build: add hardened builds with PIE (ASLR) support

Introduce a configuration option to build a "hardened" OpenWrt with
ASLR PIE support.

Add new option PKG_ASLR_PIE to enable Address Space Layout Randomization (ASLR)
by building Position Independent Executables (PIE). This new option protects
against "return-to-text" attacks.

Busybox need a special care, link is done with ld, not gcc, leading to
unknown flags. Set BUSYBOX_DEFAULT_PIE instead and disable PKG_ASLR_PIE.

If other failing packages were found, PKG_ASLR_PIE:=0 should be added to
their Makefiles.

Original Work by: Yongkui Han <yonhan@cisco.com>
Signed-off-by: Julien Dusser <julien.dusser@free.fr>

config/Config-build.in

@@ -184,6 +184,22 @@ menu "Global build settings"
 		  this per package by adding PKG_CHECK_FORMAT_SECURITY:=0 in the package
 		  Makefile.
 
+	config PKG_ASLR_PIE
+		bool
+		prompt "User space ASLR PIE compilation"
+		select BUSYBOX_DEFAULT_PIE
+		default n
+		help
+		  Add -fPIC to CFLAGS and -specs=hardened-build-ld to LDFLAGS.
+		  This enables package build as Position Independent Executables (PIE)
+		  to protect against "return-to-text" attacks. This belongs to the
+		  feature of Address Space Layout Randomisation (ASLR), which is
+		  implemented by the kernel and the ELF loader by randomising the
+		  location of memory allocations. This makes memory addresses harder
+		  to predict when an attacker is attempting a memory-corruption exploit.
+		  You can disable this per package by adding PKG_ASLR_PIE:=0 in the package
+		  Makefile.
+
 	choice
 		prompt "User space Stack-Smashing Protection"
 		depends on USE_MUSL

include/hardened-ld-pie.specs

@@ -0,0 +1,2 @@
+*self_spec:
++ %{no-pie|static|r|shared:;:-pie}

include/hardening.mk

@@ -6,6 +6,7 @@
 #
 
 PKG_CHECK_FORMAT_SECURITY ?= 1
+PKG_ASLR_PIE ?= 1
 PKG_SSP ?= 1
 PKG_FORTIFY_SOURCE ?= 1
 PKG_RELRO ?= 1
@@ -15,6 +16,12 @@ ifdef CONFIG_PKG_CHECK_FORMAT_SECURITY
     TARGET_CFLAGS += -Wformat -Werror=format-security
   endif
 endif
+ifdef CONFIG_PKG_ASLR_PIE
+  ifeq ($(strip $(PKG_ASLR_PIE)),1)
+    TARGET_CFLAGS += -fPIC
+    TARGET_LDFLAGS += -specs=$(INCLUDE_DIR)/hardened-ld-pie.specs
+  endif
+endif
 ifdef CONFIG_PKG_CC_STACKPROTECTOR_REGULAR
   ifeq ($(strip $(PKG_SSP)),1)
     TARGET_CFLAGS += -fstack-protector

package/utils/busybox/Makefile

@@ -22,6 +22,9 @@ PKG_BUILD_PARALLEL:=1
 PKG_CHECK_FORMAT_SECURITY:=0
 PKG_INSTALL:=1
 
+#Busybox use it's own PIE config flag and LDFLAGS are used with ld, not gcc.
+PKG_ASLR_PIE:=0
+
 PKG_LICENSE:=GPL-2.0
 PKG_LICENSE_FILES:=LICENSE archival/libarchive/bz/LICENSE
 PKG_CPE_ID:=cpe:/a:busybox:busybox