[PATCH] efi: libstub: Disable struct randomization

[PATCH] efi: libstub: Disable struct randomization

[Ard Biesheuvel]

The EFI stub is a wrapper around the core kernel that makes it look like
a EFI compatible PE/COFF application to the EFI firmware. EFI
applications run on top of the EFI runtime, which is heavily based on
so-called protocols, which are struct types consisting [mostly] of
function pointer members that are instantiated and recorded in a
protocol database.

These structs look like the ideal randomization candidates to the
randstruct plugin (as they only carry function pointers), but of course,
these protocols are contracts between the firmware that exposes them,
and the EFI applications (including our stubbed kernel) that invoke
them. This means that struct randomization for EFI protocols is not a
great idea, and given that the stub shares very little data with the
core kernel that is represented as a randomizable struct, we're better
off just disabling it completely here.

Reported-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
Tested-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index d0537573501e..dd8c8a218245 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -42,6 +42,13 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))
 # disable LTO
 KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
 
+#
+# struct randomization only makes sense for Linux internal types, which the EFI
+# stub code never touches, so let's turn off struct randomization for the stub
+# altogether
+#
+KBUILD_CFLAGS := $(filter-out $(RANDSTRUCT_CFLAGS), $(KBUILD_CFLAGS))
+
 GCOV_PROFILE			:= n
 # Sanitizer runtimes are unavailable and cannot be linked here.
 KASAN_SANITIZE			:= n
-- 
2.35.1

Re: [PATCH] efi: libstub: Disable struct randomization

[Kees Cook]

On Mon, Aug 22, 2022 at 07:29:53PM +0200, Ard Biesheuvel wrote:
> The EFI stub is a wrapper around the core kernel that makes it look like
> a EFI compatible PE/COFF application to the EFI firmware. EFI
> applications run on top of the EFI runtime, which is heavily based on
> so-called protocols, which are struct types consisting [mostly] of
> function pointer members that are instantiated and recorded in a
> protocol database.
> 
> These structs look like the ideal randomization candidates to the
> randstruct plugin (as they only carry function pointers), but of course,
> these protocols are contracts between the firmware that exposes them,
> and the EFI applications (including our stubbed kernel) that invoke
> them. This means that struct randomization for EFI protocols is not a
> great idea, and given that the stub shares very little data with the
> core kernel that is represented as a randomizable struct, we're better
> off just disabling it completely here.
> 
> Reported-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
> Tested-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook

Re: [PATCH] efi: libstub: Disable struct randomization

[Ard Biesheuvel]

On Mon, 22 Aug 2022 at 21:58, Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Aug 22, 2022 at 07:29:53PM +0200, Ard Biesheuvel wrote:
> > The EFI stub is a wrapper around the core kernel that makes it look like
> > a EFI compatible PE/COFF application to the EFI firmware. EFI
> > applications run on top of the EFI runtime, which is heavily based on
> > so-called protocols, which are struct types consisting [mostly] of
> > function pointer members that are instantiated and recorded in a
> > protocol database.
> >
> > These structs look like the ideal randomization candidates to the
> > randstruct plugin (as they only carry function pointers), but of course,
> > these protocols are contracts between the firmware that exposes them,
> > and the EFI applications (including our stubbed kernel) that invoke
> > them. This means that struct randomization for EFI protocols is not a
> > great idea, and given that the stub shares very little data with the
> > core kernel that is represented as a randomizable struct, we're better
> > off just disabling it completely here.
> >
> > Reported-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
> > Tested-by: Daniel Marth <daniel.marth@inso.tuwien.ac.at>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> Acked-by: Kees Cook <keescook@chromium.org>
>

I've queued this up as a fix with a cc:stable

Thanks all,
Ard.