[PATCH v9 07/15] Makefile: Add build and config option for CONFIG_FG_KASLR

Alexander Lobakin
2021-12-23T00:23:59Z

From: Kristen Carlson Accardi <kristen@linux.intel.com>

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.

alobakin:
Improve cflags management in the top Makefile: don't turn on
-f{data,function}-sections with ClangLTO as this is a no-op
provoking a full rebuild.
Add ".symtab_shndx" to the list of known sections since we are going
to support it. Otherwise LD will emit a warning when there are more
than 64k sections and CONFIG_LD_ORPHAN_WARN=y.
Turn ".text" LD script wildcard into ".text.__unused__" to make sure
all kernel code will land into our special sections.
Make FG-KASLR depend on `-z unique-symbol`. With every function being
in a separate section (randomly ordered each boot), position-based
search is impossible. This flag is likely to be widely available.

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>
Co-developed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
---
Makefile | 17 ++++++++++++++---
arch/Kconfig | 6 +++++-
include/asm-generic/vmlinux.lds.h | 20 ++++++++++++++++++--
include/linux/linkage.h | 3 ++-
init/Kconfig | 18 ++++++++++++++++--
5 files changed, 55 insertions(+), 9 deletions(-)

diff --git a/Makefile b/Makefile
index b921b1fabf70..3346269341d4 100644
--- a/Makefile
+++ b/Makefile
@@ -883,7 +883,7 @@ KBUILD_CFLAGS += -fno-inline-functions-called-once
endif

# Prefer linking with the `-z unique-symbol` if available, this eliminates
-# position-based search
+# position-based search. Also is a requirement for FG-KASLR
ifeq ($(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL)$(CONFIG_LIVEPATCH),yy)
KBUILD_LDFLAGS += -z unique-symbol
endif
@@ -892,7 +892,7 @@ endif
# `include/linux/linkage.h` for explanation. This flag is to enable GAS to
# insert the name of the previous section instead of `%S` inside .pushsection
ifdef CONFIG_HAVE_ASM_FUNCTION_SECTIONS
-ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_LTO_CLANG),)
+ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_LTO_CLANG)$(CONFIG_FG_KASLR),)
SECSUBST_AFLAGS := -Wa,--sectname-subst
KBUILD_AFLAGS_KERNEL += $(SECSUBST_AFLAGS)
KBUILD_CFLAGS_KERNEL += $(SECSUBST_AFLAGS)
@@ -906,8 +906,19 @@ KBUILD_CFLAGS_MODULE += -Wa,--sectname-subst
endif
endif # CONFIG_HAVE_ASM_FUNCTION_SECTIONS

+# ClangLTO implies `-ffunction-sections -fdata-sections`, no need
+# to specify them manually and trigger a pointless full rebuild
+ifndef CONFIG_LTO_CLANG
+ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
+KBUILD_CFLAGS_KERNEL += -ffunction-sections
+endif
+
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS_KERNEL += -fdata-sections
+endif
+endif # CONFIG_LTO_CLANG
+
ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif

diff --git a/arch/Kconfig b/arch/Kconfig
index b31a836bc252..01c026d090d4 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1316,7 +1316,11 @@ config ARCH_SUPPORTS_ASM_FUNCTION_SECTIONS
bool
help
An arch should select this if it can be built and run with its
- ASM functions placed into separate sections to improve DCE and LTO.
+ ASM functions placed into separate sections to improve DCE, LTO
+ and FG-KASLR.
+
+config ARCH_SUPPORTS_FG_KASLR
+ bool

source "kernel/gcov/Kconfig"

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e7b8a84e0e64..586465b2abb2 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -100,14 +100,12 @@
* sections to be brought in with rodata.
*/
#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
-#define TEXT_MAIN SECT_WILDCARD(.text)
#define DATA_MAIN SECT_WILDCARD(.data) .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L*
#define SDATA_MAIN SECT_WILDCARD(.sdata)
#define RODATA_MAIN SECT_WILDCARD(.rodata) .rodata..L*
#define BSS_MAIN SECT_WILDCARD(.bss) .bss..compoundliteral*
#define SBSS_MAIN SECT_WILDCARD(.sbss)
#else
-#define TEXT_MAIN .text
#define DATA_MAIN .data
#define SDATA_MAIN .sdata
#define RODATA_MAIN .rodata
@@ -115,6 +113,23 @@
#define SBSS_MAIN .sbss
#endif

+/*
+ * LTO_CLANG, LD_DEAD_CODE_DATA_ELIMINATION and 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_LTO_CLANG)) && !defined(CONFIG_FG_KASLR)
+#define TEXT_MAIN SECT_WILDCARD(.text)
+#elif defined(CONFIG_FG_KASLR)
+#define TEXT_MAIN .text.__unused__
+#else
+#define TEXT_MAIN .text
+#endif
+
/*
* GCC 4.5 and later have a 32 bytes section alignment for structures.
* Except GCC 4.9, that feels the need to align on 64 bytes.
@@ -843,6 +858,7 @@
#define ELF_DETAILS \
.comment 0 : { *(.comment) } \
.symtab 0 : { *(.symtab) } \
+ .symtab_shndx 0 : { *(.symtab_shndx) } \
.strtab 0 : { *(.strtab) } \
.shstrtab 0 : { *(.shstrtab) }

diff --git a/include/linux/linkage.h b/include/linux/linkage.h
index 0c0ddf4429dc..f3c96fb6a534 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -75,10 +75,11 @@

/*
* Allow ASM symbols to have their own unique sections if they are being
- * generated by the compiler for C functions (DCE, LTO).
+ * generated by the compiler for C functions (DCE, FG-KASLR, LTO).
*/
#if defined(CONFIG_HAVE_ASM_FUNCTION_SECTIONS) && \
((defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) && !defined(MODULE)) || \
+ (defined(CONFIG_FG_KASLR) && !defined(MODULE)) || \
(defined(CONFIG_LTO_CLANG)))

#define SYM_PUSH_SECTION(name) \
diff --git a/init/Kconfig b/init/Kconfig
index 3babc0aeac61..a74b3c3acb49 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1394,8 +1394,8 @@ config HAVE_ASM_FUNCTION_SECTIONS
This enables ASM function sections if both architecture
and toolchain supports that. It allows creating a separate
.text section for each ASM function in order to improve
- DCE and LTO (works the same way as -ffunction-sections for
- C code).
+ DCE, LTO and FG-KASLR (works the same way as -ffunction-sections
+ for C code).

config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
bool
@@ -2065,6 +2065,20 @@ config PROFILING
config TRACEPOINTS
bool

+config FG_KASLR
+ bool "Function Granular Kernel Address Space Layout Randomization"
+ depends on ARCH_SUPPORTS_FG_KASLR
+ depends on $(cc-option,-ffunction-sections)
+ depends on LD_HAS_Z_UNIQUE_SYMBOL || !LIVEPATCH
+ 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"
--
2.33.1