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Subject: Re: [PATCH 1/4] kselftests/arm64: add a basic Pointer Authentication
 test
To:     Boyan Karatotev <boyan.karatotev@arm.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     amit.kachhap@arm.com, boian4o1@gmail.com,
        Shuah Khan <shuah@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>
References: <20200828131606.7946-1-boyan.karatotev@arm.com>
 <20200828131606.7946-2-boyan.karatotev@arm.com>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <9a975ad4-802a-2f94-0555-94a03dac1343@arm.com>
Date:   Fri, 28 Aug 2020 15:28:56 +0100
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On 8/28/20 2:16 PM, Boyan Karatotev wrote:
> 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.
> 

Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>

> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
> ---
>  tools/testing/selftests/arm64/Makefile        |  2 +-
>  .../testing/selftests/arm64/pauth/.gitignore  |  1 +
>  tools/testing/selftests/arm64/pauth/Makefile  | 22 ++++++++++++
>  tools/testing/selftests/arm64/pauth/helper.h  | 10 ++++++
>  tools/testing/selftests/arm64/pauth/pac.c     | 32 +++++++++++++++++
>  .../selftests/arm64/pauth/pac_corruptor.S     | 36 +++++++++++++++++++
>  6 files changed, 102 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/arm64/pauth/.gitignore
>  create mode 100644 tools/testing/selftests/arm64/pauth/Makefile
>  create mode 100644 tools/testing/selftests/arm64/pauth/helper.h
>  create mode 100644 tools/testing/selftests/arm64/pauth/pac.c
>  create mode 100644 tools/testing/selftests/arm64/pauth/pac_corruptor.S
> 
> diff --git a/tools/testing/selftests/arm64/Makefile b/tools/testing/selftests/arm64/Makefile
> index 93b567d23c8b..525506fd97b9 100644
> --- a/tools/testing/selftests/arm64/Makefile
> +++ b/tools/testing/selftests/arm64/Makefile
> @@ -4,7 +4,7 @@
>  ARCH ?= $(shell uname -m 2>/dev/null || echo not)
>  
>  ifneq (,$(filter $(ARCH),aarch64 arm64))
> -ARM64_SUBTARGETS ?= tags signal
> +ARM64_SUBTARGETS ?= tags signal pauth
>  else
>  ARM64_SUBTARGETS :=
>  endif
> diff --git a/tools/testing/selftests/arm64/pauth/.gitignore b/tools/testing/selftests/arm64/pauth/.gitignore
> new file mode 100644
> index 000000000000..b557c916720a
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/pauth/.gitignore
> @@ -0,0 +1 @@
> +pac
> diff --git a/tools/testing/selftests/arm64/pauth/Makefile b/tools/testing/selftests/arm64/pauth/Makefile
> new file mode 100644
> index 000000000000..785c775e5e41
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/pauth/Makefile
> @@ -0,0 +1,22 @@
> +# SPDX-License-Identifier: GPL-2.0
> +# Copyright (C) 2020 ARM Limited
> +
> +CFLAGS += -mbranch-protection=pac-ret
> +
> +TEST_GEN_PROGS := pac
> +TEST_GEN_FILES := pac_corruptor.o
> +
> +include ../../lib.mk
> +
> +# pac* and aut* instructions are not available on architectures berfore
> +# ARMv8.3. Therefore target ARMv8.3 wherever they are used directly
> +$(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
> +
> diff --git a/tools/testing/selftests/arm64/pauth/helper.h b/tools/testing/selftests/arm64/pauth/helper.h
> new file mode 100644
> index 000000000000..f777f88acf0a
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/pauth/helper.h
> @@ -0,0 +1,10 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2020 ARM Limited */
> +
> +#ifndef _HELPER_H_
> +#define _HELPER_H_
> +
> +void pac_corruptor(void);
> +
> +#endif
> +
> diff --git a/tools/testing/selftests/arm64/pauth/pac.c b/tools/testing/selftests/arm64/pauth/pac.c
> new file mode 100644
> index 000000000000..ed445050f621
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/pauth/pac.c
> @@ -0,0 +1,32 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright (C) 2020 ARM Limited
> +
> +#include <sys/auxv.h>
> +#include <signal.h>
> +
> +#include "../../kselftest_harness.h"
> +#include "helper.h"
> +
> +/*
> + * Tests are ARMv8.3 compliant. They make no provisions for features present in
> + * future version of the arm architecture
> + */
> +
> +#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)
> +
> +
> +/* check that a corrupted PAC results in SIGSEGV */
> +TEST_SIGNAL(corrupt_pac, SIGSEGV)
> +{
> +	ASSERT_PAUTH_ENABLED();
> +
> +	pac_corruptor();
> +}
> +
> +TEST_HARNESS_MAIN
> +
> diff --git a/tools/testing/selftests/arm64/pauth/pac_corruptor.S b/tools/testing/selftests/arm64/pauth/pac_corruptor.S
> new file mode 100644
> index 000000000000..6a34ec23a034
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/pauth/pac_corruptor.S
> @@ -0,0 +1,36 @@
> +/* 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
> +
> +	/* make stack frame */
> +	sub sp, sp, #16
> +	stp x29, lr, [sp]
> +	mov x29, sp
> +
> +	/* prepare mask for bit to be corrupted (bit 54) */
> +	mov x1, xzr
> +	add x1, x1, #1
> +	lsl x1, x1, #54
> +
> +	/* get saved lr, corrupt selected bit, put it back */
> +	ldr x0, [sp, #8]
> +	eor x0, x0, x1
> +	str x0, [sp, #8]
> +
> +	/* remove stack frame */
> +	ldp x29, lr, [sp]
> +	add sp, sp, #16
> +
> +	autiasp
> +	ret
> +
> 

-- 
Regards,
Vincenzo