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

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

On Fri, 2019-06-07 at 11:29 -0700, Andy Lutomirski wrote:
> > On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> > 
> > > 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.
> 
> This shouldn’t be able to OOPS because it only happens at CPL 3, right?  We
> should put it into core dumps, though.
> 
> > 
> > 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
> > locations.
> 
> Given that the format depends on the VA size, this might be a good idea.  I
> bet we can reuse the special mapping infrastructure for this — the VMA could
> be a MAP_PRIVATE special mapping named [cet_legacy_bitmap] or similar, and we
> can even make special rules to core dump it intelligently if needed.  And we
> can make mremap() on it work correctly if anyone (CRIU?) cares.
> 
> Hmm.  Can we be creative and skip populating it with zeros?  The CPU should
> only ever touch a page if we miss an ENDBR on it, so, in normal operation, we
> don’t need anything to be there.  We could try to prevent anyone from
> *reading* it outside of ENDBR tracking if we want to avoid people accidentally
> wasting lots of memory by forcing it to be fully populated when the read it.
> 
> The one downside is this forces it to be per-mm, but that seems like a
> generally reasonable model anyway.
> 
> This also gives us an excellent opportunity to make it read-only as seen from
> userspace to prevent exploits from just poking it full of ones before
> redirecting execution.

GLIBC sets bits only for legacy code, and then makes the bitmap read-only.  That
avoids most issues:

  To populate bitmap pages, mprotect() is required.
  Reading zero bitmap pages would not waste more physical memory, right?

Yu-cheng