linux-kernel - Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

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Message-ID: <20200624162919.GH25945@arm.com>
Date:   Wed, 24 Jun 2020 17:29:19 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Fangrui Song <maskray@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        X86 ML <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>, Will Deacon <will@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

On Wed, Jun 24, 2020 at 05:48:41PM +0200, Ard Biesheuvel wrote:
> On Wed, 24 Jun 2020 at 17:45, Kees Cook <keescook@...omium.org> wrote:
> >
> > On Wed, Jun 24, 2020 at 05:31:06PM +0200, Ard Biesheuvel wrote:
> > > On Wed, 24 Jun 2020 at 17:21, Kees Cook <keescook@...omium.org> wrote:
> > > >
> > > > On Wed, Jun 24, 2020 at 12:46:32PM +0200, Ard Biesheuvel wrote:
> > > > > I'm not sure if there is a point to having PAC and/or BTI in the EFI
> > > > > stub, given that it runs under the control of the firmware, with its
> > > > > memory mappings and PAC configuration etc.
> > > >
> > > > Is BTI being ignored when the firmware runs?
> > >
> > > Given that it requires the 'guarded' attribute to be set in the page
> > > tables, and the fact that the UEFI spec does not require it for
> > > executables that it invokes, nor describes any means of annotating
> > > such executables as having been built with BTI annotations, I think we
> > > can safely assume that the EFI stub will execute with BTI disabled in
> > > the foreseeable future.
> >
> > yaaaaaay. *sigh* How long until EFI catches up?
> >
> > That said, BTI shouldn't _hurt_, right? If EFI ever decides to enable
> > it, we'll be ready?
> >
> 
> Sure. Although I anticipate that we'll need to set some flag in the
> PE/COFF header to enable it, and so any BTI opcodes we emit without
> that will never take effect in practice.

In the meantime, it is possible to build all the in-tree parts of EFI
for BTI, and just turn it off for out-of-tree EFI binaries?

If there's no easy way to do this though, I guess we should wait for /
push for a PE/COFF flag to describe this properly.

Cheers
---Dave