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Date:   Wed, 24 Jun 2020 17:29:19 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
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        Catalin Marinas <catalin.marinas@arm.com>,
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Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
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On Wed, Jun 24, 2020 at 05:48:41PM +0200, Ard Biesheuvel wrote:
> On Wed, 24 Jun 2020 at 17:45, Kees Cook <keescook@chromium.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@chromium.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