Re: [PATCH] binfmt_elf: Extract .note.gnu.property from an ELF file

[Dave Martin]

On Wed, Jun 26, 2019 at 10:14:07AM -0700, Andy Lutomirski wrote:
> On Thu, May 2, 2019 at 4:10 AM Dave Martin <Dave.Martin@xxxxxxx> wrote:

[...]

> > A couple of questions before I look in more detail:
> >
> > 1) Can we rely on PT_GNU_PROPERTY being present in the phdrs to describe
> > the NT_GNU_PROPERTY_TYPE_0 note?  If so, we can avoid trying to parse
> > irrelevant PT_NOTE segments.
> >
> >
> > 2) Are there standard types for things like the program property header?
> > If not, can we add something in elf.h?  We should try to coordinate with
> > libc on that.  Something like
> >
> 
> Where did PT_GNU_PROPERTY come from?  Are there actual docs for it?
> Can someone here tell us what the actual semantics of this new ELF
> thingy are?  From some searching, it seems like it's kind of an ELF
> note but kind of not.  An actual description would be fantastic.

https://github.com/hjl-tools/linux-abi/wiki/linux-abi-draft.pdf

I don't know _when_ it was added, and the description is minimal, but
it's there.

(I'd say it's fairly obvious how it should be used, but it could do with
some clarification...)

> Also, I don't think there's any actual requirement that the upstream
> kernel recognize existing CET-enabled RHEL 8 binaries as being
> CET-enabled.  I tend to think that RHEL 8 jumped the gun here.  While
> the upstream kernel should make some reasonble effort to make sure
> that RHEL 8 binaries will continue to run, I don't see why we need to
> go out of our way to keep the full set of mitigations available for
> binaries that were developed against a non-upstream kernel.

If that's an accpetable approach, it should certainly make our life
easier.

> In fact, if we handle the legacy bitmap differently from RHEL 8, we
> may *have* to make sure that we don't recognize existing RHEL 8
> binaries as CET-enabled.

Can't comment on that.  If the existing RHEL 8 binaries strictly don't
have the PT_GNU_PROPERTY phdr, then this might serve a dual purpose ...
otherwise, x86 might need some additional annotation for new binaries.

I'll leave it for others to comment.

Cheers
---Dave