kselftests/arm64: add a basic Pointer Authentication test

PAuth signs and verifies return addresses on the stack. It does so by
inserting a Pointer Authentication code (PAC) into some of the unused top
bits of an address. This is achieved by adding paciasp/autiasp instructions
at the beginning and end of a function.

This feature is partially backwards compatible with earlier versions of the
ARM architecture. To coerce the compiler into emitting fully backwards
compatible code the main file is compiled to target an earlier ARM version.
This allows the tests to check for the feature and print meaningful error
messages instead of crashing.

Add a test to verify that corrupting the return address results in a
SIGSEGV on return.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Reviewed-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200918104715.182310-2-boian4o1@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>

tools/testing/selftests/arm64/Makefile

@@ -4,7 +4,7 @@
 ARCH ?= $(shell uname -m 2>/dev/null || echo not)
 
 ifneq (,$(filter $(ARCH),aarch64 arm64))
-ARM64_SUBTARGETS ?= tags signal
+ARM64_SUBTARGETS ?= tags signal pauth
 else
 ARM64_SUBTARGETS :=
 endif

tools/testing/selftests/arm64/pauth/.gitignore

@@ -0,0 +1 @@
+pac

tools/testing/selftests/arm64/pauth/Makefile

@@ -0,0 +1,32 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2020 ARM Limited
+
+# preserve CC value from top level Makefile
+ifeq ($(CC),cc)
+CC := $(CROSS_COMPILE)gcc
+endif
+
+CFLAGS += -mbranch-protection=pac-ret
+# check if the compiler supports ARMv8.3 and branch protection with PAuth
+pauth_cc_support := $(shell if ($(CC) $(CFLAGS) -march=armv8.3-a -E -x c /dev/null -o /dev/null 2>&1) then echo "1"; fi)
+
+ifeq ($(pauth_cc_support),1)
+TEST_GEN_PROGS := pac
+TEST_GEN_FILES := pac_corruptor.o
+endif
+
+include ../../lib.mk
+
+ifeq ($(pauth_cc_support),1)
+# pac* and aut* instructions are not available on architectures berfore
+# ARMv8.3. Therefore target ARMv8.3 wherever they are used directly
+$(OUTPUT)/pac_corruptor.o: pac_corruptor.S
+	$(CC) -c $^ -o $@ $(CFLAGS) -march=armv8.3-a
+
+# when -mbranch-protection is enabled and the target architecture is ARMv8.3 or
+# greater, gcc emits pac* instructions which are not in HINT NOP space,
+# preventing the tests from occurring at all. Compile for ARMv8.2 so tests can
+# run on earlier targets and print a meaningful error messages
+$(OUTPUT)/pac: pac.c $(OUTPUT)/pac_corruptor.o
+	$(CC) $^ -o $@ $(CFLAGS) -march=armv8.2-a
+endif

tools/testing/selftests/arm64/pauth/helper.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) 2020 ARM Limited */
+
+#ifndef _HELPER_H_
+#define _HELPER_H_
+
+void pac_corruptor(void);
+
+#endif

tools/testing/selftests/arm64/pauth/pac.c

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2020 ARM Limited
+
+#include <sys/auxv.h>
+#include <signal.h>
+#include <setjmp.h>
+
+#include "../../kselftest_harness.h"
+#include "helper.h"
+
+#define ASSERT_PAUTH_ENABLED() \
+do { \
+	unsigned long hwcaps = getauxval(AT_HWCAP); \
+	/* data key instructions are not in NOP space. This prevents a SIGILL */ \
+	ASSERT_NE(0, hwcaps & HWCAP_PACA) TH_LOG("PAUTH not enabled"); \
+} while (0)
+
+sigjmp_buf jmpbuf;
+void pac_signal_handler(int signum, siginfo_t *si, void *uc)
+{
+	if (signum == SIGSEGV || signum == SIGILL)
+		siglongjmp(jmpbuf, 1);
+}
+
+/* check that a corrupted PAC results in SIGSEGV or SIGILL */
+TEST(corrupt_pac)
+{
+	struct sigaction sa;
+
+	ASSERT_PAUTH_ENABLED();
+	if (sigsetjmp(jmpbuf, 1) == 0) {
+		sa.sa_sigaction = pac_signal_handler;
+		sa.sa_flags = SA_SIGINFO | SA_RESETHAND;
+		sigemptyset(&sa.sa_mask);
+
+		sigaction(SIGSEGV, &sa, NULL);
+		sigaction(SIGILL, &sa, NULL);
+
+		pac_corruptor();
+		ASSERT_TRUE(0) TH_LOG("SIGSEGV/SIGILL signal did not occur");
+	}
+}
+
+TEST_HARNESS_MAIN

tools/testing/selftests/arm64/pauth/pac_corruptor.S

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (C) 2020 ARM Limited */
+
+.global pac_corruptor
+
+.text
+/*
+ * Corrupting a single bit of the PAC ensures the authentication will fail.  It
+ * also guarantees no possible collision. TCR_EL1.TBI0 is set by default so no
+ * top byte PAC is tested
+ */
+ pac_corruptor:
+	paciasp
+
+	/* corrupt the top bit of the PAC */
+	eor lr, lr, #1 << 53
+
+	autiasp
+	ret