From: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Add Kconfig symbols CONFIG_ARCH_SUPPORTS_FG_KASLR and
CONFIG_FG_KASLR. The first is hidden and used to indicate that
a particular architecture supports it, the second allows a user
to enable FG-KASLR when the former is set to 'y'.
Make Kbuild not consolidate function sections back into `.text`
on linking if CONFIG_FG_KASLR is enabled (even with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y) as the feature itself
relies on functions still being separated in the final vmlinux.
Alexander Lobakin:
Improve KBUILD_CFLAGS{,_MODULE} 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 it is going
to be supported by the architecture-specific code. Otherwise LD
emits 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
(on non-LLD builds).
Signed-off-by: Kristen Carlson Accardi <kristen@xxxxxxxxxxxxxxx>
Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx> # coexistence with DCE
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Tested-by: Tony Luck <tony.luck@xxxxxxxxx>
Co-developed-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
---
Makefile | 17 ++++++++++++++---
arch/Kconfig | 6 +++++-
include/asm-generic/vmlinux.lds.h | 20 ++++++++++++++++++--
include/linux/linkage.h | 9 +++++----
init/Kconfig | 19 +++++++++++++++++--
5 files changed, 59 insertions(+), 12 deletions(-)
diff --git a/Makefile b/Makefile
index fbe2d13028f4..4328d53d8b25 100644
--- a/Makefile
+++ b/Makefile
@@ -872,7 +872,7 @@ 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
@@ -881,7 +881,7 @@ 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 += $(SECSUBST_AFLAGS)
@@ -895,8 +895,19 @@ KBUILD_CFLAGS_MODULE += -Wa,--sectname-subst
endif
endif # CONFIG_HAVE_ASM_FUNCTION_SECTIONS
+# ClangLTO implies `-ffunction-sections -fdata-sections`, no need
+# to specify them manually and trigger a pointless full rebuild
+ifndef CONFIG_LTO_CLANG
+ifneq ($(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION)$(CONFIG_FG_KASLR),)
+KBUILD_CFLAGS_KERNEL += -ffunction-sections
+endif
+
+ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+KBUILD_CFLAGS_KERNEL += -fdata-sections
+endif
+endif # CONFIG_LTO_CLANG
+
ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
-KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
LDFLAGS_vmlinux += --gc-sections
endif
diff --git a/arch/Kconfig b/arch/Kconfig
index 550f0599e211..e06aeeea39f4 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1326,7 +1326,11 @@ config ARCH_SUPPORTS_ASM_FUNCTION_SECTIONS
bool
help
An arch should select this if it can be built and run with its
- asm functions placed into separate sections to improve DCE and LTO.
+ asm functions placed into separate sections to improve DCE, LTO
+ and FG-KASLR.
+
+config ARCH_SUPPORTS_FG_KASLR
+ bool
source "kernel/gcov/Kconfig"
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e7b8a84e0e64..586465b2abb2 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -100,14 +100,12 @@
* sections to be brought in with rodata.
*/
#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
-#define TEXT_MAIN SECT_WILDCARD(.text)
#define DATA_MAIN SECT_WILDCARD(.data) .data..L* .data..compoundliteral* .data.$__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 f3b966a6427e..95ca162a868c 100644
--- a/include/linux/linkage.h
+++ b/include/linux/linkage.h
@@ -75,11 +75,12 @@
/*
* Allow ASM symbols to have their own unique sections if they are being
- * generated by the compiler for C functions (DCE, LTO). Correlates with
- * the presence of the `-ffunction-section` in KBUILD_CFLAGS.
+ * generated by the compiler for C functions (DCE, FG-KASLR, LTO). Correlates
+ * with the presence of the `-ffunction-section` in KBUILD_CFLAGS.
*/
#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) \
@@ -91,13 +92,13 @@
#define __ASM_PUSH_SECTION(name) \
".pushsection %S." name ", \"ax\""
-#else /* !(CONFIG_HAVE_ASM_FUNCTION_SECTIONS && (DCE || LTO)) */
+#else /* !(CONFIG_HAVE_ASM_FUNCTION_SECTIONS && (DCE || FG_KASLR || LTO)) */
#define SYM_PUSH_SECTION(name)
#define SYM_POP_SECTION()
#define __ASM_PUSH_SECTION(name)
-#endif /* !(CONFIG_HAVE_ASM_FUNCTION_SECTIONS && (DCE || LTO)) */
+#endif /* !(CONFIG_HAVE_ASM_FUNCTION_SECTIONS && (DCE || FG_KASLR || LTO)) */
#define ASM_PUSH_SECTION(name) \
__ASM_PUSH_SECTION(__stringify(name))
diff --git a/init/Kconfig b/init/Kconfig
index 4acfc80f22df..26f9a6e52dbd 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1393,8 +1393,9 @@ config HAVE_ASM_FUNCTION_SECTIONS
help
This enables asm function sections if both architecture and
toolchain support it. It allows creating a separate section
- for each function written in assembly in order to improve DCE
- and LTO (works the same way as -ffunction-sections for C code).
+ for each function written in assembly in order to improve DCE,
+ LTO and FG-KASLR (works the same way as -ffunction-sections
+ for C code).
config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
bool
@@ -2061,6 +2062,20 @@ 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"
--
2.34.1
