Commit bf8ddcfa authored by Kristina Martsenko's avatar Kristina Martsenko Committed by Amit Kachhap

arm64: compile the kernel with ptrauth return address signing

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

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. This option is not used for GNU toolchain.

Cc: Catalin Marinas <>
Cc: Will Deacon <>
Cc: Masahiro Yamada <>
Reviewed-by: default avatarKees Cook <>
Co-developed-by: Vincenzo Frascino's avatarVincenzo Frascino <>
Signed-off-by: Vincenzo Frascino's avatarVincenzo Frascino <>
Signed-off-by: default avatarKristina Martsenko <>
[Amit: Cover leaf function, comments]
Signed-off-by: Amit Kachhap's avatarAmit Daniel Kachhap <>
parent 7b70f629
......@@ -1499,6 +1499,7 @@ config ARM64_PTR_AUTH
bool "Enable support for pointer authentication"
default y
depends on !KVM || ARM64_VHE
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.
# GCC 9 or later, clang 8 or later
def_bool $(cc-option,-mbranch-protection=pac-ret+leaf)
# 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)
menu "ARMv8.5 architectural features"
......@@ -65,6 +65,17 @@ stack_protector_prepare: prepare0
ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
# -march=armv8.3-a enables the non-nops instructions for PAC, to avoid the compiler
# to generate them and consequently to break the single image contract we pass it
# only to the assembler when clang is selected as a compiler. For the GNU toolchain
# this option is not used.
branch-prot-flags-$(CONFIG_AS_HAS_PAC) += -Wa,-march=armv8.3-a
KBUILD_CFLAGS += $(branch-prot-flags-y)
KBUILD_CPPFLAGS += -mbig-endian
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