efi: libstub: Disable struct randomization

commit 1a3887924a7e6edd331be76da7bf4c1e8eab4b1e upstream.

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.

Cc: <stable@vger.kernel.org> # v4.14+
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index a2ae9c3b95793821c305c4c3e8d125cbce783bd7..433e11dab4a87de086f676e52fc19f2a3cb1a09c 100644 (file)
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -37,6 +37,13 @@ KBUILD_CFLAGS                        := $(cflags-y) -Os -DDISABLE_BRANCH_PROFILING \
                                   $(call cc-option,-fno-addrsig) \
                                   -D__DISABLE_EXPORTS
 
+#
+# 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))
+
 # remove SCS flags from all objects in this directory
 KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_SCS), $(KBUILD_CFLAGS))