ESP32_Serial_Swiss_Army_Knife/src/web_admin_tickets.c

/* SPDX-License-Identifier: GPL-3.0-only */
#include "web_admin_tickets.h"

#include <limits.h>
#include <string.h>
#include "esp_timer.h"
#include "freertos/FreeRTOS.h"
#include "mbedtls/sha256.h"
#include "secure_random.h"

typedef struct {
    uint64_t generation;
    web_session_id_t id;
    int64_t expires_at_us;
    user_principal_t principal;
    uint8_t digest[32];
} ticket_t;

static portMUX_TYPE s_lock = portMUX_INITIALIZER_UNLOCKED;
static struct {
    ticket_t tickets[WEB_ADMIN_TICKET_CAPACITY];
    uint64_t epoch;
    uint64_t generation;
    uint32_t issued, consumed, rejected, capacity_rejections;
    bool ready;
} s_state;

static void increment(uint32_t *counter)
{
    if (*counter != UINT32_MAX) ++*counter;
}

static bool equal_digest(const uint8_t *a, const uint8_t *b)
{
    volatile uint8_t difference = 0;
    for (size_t i = 0; i < 32; ++i) difference |= a[i] ^ b[i];
    return difference == 0;
}

static bool admin(const user_principal_t *p)
{
    return p != NULL && p->role == USER_ROLE_ADMIN &&
        p->method == USER_AUTH_METHOD_PASSWORD && p->user_id != 0 &&
        p->auth_generation != 0 && p->username_length != 0 &&
        p->username_length <= USER_DATABASE_USERNAME_CAPACITY;
}

static bool same_principal(const user_principal_t *a, const user_principal_t *b)
{
    return admin(b) && a->user_id == b->user_id &&
        a->auth_generation == b->auth_generation && a->role == b->role &&
        a->method == b->method && a->username_length == b->username_length &&
        memcmp(a->username, b->username, a->username_length) == 0;
}

static bool current(web_session_id_t id, const user_principal_t *p)
{
    bool valid = false;
    return id != 0 && admin(p) &&
        web_session_store_check_principal(id, p, &valid) == ESP_OK && valid;
}

static void expire_locked(int64_t now)
{
    for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i) {
        ticket_t *t = &s_state.tickets[i];
        if (t->generation && t->expires_at_us <= now) secure_wipe(t, sizeof(*t));
    }
}

/* Fixed two-slot walk. Generation prevents an external check from deleting a
 * replacement, including when RNG returns the same bytes on a later issue. */
static void prune(void)
{
    for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i) {
        ticket_t copy = {0};
        int64_t now = esp_timer_get_time();
        taskENTER_CRITICAL(&s_lock);
        expire_locked(now);
        copy = s_state.tickets[i];
        taskEXIT_CRITICAL(&s_lock);
        if (copy.generation && !current(copy.id, &copy.principal)) {
            taskENTER_CRITICAL(&s_lock);
            if (s_state.tickets[i].generation == copy.generation)
                secure_wipe(&s_state.tickets[i], sizeof(ticket_t));
            taskEXIT_CRITICAL(&s_lock);
        }
        secure_wipe(&copy, sizeof(copy));
    }
}

static void advance_epoch_locked(void)
{
    if (s_state.epoch != UINT64_MAX) ++s_state.epoch;
    if (s_state.epoch == UINT64_MAX) {
        s_state.ready = false;
        secure_wipe(s_state.tickets, sizeof(s_state.tickets));
    }
}

void web_admin_tickets_start(void)
{
    taskENTER_CRITICAL(&s_lock);
    if (!s_state.ready && s_state.epoch != UINT64_MAX &&
        s_state.generation != UINT64_MAX) {
        advance_epoch_locked();
        s_state.ready = s_state.epoch != UINT64_MAX;
    }
    taskEXIT_CRITICAL(&s_lock);
}

void web_admin_tickets_stop(void)
{
    taskENTER_CRITICAL(&s_lock);
    advance_epoch_locked();
    s_state.ready = false;
    secure_wipe(s_state.tickets, sizeof(s_state.tickets));
    taskEXIT_CRITICAL(&s_lock);
}

static bool capture_epoch(uint64_t *epoch)
{
    taskENTER_CRITICAL(&s_lock);
    *epoch = s_state.epoch;
    bool ready = s_state.ready;
    taskEXIT_CRITICAL(&s_lock);
    return ready;
}

static esp_err_t result(esp_err_t error)
{
    if (error != ESP_OK) {
        taskENTER_CRITICAL(&s_lock);
        increment(&s_state.rejected);
        taskEXIT_CRITICAL(&s_lock);
    }
    return error;
}

esp_err_t web_admin_tickets_issue(web_session_id_t id,
    const user_principal_t *principal, char token[WEB_ADMIN_TICKET_LENGTH + 1U])
{
    ticket_t candidate = {0};
    uint8_t random[32] = {0};
    uint64_t epoch = 0;
    esp_err_t error = ESP_ERR_INVALID_ARG;
    if (token == NULL) return result(error);
    secure_wipe(token, WEB_ADMIN_TICKET_LENGTH + 1U);
    if (id == 0 || principal == NULL) goto done;
    error = ESP_ERR_INVALID_STATE;
    if (!capture_epoch(&epoch) || !current(id, principal)) goto done;
    candidate.id = id;
    candidate.principal = *principal;
    prune();
    if (!current(id, &candidate.principal)) goto done;
    error = secure_random_fill(random, sizeof(random));
    /* Recheck even when crypto fails; never use an old authorization result. */
    bool valid = current(id, &candidate.principal);
    if (error != ESP_OK) goto done;
    error = ESP_ERR_INVALID_STATE;
    if (!valid) goto done;
    static const char hex[] = "0123456789abcdef";
    for (size_t i = 0; i < sizeof(random); ++i) {
        token[2 * i] = hex[random[i] >> 4];
        token[2 * i + 1] = hex[random[i] & 15];
    }
    int crypto = mbedtls_sha256(random, sizeof(random), candidate.digest, 0);
    valid = current(id, &candidate.principal);
    error = crypto == 0 ? ESP_ERR_INVALID_STATE : ESP_FAIL;
    if (crypto != 0 || !valid) goto done;
    int64_t now = esp_timer_get_time();
    taskENTER_CRITICAL(&s_lock);
    expire_locked(now);
    if (s_state.ready && epoch == s_state.epoch && now >= 0 &&
        now <= INT64_MAX - WEB_ADMIN_TICKET_LIFETIME_US &&
        s_state.generation != UINT64_MAX) {
        ticket_t *free_slot = NULL;
        bool duplicate = false;
        for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i) {
            ticket_t *t = &s_state.tickets[i];
            if (!t->generation) free_slot = t;
            else if (equal_digest(t->digest, candidate.digest)) duplicate = true;
        }
        if (duplicate) error = ESP_FAIL;
        else if (free_slot == NULL) {
            increment(&s_state.capacity_rejections);
            error = ESP_ERR_NO_MEM;
        } else {
            candidate.generation = ++s_state.generation;
            candidate.expires_at_us = now + WEB_ADMIN_TICKET_LIFETIME_US;
            *free_slot = candidate;
            increment(&s_state.issued);
            error = ESP_OK;
        }
    }
    taskEXIT_CRITICAL(&s_lock);
done:
    secure_wipe(random, sizeof(random));
    secure_wipe(&candidate, sizeof(candidate));
    if (error != ESP_OK) secure_wipe(token, WEB_ADMIN_TICKET_LENGTH + 1U);
    return result(error);
}

static int unhex(char c)
{
    if (c >= '0' && c <= '9') return c - '0';
    if (c >= 'a' && c <= 'f') return c - 'a' + 10;
    if (c >= 'A' && c <= 'F') return c - 'A' + 10;
    return -1;
}

esp_err_t web_admin_tickets_consume(const char *token, web_session_id_t id,
    const user_principal_t *principal)
{
    uint8_t bytes[32] = {0}, digest[32] = {0};
    ticket_t found = {0};
    uint64_t epoch = 0;
    esp_err_t error = ESP_ERR_INVALID_ARG;
    if (token == NULL) goto done;
    for (size_t i = 0; i < WEB_ADMIN_TICKET_LENGTH; ++i) {
        int n = unhex(token[i]);
        if (n < 0) goto done;
        bytes[i / 2] |= (uint8_t)(n << ((i % 2 == 0) ? 4 : 0));
    }
    if (token[WEB_ADMIN_TICKET_LENGTH] != '\0') goto done;
    error = ESP_ERR_INVALID_STATE;
    if (!capture_epoch(&epoch)) goto done;
    bool before = current(id, principal);
    if (mbedtls_sha256(bytes, sizeof(bytes), digest, 0) != 0) {
        (void)current(id, principal);
        error = ESP_FAIL;
        goto done;
    }
    int64_t now = esp_timer_get_time();
    taskENTER_CRITICAL(&s_lock);
    expire_locked(now);
    if (s_state.ready && epoch == s_state.epoch) {
        error = ESP_ERR_NOT_FOUND;
        for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i) {
            ticket_t *t = &s_state.tickets[i];
            if (t->generation && equal_digest(t->digest, digest)) {
                found = *t;
                secure_wipe(t, sizeof(*t));
                increment(&s_state.consumed);
                break;
            }
        }
    }
    taskEXIT_CRITICAL(&s_lock);
    /* Burn precedes acting on either currentness result or identity binding. */
    bool after = current(id, principal);
    if (found.generation) {
        now = esp_timer_get_time();
        taskENTER_CRITICAL(&s_lock);
        error = before && after && found.id == id &&
            same_principal(&found.principal, principal) && s_state.ready &&
            epoch == s_state.epoch && now < found.expires_at_us ?
            ESP_OK : ESP_ERR_INVALID_STATE;
        taskEXIT_CRITICAL(&s_lock);
    }
done:
    secure_wipe(bytes, sizeof(bytes));
    secure_wipe(digest, sizeof(digest));
    secure_wipe(&found, sizeof(found));
    return result(error);
}

void web_admin_tickets_revoke(web_session_id_t id, const uint8_t *username,
    size_t length)
{
    taskENTER_CRITICAL(&s_lock);
    advance_epoch_locked();
    for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i) {
        ticket_t *t = &s_state.tickets[i];
        bool match = id != 0 ? t->id == id : username == NULL ||
            (length == t->principal.username_length &&
             length <= USER_DATABASE_USERNAME_CAPACITY &&
             memcmp(username, t->principal.username, length) == 0);
        if (match) secure_wipe(t, sizeof(*t));
    }
    taskEXIT_CRITICAL(&s_lock);
}

void web_admin_tickets_get_snapshot(web_admin_tickets_snapshot_t *snapshot)
{
    if (snapshot == NULL) return;
    prune();
    int64_t now = esp_timer_get_time();
    taskENTER_CRITICAL(&s_lock);
    expire_locked(now);
    *snapshot = (web_admin_tickets_snapshot_t) {
        .issued = s_state.issued, .consumed = s_state.consumed,
        .rejected = s_state.rejected,
        .capacity_rejections = s_state.capacity_rejections,
        .storage_bytes = sizeof(s_state) + sizeof(s_lock), .ready = s_state.ready,
    };
    for (size_t i = 0; i < WEB_ADMIN_TICKET_CAPACITY; ++i)
        if (s_state.tickets[i].generation) ++snapshot->active;
    taskEXIT_CRITICAL(&s_lock);
}