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Subject[PATCH v4 04/10] x86: Makefile: Add build and config option for CONFIG_FG_KASLR
Date
Allow user to select CONFIG_FG_KASLR if dependencies are met. Change
the make file to build with -ffunction-sections if CONFIG_FG_KASLR.

While the only architecture that supports CONFIG_FG_KASLR does not
currently enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION, make sure these
2 features play nicely together for the future by ensuring that if
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is selected when used with
CONFIG_FG_KASLR the function sections will not be consolidated back
into .text. Thanks to Kees Cook for the dead code elimination changes.

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Tony Luck <tony.luck@intel.com>
---
Makefile | 6 +++++-
arch/x86/Kconfig | 4 ++++
include/asm-generic/vmlinux.lds.h | 16 ++++++++++++++--
init/Kconfig | 14 ++++++++++++++
4 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 0b5f8538bde5..66427b12de53 100644
--- a/Makefile
+++ b/Makefile
@@ -872,7 +872,7 @@ KBUILD_CFLAGS += $(call cc-option, -fno-inline-functions-called-once)
endif

ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
+KBUILD_CFLAGS_KERNEL += -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif

@@ -880,6 +880,10 @@ ifdef CONFIG_LIVEPATCH
KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
endif

+ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
+KBUILD_CFLAGS += -ffunction-sections
+endif
+
ifdef CONFIG_SHADOW_CALL_STACK
CC_FLAGS_SCS := -fsanitize=shadow-call-stack
KBUILD_CFLAGS += $(CC_FLAGS_SCS)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 883da0abf779..e7a2db3e270d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -372,6 +372,10 @@ config CC_HAS_SANE_STACKPROTECTOR
We have to make sure stack protector is unconditionally disabled if
the compiler produces broken code.

+config ARCH_HAS_FG_KASLR
+ def_bool y
+ depends on RANDOMIZE_BASE && X86_64
+
menu "Processor type and features"

config ZONE_DMA
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index db600ef218d7..a5552cf28d5d 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -93,14 +93,12 @@
* sections to be brought in with rodata.
*/
#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*
#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*
#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]*
#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
#define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]*
#else
-#define TEXT_MAIN .text
#define DATA_MAIN .data
#define SDATA_MAIN .sdata
#define RODATA_MAIN .rodata
@@ -108,6 +106,20 @@
#define SBSS_MAIN .sbss
#endif

+/*
+ * Both LD_DEAD_CODE_DATA_ELIMINATION and CONFIG_FG_KASLR options enable
+ * -ffunction-sections, which produces separately named .text sections. In
+ * the case of CONFIG_FG_KASLR, they need to stay distict so they can be
+ * separately randomized. Without CONFIG_FG_KASLR, the separate .text
+ * sections can be collected back into a common section, which makes the
+ * resulting image slightly smaller
+ */
+#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) && !defined(CONFIG_FG_KASLR)
+#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#else
+#define TEXT_MAIN .text
+#endif
+
/*
* Align to a 32 byte boundary equal to the
* alignment gcc 4.5 uses for a struct
diff --git a/init/Kconfig b/init/Kconfig
index 0498af567f70..82f042a1062f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1990,6 +1990,20 @@ config PROFILING
config TRACEPOINTS
bool

+config FG_KASLR
+ bool "Function Granular Kernel Address Space Layout Randomization"
+ depends on $(cc-option, -ffunction-sections)
+ depends on ARCH_HAS_FG_KASLR
+ default n
+ help
+ This option improves the randomness of the kernel text
+ over basic Kernel Address Space Layout Randomization (KASLR)
+ by reordering the kernel text at boot time. This feature
+ uses information generated at compile time to re-layout the
+ kernel text section at boot time at function level granularity.
+
+ If unsure, say N.
+
endmenu # General setup

source "arch/Kconfig"
--
2.20.1
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