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

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Message-ID: <20190627092715.GB2790@e103592.cambridge.arm.com>
Date:   Thu, 27 Jun 2019 10:27:17 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>
Subject: 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 <Dave.Martin@....com> 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