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Message-ID: <ebcefdf0-a71b-3b67-b133-3f47419f9ec8@arm.com>
Date: Thu, 3 Sep 2020 11:12:02 +0100
From: Boyan Karatotev <boyan.karatotev@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
Will Deacon <will@...nel.org>, boian4o1@...il.com,
Catalin Marinas <catalin.marinas@....com>,
amit.kachhap@....com, vincenzo.frascino@....com,
Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 1/4] kselftests/arm64: add a basic Pointer Authentication
test
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@...nel.org>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Signed-off-by: Boyan Karatotev <boyan.karatotev@....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
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