point this to garbage.
> Does it then fault or what?

Yeah, I think you'll fault with a rather mysterious CR2 value since
you'll go look at the instruction that faulted and not see any
references to the CR2 value.

I think this new MSR probably needs to get included in oops output when
CET is enabled.

Why don't we require that a VMA be in place for the entire bitmap?
Don't we need a "get" prctl function too in case something like a JIT is
running and needs to find the location of this bitmap to set bits itself?

Or, do we just go whole-hog and have the kernel manage the bitmap
itself. Our interface here could be:

prctl(PR_MARK_CODE_AS_LEGACY, start, size);

and then have the kernel allocate and set the bitmap for those code