linux-kernel - Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file

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Message-ID: <20190618162005.GF2790@e103592.cambridge.arm.com>
Date:   Tue, 18 Jun 2019 17:20:07 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an
 ELF file

On Tue, Jun 18, 2019 at 09:00:35AM -0700, Yu-cheng Yu wrote:
> On Tue, 2019-06-18 at 18:05 +0200, Florian Weimer wrote:
> > * Yu-cheng Yu:
> > 
> > > > I assumed that it would also parse the main executable and make
> > > > adjustments based on that.
> > > 
> > > Yes, Linux also looks at the main executable's header, but not its
> > > NT_GNU_PROPERTY_TYPE_0 if there is a loader.
> > > 
> > > > 
> > > > ld.so can certainly provide whatever the kernel needs.  We need to tweak
> > > > the existing loader anyway.
> > > > 
> > > > No valid statically-linked binaries exist today, so this is not a
> > > > consideration at this point.
> > > 
> > > So from kernel, we look at only PT_GNU_PROPERTY?
> > 
> > If you don't parse notes/segments in the executable for CET, then yes.
> > We can put PT_GNU_PROPERTY into the loader.
> 
> Thanks!

Would this require the kernel and ld.so to be updated in a particular
order to avoid breakage?  I don't know enough about RHEL to know how
controversial that might be.

Also:

What about static binaries distrubited as part of RHEL?

A user would also reasonably expect static binaries built using the
distro toolchain to work on top of the distro kernel...  which might
be broken by this.


(When I say "broken" I mean that the binary would run, but CET
protections would be silently turned off.)

Cheers
---Dave