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

[Yu-cheng Yu]

On Fri, 2019-06-07 at 13:43 -0700, Andy Lutomirski wrote:
> > On Jun 7, 2019, at 12:49 PM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > 
> > On Fri, 2019-06-07 at 11:29 -0700, Andy Lutomirski wrote:
> > > > On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@xxxxxxxxx> 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 shouldnât 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 â 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
> > > donât 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:
> 
> How does glibc know the linear address space size?  We donât want LA64 to
> break old binaries because the address calculation changed.

When an application starts, its highest stack address is determined.
It uses that as the maximum the bitmap needs to cover.

Yu-cheng