randstruct: Move seed generation into scripts/basic/

To enable Clang randstruct support, move the structure layout
randomization seed generation out of scripts/gcc-plugins/ into
scripts/basic/ so it happens early enough that it can be used by either
compiler implementation. The gcc-plugin still builds its own header file,
but now does so from the common "randstruct.seed" file.

Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-6-keescook@chromium.org

Documentation/dontdiff

@@ -211,6 +211,7 @@ r200_reg_safe.h
 r300_reg_safe.h
 r420_reg_safe.h
 r600_reg_safe.h
+randstruct.seed
 randomize_layout_hash.h
 randomize_layout_seed.h
 recordmcount

Documentation/kbuild/reproducible-builds.rst

@@ -100,8 +100,9 @@ Structure randomisation
 -----------------------
 
 If you enable ``CONFIG_RANDSTRUCT``, you will need to pre-generate
-the random seed in ``scripts/gcc-plugins/randomize_layout_seed.h``
+the random seed in ``scripts/basic/randstruct.seed`` so the same
-so the same value is used in rebuilds.
+value is used by each build. See ``scripts/gen-randstruct-seed.sh``
+for details.
 
 Debug info conflicts
 --------------------

include/linux/vermagic.h

@@ -33,7 +33,7 @@
 #define MODULE_VERMAGIC_MODVERSIONS ""
 #endif
 #ifdef RANDSTRUCT
-#include <generated/randomize_layout_hash.h>
+#include <generated/randstruct_hash.h>
 #define MODULE_RANDSTRUCT "RANDSTRUCT_" RANDSTRUCT_HASHED_SEED
 #else
 #define MODULE_RANDSTRUCT

scripts/basic/.gitignore

@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
 /fixdep
+/randstruct.seed

scripts/basic/Makefile

@@ -3,3 +3,14 @@
 # fixdep: used to generate dependency information during build process
 
 hostprogs-always-y	+= fixdep
+
+# randstruct: the seed is needed before building the gcc-plugin or
+# before running a Clang kernel build.
+gen-randstruct-seed	:= $(srctree)/scripts/gen-randstruct-seed.sh
+quiet_cmd_create_randstruct_seed = GENSEED $@
+cmd_create_randstruct_seed = \
+	$(CONFIG_SHELL) $(gen-randstruct-seed) \
+		$@ $(objtree)/include/generated/randstruct_hash.h
+$(obj)/randstruct.seed: $(gen-randstruct-seed) FORCE
+	$(call if_changed,create_randstruct_seed)
+always-$(CONFIG_RANDSTRUCT) += randstruct.seed

scripts/gcc-plugins/Makefile

@@ -1,12 +1,17 @@
 # SPDX-License-Identifier: GPL-2.0
 
-$(obj)/randomize_layout_plugin.so: $(objtree)/$(obj)/randomize_layout_seed.h
+$(obj)/randomize_layout_plugin.so: $(obj)/randomize_layout_seed.h
-quiet_cmd_create_randomize_layout_seed = GENSEED $@
+quiet_cmd_create_randomize_layout_seed = SEEDHDR $@
 cmd_create_randomize_layout_seed = \
-  $(CONFIG_SHELL) $(srctree)/$(src)/gen-random-seed.sh $@ $(objtree)/include/generated/randomize_layout_hash.h
+	SEED=$$(cat $(filter-out FORCE,$^) </dev/null); \
-$(objtree)/$(obj)/randomize_layout_seed.h: FORCE
+	echo '/*' > $@; \
+	echo ' * This file is automatically generated. Keep it private.' >> $@; \
+	echo ' * Exposing this value will expose the layout of randomized structures.' >> $@; \
+	echo ' */' >> $@; \
+	echo "const char *randstruct_seed = \"$$SEED\";" >> $@
+$(obj)/randomize_layout_seed.h: $(objtree)/scripts/basic/randstruct.seed FORCE
 	$(call if_changed,create_randomize_layout_seed)
-targets += randomize_layout_seed.h randomize_layout_hash.h
+targets += randomize_layout_seed.h
 
 # Build rules for plugins
 #

scripts/gcc-plugins/gen-random-seed.sh

@@ -1,9 +0,0 @@
-#!/bin/sh
-# SPDX-License-Identifier: GPL-2.0
-
-if [ ! -f "$1" ]; then
-	SEED=`od -A n -t x8 -N 32 /dev/urandom | tr -d ' \n'`
-	echo "const char *randstruct_seed = \"$SEED\";" > "$1"
-	HASH=`echo -n "$SEED" | sha256sum | cut -d" " -f1 | tr -d ' \n'`
-	echo "#define RANDSTRUCT_HASHED_SEED \"$HASH\"" > "$2"
-fi

scripts/gen-randstruct-seed.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+SEED=$(od -A n -t x8 -N 32 /dev/urandom | tr -d ' \n')
+echo "$SEED" > "$1"
+HASH=$(echo -n "$SEED" | sha256sum | cut -d" " -f1)
+echo "#define RANDSTRUCT_HASHED_SEED \"$HASH\"" > "$2"

security/Kconfig.hardening

@@ -284,10 +284,11 @@ choice
 	  tools like Volatility against the system (unless the kernel
 	  source tree isn't cleaned after kernel installation).
 
-	  The seed used for compilation is located at
+	  The seed used for compilation is in scripts/basic/randomize.seed.
-	  scripts/randomize_layout_seed.h. It remains after a "make clean"
+	  It remains after a "make clean" to allow for external modules to
-	  to allow for external modules to be compiled with the existing
+	  be compiled with the existing seed and will be removed by a
-	  seed and will be removed by a "make mrproper" or "make distclean".
+	  "make mrproper" or "make distclean". This file should not be made
+	  public, or the structure layout can be determined.
 
 	config RANDSTRUCT_NONE
 		bool "Disable structure layout randomization"