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

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>,
	linux-arch <linux-arch@vger.kernel.org>,
	linux-efi <linux-efi@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>, Fangrui Song <maskray@google.com>,
	Peter Collingbourne <pcc@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Masahiro Yamada <masahiroy@kernel.org>, X86 ML <x86@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Ingo Molnar <mingo@redhat.com>, James Morse <james.morse@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@suse.de>, Will Deacon <will@kernel.org>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
Date: Wed, 24 Jun 2020 17:29:19 +0100	[thread overview]
Message-ID: <20200624162919.GH25945@arm.com> (raw)
In-Reply-To: <CAMj1kXHqBs44uukRSdFwA_hcmX_yKVfjqdv9RoPbbu-6Wz+RaA@mail.gmail.com>

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