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Subject: Re: [PATCH 1/4] kselftests/arm64: add a basic Pointer Authentication
 test
To:     Dave Martin <Dave.Martin@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
        Will Deacon <will@kernel.org>, boian4o1@gmail.com,
        Catalin Marinas <catalin.marinas@arm.com>,
        amit.kachhap@arm.com, vincenzo.frascino@arm.com,
        Shuah Khan <shuah@kernel.org>
References: <20200828131606.7946-1-boyan.karatotev@arm.com>
 <20200828131606.7946-2-boyan.karatotev@arm.com>
 <20200902164858.GI6642@arm.com>
From:   Boyan Karatotev <boyan.karatotev@arm.com>
Message-ID: <ebcefdf0-a71b-3b67-b133-3f47419f9ec8@arm.com>
Date:   Thu, 3 Sep 2020 11:12:02 +0100
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On 02/09/2020 17:49, Dave Martin wrote:
> On Fri, Aug 28, 2020 at 02:16:03PM +0100, 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.
>>
>> 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]
> 
> Nit: if respinning, you can optimise a few sequences of this sort, e.g.
> 
> 	stp	x29, lr, [sp, #-16]!
> 
>> +	mov x29, sp
>> +
>> +	/* prepare mask for bit to be corrupted (bit 54) */
>> +	mov x1, xzr
>> +	add x1, x1, #1
>> +	lsl x1, x1, #54
> 
> Nit:
> 
> 	mov	x1, #1 << 54
Thank you for this, didn't know I could do it this way.
> 
> but anyway, the logic operations can encode most simple bitmasks
> directly as immediate operands, so you can skip this and just do
> 
>> +
>> +	/* get saved lr, corrupt selected bit, put it back */
>> +	ldr x0, [sp, #8]
>> +	eor x0, x0, x1
> 
> 	eor	x0, x0, #1 << 54
> 
>> +	str x0, [sp, #8]
>> +
>> +	/* remove stack frame */
>> +	ldp x29, lr, [sp]
>> +	add sp, sp, #16
> 
> 	ldp	x29, lr, [sp], #16
> 
> [...]
> 
> Actually, since there are no leaf nested function calls and no trap is
> expected until the function returns (so backtracing in the middle of
> this function is unlikely to be needed), could we optimise this whole
> thing down to the following?
> 
I suppose you're right. The intent was to emulate a c function but there
really is no point in doing all this extra work. Will change it.
> pac_corruptor:
> 	paciasp
> 	eor	lr, lr, #1 << 53
> 	autiasp
> 	ret
> 
> Cheers
> ---Dave
> 


-- 
Regards,
Boyan