[PATCH v10 07/15] Makefile: add config options and build scripts for FG-KASLR

[Alexander Lobakin]

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