xnu-7195.101.1.tar.gz

[apple/xnu.git] / osfmk / prng / entropy.c

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#include <libkern/crypto/sha2.h>
#include <libkern/crypto/crypto_internal.h>
#include <os/atomic_private.h>
#include <kern/assert.h>
#include <kern/percpu.h>
#include <kern/zalloc.h>
#include <kern/lock_group.h>
#include <kern/locks.h>
#include <kern/misc_protos.h>
#include <pexpert/pexpert.h>
#include <prng/entropy.h>
#include <machine/machine_routines.h>
#include <libkern/section_keywords.h>
#include <sys/cdefs.h>

// The number of samples we can hold in an entropy buffer.
#define ENTROPY_MAX_SAMPLE_COUNT (2048)

// The state for a per-CPU entropy buffer.
typedef struct entropy_cpu_data {
        // A buffer to hold entropy samples.
        entropy_sample_t samples[ENTROPY_MAX_SAMPLE_COUNT];

        // A count of samples resident in the buffer. It also functions as
        // an index to the buffer. All entries at indices less than the
        // sample count are considered valid for consumption by the
        // reader. The reader resets this to zero after consuming the
        // available entropy.
        uint32_t _Atomic sample_count;
} entropy_cpu_data_t;

// This structure holds the state for an instance of a FIPS continuous
// health test. In practice, we do not expect these tests to fail.
typedef struct entropy_health_test {
        // The initial sample observed in this test instance. Tests look
        // for some repetition of the sample, either consecutively or
        // within a window.
        entropy_sample_t init_observation;

        // The count of times the initial observation has recurred within
        // the span of the current test.
        uint64_t observation_count;

        // The statistics are only relevant for telemetry and parameter
        // tuning. They do not drive any actual logic in the module.
        entropy_health_stats_t *stats;
} entropy_health_test_t;

typedef enum health_test_result {
        health_test_failure,
        health_test_success
} health_test_result_t;

// Along with various counters and the buffer itself, this includes
// the state for two FIPS continuous health tests.
typedef struct entropy_data {
        // State for a SHA256 computation. This is used to accumulate
        // entropy samples from across all CPUs. It is finalized when
        // entropy is provided to the consumer of this module.
        SHA256_CTX sha256_ctx;

        // Since the corecrypto kext is not loaded when this module is
        // initialized, we cannot initialize the SHA256 state at that
        // time. Instead, we initialize it lazily during entropy
        // consumption. This flag tracks whether initialization is
        // complete.
        bool sha256_ctx_init;

        // A total count of entropy samples that have passed through this
        // structure. It is incremented as new samples are accumulated
        // from the various per-CPU structures. The "current" count of
        // samples is the difference between this field and the "read"
        // sample count below (which see).
        uint64_t total_sample_count;

        // Initially zero, this flag is reset to the current sample count
        // if and when we fail a health test. We consider the startup
        // health tests to be complete when the difference between the
        // total sample count and this field is at least 1024. In other
        // words, we must accumulate 1024 good samples to demonstrate
        // viability. We refuse to provide any entropy before that
        // threshold is reached.
        uint64_t startup_sample_count;

        // The count of samples from the last time we provided entropy to
        // the kernel RNG. We use this to compute how many new samples we
        // have to contribute. This value is also reset to the current
        // sample count in case of health test failure.
        uint64_t read_sample_count;

        // The lock group for this structure; see below.
        lck_grp_t lock_group;

        // This structure accumulates entropy samples from across all CPUs
        // for a single point of consumption protected by a mutex.
        lck_mtx_t mutex;

        // State for the Repetition Count Test.
        entropy_health_test_t repetition_count_test;

        // State for the Adaptive Proportion Test.
        entropy_health_test_t adaptive_proportion_test;
} entropy_data_t;

static entropy_cpu_data_t PERCPU_DATA(entropy_cpu_data);

int entropy_health_startup_done;
entropy_health_stats_t entropy_health_rct_stats;
entropy_health_stats_t entropy_health_apt_stats;

static entropy_data_t entropy_data = {
        .repetition_count_test = {
                .init_observation = -1,
                .stats = &entropy_health_rct_stats,
        },
        .adaptive_proportion_test = {
                .init_observation = -1,
                .stats = &entropy_health_apt_stats,
        },
};

__security_const_late entropy_sample_t *entropy_analysis_buffer;
__security_const_late uint32_t entropy_analysis_buffer_size;
static __security_const_late uint32_t entropy_analysis_max_sample_count;
static uint32_t entropy_analysis_sample_count;

__startup_func
static void
entropy_analysis_init(uint32_t sample_count)
{
        entropy_analysis_max_sample_count = sample_count;
        entropy_analysis_buffer_size = sample_count * sizeof(entropy_sample_t);
        entropy_analysis_buffer = zalloc_permanent(entropy_analysis_buffer_size, ZALIGN(entropy_sample_t));
}

__startup_func
void
entropy_init(void)
{
        lck_grp_init(&entropy_data.lock_group, "entropy-data", LCK_GRP_ATTR_NULL);
        lck_mtx_init(&entropy_data.mutex, &entropy_data.lock_group, LCK_ATTR_NULL);

        // The below path is used only for testing. This boot arg is used
        // to collect raw entropy samples for offline analysis. The "ebsz"
        // name is supported only until dependent tools can be updated to
        // use the more descriptive "entropy-analysis-sample-count".
        uint32_t sample_count = 0;
        if (__improbable(PE_parse_boot_argn("entropy-analysis-sample-count", &sample_count, sizeof(sample_count)))) {
                entropy_analysis_init(sample_count);
        } else if (__improbable(PE_parse_boot_argn("ebsz", &sample_count, sizeof(sample_count)))) {
                entropy_analysis_init(sample_count);
        }
}

void
entropy_collect(void)
{
        // This function is called from within the interrupt handler, so
        // we do not need to disable interrupts.

        entropy_cpu_data_t *e = PERCPU_GET(entropy_cpu_data);

        uint32_t sample_count = os_atomic_load(&e->sample_count, relaxed);

        assert(sample_count <= ENTROPY_MAX_SAMPLE_COUNT);

        // If the buffer is full, we return early without collecting
        // entropy.
        if (sample_count == ENTROPY_MAX_SAMPLE_COUNT) {
                return;
        }

        e->samples[sample_count] = (entropy_sample_t)ml_get_timebase_entropy();

        // If the consumer has reset the sample count on us, the only
        // consequence is a dropped sample. We effectively abort the
        // entropy collection in this case.
        (void)os_atomic_cmpxchg(&e->sample_count, sample_count, sample_count + 1, release);
}

// For information on the following tests, see NIST SP 800-90B 4
// Health Tests. These tests are intended to detect catastrophic
// degradations in entropy. As noted in that document:
//
// > Health tests are expected to raise an alarm in three cases:
// > 1. When there is a significant decrease in the entropy of the
// > outputs,
// > 2. When noise source failures occur, or
// > 3. When hardware fails, and implementations do not work
// > correctly.
//
// Each entropy accumulator declines to release entropy until the
// startup tests required by NIST are complete. In the event that a
// health test does fail, all entropy accumulators are reset and
// decline to release further entropy until their startup tests can be
// repeated.

static health_test_result_t
add_observation(entropy_health_test_t *t, uint64_t bound)
{
        t->observation_count += 1;
        t->stats->max_observation_count = MAX(t->stats->max_observation_count, (uint32_t)t->observation_count);
        if (__improbable(t->observation_count >= bound)) {
                t->stats->failure_count += 1;
                return health_test_failure;
        }

        return health_test_success;
}

static void
reset_test(entropy_health_test_t *t, entropy_sample_t observation)
{
        t->stats->reset_count += 1;
        t->init_observation = observation;
        t->observation_count = 1;
        t->stats->max_observation_count = MAX(t->stats->max_observation_count, (uint32_t)t->observation_count);
}

// 4.4.1 Repetition Count Test
//
// Like the name implies, this test counts consecutive occurrences of
// the same value.
//
// We compute the bound C as:
//
// A = 2^-128
// H = 1
// C = 1 + ceil(-log(A, 2) / H) = 129
//
// With A the acceptable chance of false positive and H a conservative
// estimate for the entropy (in bits) of each sample.

#define REPETITION_COUNT_BOUND (129)

static health_test_result_t
repetition_count_test(entropy_sample_t observation)
{
        entropy_health_test_t *t = &entropy_data.repetition_count_test;

        if (t->init_observation == observation) {
                return add_observation(t, REPETITION_COUNT_BOUND);
        } else {
                reset_test(t, observation);
        }

        return health_test_success;
}

// 4.4.2 Adaptive Proportion Test
//
// This test counts occurrences of a value within a window of samples.
//
// We use a non-binary alphabet, giving us a window size of 512. (In
// particular, we consider the least-significant byte of each time
// sample.)
//
// Assuming one bit of entropy, we can compute the binomial cumulative
// distribution function over 512 trials in SageMath as:
//
// k = var('k')
// f(x) = sum(binomial(512, k), k, x, 512) / 2^512
//
// We compute the bound C as the minimal x for which:
//
// f(x) < 2^-128
//
// Is true.
//
// Empirically, we have C = 400.

#define ADAPTIVE_PROPORTION_BOUND (400)
#define ADAPTIVE_PROPORTION_WINDOW (512)

// This mask definition requires the window be a power of two.
static_assert(__builtin_popcount(ADAPTIVE_PROPORTION_WINDOW) == 1);
#define ADAPTIVE_PROPORTION_INDEX_MASK (ADAPTIVE_PROPORTION_WINDOW - 1)

static health_test_result_t
adaptive_proportion_test(entropy_sample_t observation, uint32_t offset)
{
        entropy_health_test_t *t = &entropy_data.adaptive_proportion_test;

        // We work in windows of size ADAPTIVE_PROPORTION_WINDOW, so we
        // can compute our index by taking the entropy buffer's overall
        // sample count plus the offset of this observation modulo the
        // window size.
        uint32_t index = (entropy_data.total_sample_count + offset) & ADAPTIVE_PROPORTION_INDEX_MASK;

        if (index == 0) {
                reset_test(t, observation);
        } else if (t->init_observation == observation) {
                return add_observation(t, ADAPTIVE_PROPORTION_BOUND);
        }

        return health_test_success;
}

static health_test_result_t
entropy_health_test(uint32_t sample_count, entropy_sample_t *samples)
{
        health_test_result_t result = health_test_success;

        for (uint32_t i = 0; i < sample_count; i += 1) {
                // We only consider the low bits of each sample, since that is
                // where we expect the entropy to be concentrated.
                entropy_sample_t observation = samples[i] & 0xff;

                if (__improbable(repetition_count_test(observation) == health_test_failure)) {
                        result = health_test_failure;
                }

                if (__improbable(adaptive_proportion_test(observation, i) == health_test_failure)) {
                        result = health_test_failure;
                }
        }

        return result;
}

static void
entropy_analysis_store(uint32_t sample_count, entropy_sample_t *samples)
{
        lck_mtx_assert(&entropy_data.mutex, LCK_MTX_ASSERT_OWNED);

        sample_count = MIN(sample_count, (entropy_analysis_max_sample_count - entropy_analysis_sample_count));
        if (sample_count == 0) {
                return;
        }

        size_t size = sample_count * sizeof(samples[0]);
        memcpy(&entropy_analysis_buffer[entropy_analysis_sample_count], samples, size);
        entropy_analysis_sample_count += sample_count;
}

int32_t
entropy_provide(size_t *entropy_size, void *entropy, __unused void *arg)
{
#if (DEVELOPMENT || DEBUG)
        if (*entropy_size < SHA256_DIGEST_LENGTH) {
                panic("[entropy_provide] recipient entropy buffer is too small\n");
        }
#endif

        int32_t sample_count = 0;
        *entropy_size = 0;

        // The first call to this function comes while the corecrypto kext
        // is being loaded. We require SHA256 to accumulate entropy
        // samples.
        if (__improbable(!g_crypto_funcs)) {
                return sample_count;
        }

        // There is only one consumer (the kernel PRNG), but they could
        // try to consume entropy from different threads. We simply fail
        // if a consumption is already in progress.
        if (!lck_mtx_try_lock(&entropy_data.mutex)) {
                return sample_count;
        }

        // This only happens on the first call to this function. We cannot
        // perform this initialization in entropy_init because the
        // corecrypto kext is not loaded yet.
        if (__improbable(!entropy_data.sha256_ctx_init)) {
                SHA256_Init(&entropy_data.sha256_ctx);
                entropy_data.sha256_ctx_init = true;
        }

        health_test_result_t health_test_result = health_test_success;

        // We accumulate entropy from all CPUs.
        percpu_foreach(e, entropy_cpu_data) {
                // On each CPU, the sample count functions as an index into
                // the entropy buffer. All samples before that index are valid
                // for consumption.
                uint32_t cpu_sample_count = os_atomic_load(&e->sample_count, acquire);

                assert(cpu_sample_count <= ENTROPY_MAX_SAMPLE_COUNT);

                // The health test depends in part on the current state of
                // the entropy data, so we test the new sample before
                // accumulating it.
                if (__improbable(entropy_health_test(cpu_sample_count, e->samples) == health_test_failure)) {
                        health_test_result = health_test_failure;
                }

                // We accumulate the samples regardless of whether the test
                // failed. It cannot hurt.
                entropy_data.total_sample_count += cpu_sample_count;
                SHA256_Update(&entropy_data.sha256_ctx, e->samples, cpu_sample_count * sizeof(e->samples[0]));

                // This code path is only used for testing. Its use is governed by
                // a boot arg; see its initialization above.
                if (__improbable(entropy_analysis_buffer)) {
                        entropy_analysis_store(cpu_sample_count, e->samples);
                }

                // "Drain" the per-CPU buffer by resetting its sample count.
                os_atomic_store(&e->sample_count, 0, relaxed);
        }

        // We expect this never to happen.
        //
        // But if it does happen, we need to return negative to signal the
        // consumer (i.e. the kernel PRNG) that there has been a failure.
        if (__improbable(health_test_result == health_test_failure)) {
                entropy_health_startup_done = 0;
                entropy_data.startup_sample_count = entropy_data.total_sample_count;
                entropy_data.read_sample_count = entropy_data.total_sample_count;
                sample_count = -1;
                goto out;
        }

        // FIPS requires we pass our startup health tests before providing
        // any entropy. This condition is only true during startup and in
        // case of reset due to test failure.
        if (__improbable((entropy_data.total_sample_count - entropy_data.startup_sample_count) < 1024)) {
                goto out;
        }

        entropy_health_startup_done = 1;

        // The count of new samples from the consumer's perspective.
        int32_t n = (int32_t)(entropy_data.total_sample_count - entropy_data.read_sample_count);

        // For performance reasons, we require a small threshold of
        // samples to have built up before we provide any to the PRNG.
        if (n < 32) {
                goto out;
        }

        SHA256_Final(entropy, &entropy_data.sha256_ctx);
        SHA256_Init(&entropy_data.sha256_ctx);
        entropy_data.read_sample_count = entropy_data.total_sample_count;

        sample_count = n;
        *entropy_size = SHA256_DIGEST_LENGTH;

out:
        lck_mtx_unlock(&entropy_data.mutex);

        return sample_count;
}