From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753364AbcFOTCz (ORCPT ); Wed, 15 Jun 2016 15:02:55 -0400 Received: from mail-pa0-f54.google.com ([209.85.220.54]:33844 "EHLO mail-pa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751591AbcFOTCx (ORCPT ); Wed, 15 Jun 2016 15:02:53 -0400 Date: Wed, 15 Jun 2016 12:02:46 -0700 From: Kees Cook To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, x86@kernel.org Subject: [PATCH v10] x86/boot: Refuse to build with data relocations Message-ID: <20160615190246.GA25889@www.outflux.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The compressed kernel is built with -fPIC/-fPIE so that it can run in any location a bootloader happens to put it. However, since ELF relocation processing is not happening (and all the relocation information has already been stripped at link time), none of the code can use data relocations (e.g. static assignments of pointers). This is already noted in a warning comment at the top of misc.c, but this adds an explicit check for the condition during the linking stage to block any such bugs from appearing. If this was in place with the earlier bug in pagetable.c, the build would fail like this: ... CC arch/x86/boot/compressed/pagetable.o DATAREL arch/x86/boot/compressed/vmlinux error: arch/x86/boot/compressed/pagetable.o has data relocations! make[2]: *** [arch/x86/boot/compressed/vmlinux] Error 1 ... A clean build shows: ... CC arch/x86/boot/compressed/pagetable.o DATAREL arch/x86/boot/compressed/vmlinux LD arch/x86/boot/compressed/vmlinux ... Suggested-by: Ingo Molnar Signed-off-by: Kees Cook --- This is a resend, for -next --- arch/x86/boot/compressed/Makefile | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile index f1356889204e..536ccfcc01c6 100644 --- a/arch/x86/boot/compressed/Makefile +++ b/arch/x86/boot/compressed/Makefile @@ -85,7 +85,25 @@ vmlinux-objs-$(CONFIG_EFI_STUB) += $(obj)/eboot.o $(obj)/efi_stub_$(BITS).o \ $(objtree)/drivers/firmware/efi/libstub/lib.a vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o +# The compressed kernel is built with -fPIC/-fPIE so that a boot loader +# can place it anywhere in memory and it will still run. However, since +# it is executed as-is without any ELF relocation processing performed +# (and has already had all relocation sections stripped from the binary), +# none of the code can use data relocations (e.g. static assignments of +# pointer values), since they will be meaningless at runtime. This check +# will refuse to link the vmlinux if any of these relocations are found. +quiet_cmd_check_data_rel = DATAREL $@ +define cmd_check_data_rel + for obj in $(filter %.o,$^); do \ + readelf -S $$obj | grep -qF .rel.local && { \ + echo "error: $$obj has data relocations!" >&2; \ + exit 1; \ + } || true; \ + done +endef + $(obj)/vmlinux: $(vmlinux-objs-y) FORCE + $(call if_changed,check_data_rel) $(call if_changed,ld) OBJCOPYFLAGS_vmlinux.bin := -R .comment -S -- 2.7.4 -- Kees Cook Chrome OS & Brillo Security