Re: [PATCH 4/4] kselftests/arm64: add PAuth tests for single threaded consistency and key uniqueness

[Vincenzo Frascino]

On 8/28/20 2:16 PM, Boyan Karatotev wrote:
> PAuth adds 5 different keys that can be used to sign addresses.
> 
> Add a test that verifies that the kernel initializes them uniquely and
> preserves them across context switches.
>

Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@xxxxxxx>

> Cc: Shuah Khan <shuah@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Boyan Karatotev <boyan.karatotev@xxxxxxx>
> ---
>  tools/testing/selftests/arm64/pauth/pac.c | 116 ++++++++++++++++++++++
>  1 file changed, 116 insertions(+)
> 
> diff --git a/tools/testing/selftests/arm64/pauth/pac.c b/tools/testing/selftests/arm64/pauth/pac.c
> index 16dea47b11c7..718f49adc275 100644
> --- a/tools/testing/selftests/arm64/pauth/pac.c
> +++ b/tools/testing/selftests/arm64/pauth/pac.c
> @@ -1,10 +1,13 @@
>  // SPDX-License-Identifier: GPL-2.0
>  // Copyright (C) 2020 ARM Limited
>  
> +#define _GNU_SOURCE
> +
>  #include <sys/auxv.h>
>  #include <sys/types.h>
>  #include <sys/wait.h>
>  #include <signal.h>
> +#include <sched.h>
>  
>  #include "../../kselftest_harness.h"
>  #include "helper.h"
> @@ -21,6 +24,7 @@
>   * The VA space size is 48 bits. Bigger is opt-in.
>   */
>  #define PAC_MASK (~0xff80ffffffffffff)
> +#define ARBITRARY_VALUE (0x1234)
>  #define ASSERT_PAUTH_ENABLED() \
>  do { \
>  	unsigned long hwcaps = getauxval(AT_HWCAP); \
> @@ -66,13 +70,36 @@ int are_same(struct signatures *old, struct signatures *new, int nkeys)
>  	return res;
>  }
>  
> +int are_unique(struct signatures *sign, int nkeys)
> +{
> +	size_t vals[nkeys];
> +
> +	vals[0] = sign->keyia & PAC_MASK;
> +	vals[1] = sign->keyib & PAC_MASK;
> +	vals[2] = sign->keyda & PAC_MASK;
> +	vals[3] = sign->keydb & PAC_MASK;
> +
> +	if (nkeys >= 4)
> +		vals[4] = sign->keyg & PAC_MASK;
> +
> +	for (int i = 0; i < nkeys - 1; i++) {
> +		for (int j = i + 1; j < nkeys; j++) {
> +			if (vals[i] == vals[j])
> +				return 0;
> +		}
> +	}
> +	return 1;
> +}
> +
>  int exec_sign_all(struct signatures *signed_vals, size_t val)
>  {
>  	int new_stdin[2];
>  	int new_stdout[2];
>  	int status;
> +	int i;
>  	ssize_t ret;
>  	pid_t pid;
> +	cpu_set_t mask;
>  
>  	ret = pipe(new_stdin);
>  	if (ret == -1) {
> @@ -86,6 +113,20 @@ int exec_sign_all(struct signatures *signed_vals, size_t val)
>  		return -1;
>  	}
>  
> +	/*
> +	 * pin this process and all its children to a single CPU, so it can also
> +	 * guarantee a context switch with its child
> +	 */
> +	sched_getaffinity(0, sizeof(mask), &mask);
> +
> +	for (i = 0; i < sizeof(cpu_set_t); i++)
> +		if (CPU_ISSET(i, &mask))
> +			break;
> +
> +	CPU_ZERO(&mask);
> +	CPU_SET(i, &mask);
> +	sched_setaffinity(0, sizeof(mask), &mask);
> +
>  	pid = fork();
>  	// child
>  	if (pid == 0) {
> @@ -192,6 +233,38 @@ TEST(pac_instructions_not_nop_generic)
>  	ASSERT_NE(0, keyg)  TH_LOG("keyg instructions did nothing");
>  }
>  
> +TEST(single_thread_unique_keys)
> +{
> +	int unique = 0;
> +	int nkeys = NKEYS;
> +	struct signatures signed_vals;
> +	unsigned long hwcaps = getauxval(AT_HWCAP);
> +
> +	/* generic and data key instructions are not in NOP space. This prevents a SIGILL */
> +	ASSERT_NE(0, hwcaps & HWCAP_PACA) TH_LOG("PAUTH not enabled");
> +	if (!(hwcaps & HWCAP_PACG)) {
> +		TH_LOG("WARNING: Generic PAUTH not enabled. Skipping generic key checks");
> +		nkeys = NKEYS - 1;
> +	}
> +
> +	/*
> +	 * The PAC field is up to 7 bits. Even with unique keys there is about
> +	 * 5% chance for a collision.  This chance rapidly increases the fewer
> +	 * bits there are, a comparison of the keys directly will be more
> +	 * reliable All signed values need to be unique at least once out of n
> +	 * attempts to be certain that the keys are unique
> +	 */
> +	for (int i = 0; i < PAC_COLLISION_ATTEMPTS; i++) {
> +		if (nkeys == NKEYS)
> +			sign_all(&signed_vals, i);
> +		else
> +			sign_specific(&signed_vals, i);
> +		unique |= are_unique(&signed_vals, nkeys);
> +	}
> +
> +	ASSERT_EQ(1, unique) TH_LOG("keys clashed every time");
> +}
> +
>  /*
>   * fork() does not change keys. Only exec() does so call a worker program.
>   * Its only job is to sign a value and report back the resutls
> @@ -227,5 +300,48 @@ TEST(exec_unique_keys)
>  	ASSERT_EQ(1, different) TH_LOG("exec() did not change keys");
>  }
>  
> +TEST(context_switch_keep_keys)
> +{
> +	int ret;
> +	struct signatures trash;
> +	struct signatures before;
> +	struct signatures after;
> +
> +	ASSERT_PAUTH_ENABLED();
> +
> +	sign_specific(&before, ARBITRARY_VALUE);
> +
> +	/* will context switch with a process with different keys at least once */
> +	ret = exec_sign_all(&trash, ARBITRARY_VALUE);
> +	ASSERT_EQ(0, ret) TH_LOG("failed to run worker");
> +
> +	sign_specific(&after, ARBITRARY_VALUE);
> +
> +	ASSERT_EQ(before.keyia, after.keyia) TH_LOG("keyia changed after context switching");
> +	ASSERT_EQ(before.keyib, after.keyib) TH_LOG("keyib changed after context switching");
> +	ASSERT_EQ(before.keyda, after.keyda) TH_LOG("keyda changed after context switching");
> +	ASSERT_EQ(before.keydb, after.keydb) TH_LOG("keydb changed after context switching");
> +}
> +
> +TEST(context_switch_keep_keys_generic)
> +{
> +	int ret;
> +	struct signatures trash;
> +	size_t before;
> +	size_t after;
> +
> +	ASSERT_GENERIC_PAUTH_ENABLED();
> +
> +	before = keyg_sign(ARBITRARY_VALUE);
> +
> +	/* will context switch with a process with different keys at least once */
> +	ret = exec_sign_all(&trash, ARBITRARY_VALUE);
> +	ASSERT_EQ(0, ret) TH_LOG("failed to run worker");
> +
> +	after = keyg_sign(ARBITRARY_VALUE);
> +
> +	ASSERT_EQ(before, after) TH_LOG("keyg changed after context switching");
> +}
> +
>  TEST_HARNESS_MAIN
>  
> 

-- 
Regards,
Vincenzo