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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 24 Jun 2020 17:48:41 +0200
Message-ID: <CAMj1kXHqBs44uukRSdFwA_hcmX_yKVfjqdv9RoPbbu-6Wz+RaA@mail.gmail.com>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
To:     Kees Cook <keescook@chromium.org>
Cc:     Will Deacon <will@kernel.org>, Fangrui Song <maskray@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Collingbourne <pcc@google.com>,
        James Morse <james.morse@arm.com>,
        Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>, X86 ML <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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.