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Date:   Wed, 24 Jun 2020 18:16:13 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        linux-efi <linux-efi@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Will Deacon <will@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Fangrui Song <maskray@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        X86 ML <x86@kernel.org>, Russell King <linux@armlinux.org.uk>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@suse.de>,
        Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Collingbourne <pcc@google.com>,
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        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        James Morse <james.morse@arm.com>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
Message-ID: <20200624171613.GJ25945@arm.com>
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On Wed, Jun 24, 2020 at 06:40:48PM +0200, Ard Biesheuvel wrote:
> On Wed, 24 Jun 2020 at 18:29, Dave Martin <Dave.Martin@arm.com> wrote:
> >
> > 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?
> >
> 
> Not sure I understand the question. What do you mean by out-of-tree
> EFI binaries? And how would the firmware (which is out of tree itself,
> and is in charge of the page tables, vector table, timer interrupt etc
> when the EFI stub executes) distinguish such binaries from the EFI
> stub?

I'm not an EFI expert, but I'm guessing that you configure EFI with
certain compiler flags and build it.  Possibly some standalone EFI
executables are built out of the same tree and shipped with the
firmware from the same build, but I'm speculating.  If not, we can just
run all EFI executables with BTI off.

> > 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.
> >
> 
> Yeah good point. I will take this to the forum.

In the interim, we could set the GP bit in EFI's page tables for the
executable code from the firmware image if we want this protection, but
turn it off in pages mapping the executable code of EFI executables.
This is better than nothing.

Cheers
---Dave