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

On Wed, Jun 26, 2019 at 10:14:07AM -0700, Andy Lutomirski wrote:

On Thu, May 2, 2019 at 4:10 AM Dave Martin [off-list ref] wrote:

[...]

quoted

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