From df0bd42fdeb76c9bc51b816c3df699db123c0024 Mon Sep 17 00:00:00 2001
From: Julien Dusser <julien.dusser@free.fr>
Date: Mon, 8 Jan 2018 23:47:06 +0100
Subject: [PATCH] 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         | 16 ++++++++++++++++
 include/hardened-ld-pie.specs  |  2 ++
 include/hardening.mk           |  7 +++++++
 package/utils/busybox/Makefile |  3 +++
 4 files changed, 28 insertions(+)
 create mode 100644 include/hardened-ld-pie.specs

diff --git a/config/Config-build.in b/config/Config-build.in
index 7ec7653a9a..660da1c47f 100644
--- a/config/Config-build.in
+++ b/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
diff --git a/include/hardened-ld-pie.specs b/include/hardened-ld-pie.specs
new file mode 100644
index 0000000000..7317b19a17
--- /dev/null
+++ b/include/hardened-ld-pie.specs
@@ -0,0 +1,2 @@
+*self_spec:
++ %{no-pie|static|r|shared:;:-pie}
diff --git a/include/hardening.mk b/include/hardening.mk
index c277081c51..06a61789ef 100644
--- a/include/hardening.mk
+++ b/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
diff --git a/package/utils/busybox/Makefile b/package/utils/busybox/Makefile
index 8866756aea..4f85cc9614 100644
--- a/package/utils/busybox/Makefile
+++ b/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
-- 
2.30.2