Subject: [PATCH v6 17/18] arm64: compile the kernel with ptrauth return address signing

From: Amit Daniel Kachhap <[email-masked]>
Date: Fri, 6 Mar 2020 12:05:24 +0530

From: Kristina Martsenko <[email-masked]>

Compile all functions with two ptrauth instructions: PACIASP in the
prologue to sign the return address, and AUTIASP in the epilogue to
authenticate the return address (from the stack). If authentication
fails, the return will cause an instruction abort to be taken, followed
by an oops and killing the task.

This should help protect the kernel against attacks using
return-oriented programming. As ptrauth protects the return address, it
can also serve as a replacement for CONFIG_STACKPROTECTOR, although note
that it does not protect other parts of the stack.

The new instructions are in the HINT encoding space, so on a system
without ptrauth they execute as NOPs.

CONFIG_ARM64_PTR_AUTH now not only enables ptrauth for userspace and KVM
guests, but also automatically builds the kernel with ptrauth
instructions if the compiler supports it. If there is no compiler
support, we do not warn that the kernel was built without ptrauth
instructions.

GCC 7 and 8 support the -msign-return-address option, while GCC 9
deprecates that option and replaces it with -mbranch-protection. Support
both options.

Clang uses an external assembler hence this patch makes sure that the
correct parameters (-march=armv8.3-a) are passed down to help it recognize
the ptrauth instructions.

Cc: Catalin Marinas <[email-masked]>
Cc: Will Deacon <[email-masked]>
Cc: Masahiro Yamada <[email-masked]>
Reviewed-by: Kees Cook <[email-masked]>
Co-developed-by: Vincenzo Frascino <[email-masked]>
Signed-off-by: Vincenzo Frascino <[email-masked]>
Signed-off-by: Kristina Martsenko <[email-masked]>
[Amit: Cover leaf function, comments]
Signed-off-by: Amit Daniel Kachhap <[email-masked]>
---
Changes since v5:
 * Clarified assembler option for GNU toochain.

 arch/arm64/Kconfig  | 20 +++++++++++++++++++-
 arch/arm64/Makefile | 11 +++++++++++
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 115ceea..0f3ea01 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1499,6 +1499,7 @@ config ARM64_PTR_AUTH
 	bool "Enable support for pointer authentication"
 	default y
 	depends on !KVM || ARM64_VHE
+	depends on (CC_HAS_SIGN_RETURN_ADDRESS || CC_HAS_BRANCH_PROT_PAC_RET) && AS_HAS_PAC
 	help
 	  Pointer authentication (part of the ARMv8.3 Extensions) provides
 	  instructions for signing and authenticating pointers against secret
@@ -1506,11 +1507,17 @@ config ARM64_PTR_AUTH
 	  and other attacks.
 
 	  This option enables these instructions at EL0 (i.e. for userspace).
-
 	  Choosing this option will cause the kernel to initialise secret keys
 	  for each process at exec() time, with these keys being
 	  context-switched along with the process.
 
+	  If the compiler supports the -mbranch-protection or
+	  -msign-return-address flag (e.g. GCC 7 or later), then this option
+	  will also cause the kernel itself to be compiled with return address
+	  protection. In this case, and if the target hardware is known to
+	  support pointer authentication, then CONFIG_STACKPROTECTOR can be
+	  disabled with minimal loss of protection.
+
 	  The feature is detected at runtime. If the feature is not present in
 	  hardware it will not be advertised to userspace/KVM guest nor will it
 	  be enabled. However, KVM guest also require VHE mode and hence
@@ -1522,6 +1529,17 @@ config ARM64_PTR_AUTH
 	  but with the feature disabled. On such a system, this option should
 	  not be selected.
 
+config CC_HAS_BRANCH_PROT_PAC_RET
+	# GCC 9 or later, clang 8 or later
+	def_bool $(cc-option,-mbranch-protection=pac-ret+leaf)
+
+config CC_HAS_SIGN_RETURN_ADDRESS
+	# GCC 7, 8
+	def_bool $(cc-option,-msign-return-address=all)
+
+config AS_HAS_PAC
+	def_bool $(as-option,-Wa$(comma)-march=armv8.3-a)
+
 endmenu
 
 menu "ARMv8.5 architectural features"
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index dca1a97..f15f92b 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -65,6 +65,17 @@ stack_protector_prepare: prepare0
 					include/generated/asm-offsets.h))
 endif
 
+ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
+branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
+branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
+# -march=armv8.3-a enables the non-nops instructions for PAC, to avoid the
+# compiler to generate them and consequently to break the single image contract
+# we pass it only to the assembler. This option is utilized only in case of non
+# integrated assemblers.
+branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a
+KBUILD_CFLAGS += $(branch-prot-flags-y)
+endif
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
-- 
2.7.4

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