linux/security/yama/yama_lsm.c
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   1// SPDX-License-Identifier: GPL-2.0-only
   2/*
   3 * Yama Linux Security Module
   4 *
   5 * Author: Kees Cook <keescook@chromium.org>
   6 *
   7 * Copyright (C) 2010 Canonical, Ltd.
   8 * Copyright (C) 2011 The Chromium OS Authors.
   9 */
  10
  11#include <linux/lsm_hooks.h>
  12#include <linux/sysctl.h>
  13#include <linux/ptrace.h>
  14#include <linux/prctl.h>
  15#include <linux/ratelimit.h>
  16#include <linux/workqueue.h>
  17#include <linux/string_helpers.h>
  18#include <linux/task_work.h>
  19#include <linux/sched.h>
  20#include <linux/spinlock.h>
  21
  22#define YAMA_SCOPE_DISABLED     0
  23#define YAMA_SCOPE_RELATIONAL   1
  24#define YAMA_SCOPE_CAPABILITY   2
  25#define YAMA_SCOPE_NO_ATTACH    3
  26
  27static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
  28
  29/* describe a ptrace relationship for potential exception */
  30struct ptrace_relation {
  31        struct task_struct *tracer;
  32        struct task_struct *tracee;
  33        bool invalid;
  34        struct list_head node;
  35        struct rcu_head rcu;
  36};
  37
  38static LIST_HEAD(ptracer_relations);
  39static DEFINE_SPINLOCK(ptracer_relations_lock);
  40
  41static void yama_relation_cleanup(struct work_struct *work);
  42static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
  43
  44struct access_report_info {
  45        struct callback_head work;
  46        const char *access;
  47        struct task_struct *target;
  48        struct task_struct *agent;
  49};
  50
  51static void __report_access(struct callback_head *work)
  52{
  53        struct access_report_info *info =
  54                container_of(work, struct access_report_info, work);
  55        char *target_cmd, *agent_cmd;
  56
  57        target_cmd = kstrdup_quotable_cmdline(info->target, GFP_KERNEL);
  58        agent_cmd = kstrdup_quotable_cmdline(info->agent, GFP_KERNEL);
  59
  60        pr_notice_ratelimited(
  61                "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
  62                info->access, target_cmd, info->target->pid, agent_cmd,
  63                info->agent->pid);
  64
  65        kfree(agent_cmd);
  66        kfree(target_cmd);
  67
  68        put_task_struct(info->agent);
  69        put_task_struct(info->target);
  70        kfree(info);
  71}
  72
  73/* defers execution because cmdline access can sleep */
  74static void report_access(const char *access, struct task_struct *target,
  75                                struct task_struct *agent)
  76{
  77        struct access_report_info *info;
  78        char agent_comm[sizeof(agent->comm)];
  79
  80        assert_spin_locked(&target->alloc_lock); /* for target->comm */
  81
  82        if (current->flags & PF_KTHREAD) {
  83                /* I don't think kthreads call task_work_run() before exiting.
  84                 * Imagine angry ranting about procfs here.
  85                 */
  86                pr_notice_ratelimited(
  87                    "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
  88                    access, target->comm, target->pid,
  89                    get_task_comm(agent_comm, agent), agent->pid);
  90                return;
  91        }
  92
  93        info = kmalloc(sizeof(*info), GFP_ATOMIC);
  94        if (!info)
  95                return;
  96        init_task_work(&info->work, __report_access);
  97        get_task_struct(target);
  98        get_task_struct(agent);
  99        info->access = access;
 100        info->target = target;
 101        info->agent = agent;
 102        if (task_work_add(current, &info->work, true) == 0)
 103                return; /* success */
 104
 105        WARN(1, "report_access called from exiting task");
 106        put_task_struct(target);
 107        put_task_struct(agent);
 108        kfree(info);
 109}
 110
 111/**
 112 * yama_relation_cleanup - remove invalid entries from the relation list
 113 *
 114 */
 115static void yama_relation_cleanup(struct work_struct *work)
 116{
 117        struct ptrace_relation *relation;
 118
 119        spin_lock(&ptracer_relations_lock);
 120        rcu_read_lock();
 121        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 122                if (relation->invalid) {
 123                        list_del_rcu(&relation->node);
 124                        kfree_rcu(relation, rcu);
 125                }
 126        }
 127        rcu_read_unlock();
 128        spin_unlock(&ptracer_relations_lock);
 129}
 130
 131/**
 132 * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
 133 * @tracer: the task_struct of the process doing the ptrace
 134 * @tracee: the task_struct of the process to be ptraced
 135 *
 136 * Each tracee can have, at most, one tracer registered. Each time this
 137 * is called, the prior registered tracer will be replaced for the tracee.
 138 *
 139 * Returns 0 if relationship was added, -ve on error.
 140 */
 141static int yama_ptracer_add(struct task_struct *tracer,
 142                            struct task_struct *tracee)
 143{
 144        struct ptrace_relation *relation, *added;
 145
 146        added = kmalloc(sizeof(*added), GFP_KERNEL);
 147        if (!added)
 148                return -ENOMEM;
 149
 150        added->tracee = tracee;
 151        added->tracer = tracer;
 152        added->invalid = false;
 153
 154        spin_lock(&ptracer_relations_lock);
 155        rcu_read_lock();
 156        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 157                if (relation->invalid)
 158                        continue;
 159                if (relation->tracee == tracee) {
 160                        list_replace_rcu(&relation->node, &added->node);
 161                        kfree_rcu(relation, rcu);
 162                        goto out;
 163                }
 164        }
 165
 166        list_add_rcu(&added->node, &ptracer_relations);
 167
 168out:
 169        rcu_read_unlock();
 170        spin_unlock(&ptracer_relations_lock);
 171        return 0;
 172}
 173
 174/**
 175 * yama_ptracer_del - remove exceptions related to the given tasks
 176 * @tracer: remove any relation where tracer task matches
 177 * @tracee: remove any relation where tracee task matches
 178 */
 179static void yama_ptracer_del(struct task_struct *tracer,
 180                             struct task_struct *tracee)
 181{
 182        struct ptrace_relation *relation;
 183        bool marked = false;
 184
 185        rcu_read_lock();
 186        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 187                if (relation->invalid)
 188                        continue;
 189                if (relation->tracee == tracee ||
 190                    (tracer && relation->tracer == tracer)) {
 191                        relation->invalid = true;
 192                        marked = true;
 193                }
 194        }
 195        rcu_read_unlock();
 196
 197        if (marked)
 198                schedule_work(&yama_relation_work);
 199}
 200
 201/**
 202 * yama_task_free - check for task_pid to remove from exception list
 203 * @task: task being removed
 204 */
 205static void yama_task_free(struct task_struct *task)
 206{
 207        yama_ptracer_del(task, task);
 208}
 209
 210/**
 211 * yama_task_prctl - check for Yama-specific prctl operations
 212 * @option: operation
 213 * @arg2: argument
 214 * @arg3: argument
 215 * @arg4: argument
 216 * @arg5: argument
 217 *
 218 * Return 0 on success, -ve on error.  -ENOSYS is returned when Yama
 219 * does not handle the given option.
 220 */
 221static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 222                           unsigned long arg4, unsigned long arg5)
 223{
 224        int rc = -ENOSYS;
 225        struct task_struct *myself = current;
 226
 227        switch (option) {
 228        case PR_SET_PTRACER:
 229                /* Since a thread can call prctl(), find the group leader
 230                 * before calling _add() or _del() on it, since we want
 231                 * process-level granularity of control. The tracer group
 232                 * leader checking is handled later when walking the ancestry
 233                 * at the time of PTRACE_ATTACH check.
 234                 */
 235                rcu_read_lock();
 236                if (!thread_group_leader(myself))
 237                        myself = rcu_dereference(myself->group_leader);
 238                get_task_struct(myself);
 239                rcu_read_unlock();
 240
 241                if (arg2 == 0) {
 242                        yama_ptracer_del(NULL, myself);
 243                        rc = 0;
 244                } else if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
 245                        rc = yama_ptracer_add(NULL, myself);
 246                } else {
 247                        struct task_struct *tracer;
 248
 249                        tracer = find_get_task_by_vpid(arg2);
 250                        if (!tracer) {
 251                                rc = -EINVAL;
 252                        } else {
 253                                rc = yama_ptracer_add(tracer, myself);
 254                                put_task_struct(tracer);
 255                        }
 256                }
 257
 258                put_task_struct(myself);
 259                break;
 260        }
 261
 262        return rc;
 263}
 264
 265/**
 266 * task_is_descendant - walk up a process family tree looking for a match
 267 * @parent: the process to compare against while walking up from child
 268 * @child: the process to start from while looking upwards for parent
 269 *
 270 * Returns 1 if child is a descendant of parent, 0 if not.
 271 */
 272static int task_is_descendant(struct task_struct *parent,
 273                              struct task_struct *child)
 274{
 275        int rc = 0;
 276        struct task_struct *walker = child;
 277
 278        if (!parent || !child)
 279                return 0;
 280
 281        rcu_read_lock();
 282        if (!thread_group_leader(parent))
 283                parent = rcu_dereference(parent->group_leader);
 284        while (walker->pid > 0) {
 285                if (!thread_group_leader(walker))
 286                        walker = rcu_dereference(walker->group_leader);
 287                if (walker == parent) {
 288                        rc = 1;
 289                        break;
 290                }
 291                walker = rcu_dereference(walker->real_parent);
 292        }
 293        rcu_read_unlock();
 294
 295        return rc;
 296}
 297
 298/**
 299 * ptracer_exception_found - tracer registered as exception for this tracee
 300 * @tracer: the task_struct of the process attempting ptrace
 301 * @tracee: the task_struct of the process to be ptraced
 302 *
 303 * Returns 1 if tracer has a ptracer exception ancestor for tracee.
 304 */
 305static int ptracer_exception_found(struct task_struct *tracer,
 306                                   struct task_struct *tracee)
 307{
 308        int rc = 0;
 309        struct ptrace_relation *relation;
 310        struct task_struct *parent = NULL;
 311        bool found = false;
 312
 313        rcu_read_lock();
 314
 315        /*
 316         * If there's already an active tracing relationship, then make an
 317         * exception for the sake of other accesses, like process_vm_rw().
 318         */
 319        parent = ptrace_parent(tracee);
 320        if (parent != NULL && same_thread_group(parent, tracer)) {
 321                rc = 1;
 322                goto unlock;
 323        }
 324
 325        /* Look for a PR_SET_PTRACER relationship. */
 326        if (!thread_group_leader(tracee))
 327                tracee = rcu_dereference(tracee->group_leader);
 328        list_for_each_entry_rcu(relation, &ptracer_relations, node) {
 329                if (relation->invalid)
 330                        continue;
 331                if (relation->tracee == tracee) {
 332                        parent = relation->tracer;
 333                        found = true;
 334                        break;
 335                }
 336        }
 337
 338        if (found && (parent == NULL || task_is_descendant(parent, tracer)))
 339                rc = 1;
 340
 341unlock:
 342        rcu_read_unlock();
 343
 344        return rc;
 345}
 346
 347/**
 348 * yama_ptrace_access_check - validate PTRACE_ATTACH calls
 349 * @child: task that current task is attempting to ptrace
 350 * @mode: ptrace attach mode
 351 *
 352 * Returns 0 if following the ptrace is allowed, -ve on error.
 353 */
 354static int yama_ptrace_access_check(struct task_struct *child,
 355                                    unsigned int mode)
 356{
 357        int rc = 0;
 358
 359        /* require ptrace target be a child of ptracer on attach */
 360        if (mode & PTRACE_MODE_ATTACH) {
 361                switch (ptrace_scope) {
 362                case YAMA_SCOPE_DISABLED:
 363                        /* No additional restrictions. */
 364                        break;
 365                case YAMA_SCOPE_RELATIONAL:
 366                        rcu_read_lock();
 367                        if (!pid_alive(child))
 368                                rc = -EPERM;
 369                        if (!rc && !task_is_descendant(current, child) &&
 370                            !ptracer_exception_found(current, child) &&
 371                            !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 372                                rc = -EPERM;
 373                        rcu_read_unlock();
 374                        break;
 375                case YAMA_SCOPE_CAPABILITY:
 376                        rcu_read_lock();
 377                        if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
 378                                rc = -EPERM;
 379                        rcu_read_unlock();
 380                        break;
 381                case YAMA_SCOPE_NO_ATTACH:
 382                default:
 383                        rc = -EPERM;
 384                        break;
 385                }
 386        }
 387
 388        if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
 389                report_access("attach", child, current);
 390
 391        return rc;
 392}
 393
 394/**
 395 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 396 * @parent: task that will become the ptracer of the current task
 397 *
 398 * Returns 0 if following the ptrace is allowed, -ve on error.
 399 */
 400static int yama_ptrace_traceme(struct task_struct *parent)
 401{
 402        int rc = 0;
 403
 404        /* Only disallow PTRACE_TRACEME on more aggressive settings. */
 405        switch (ptrace_scope) {
 406        case YAMA_SCOPE_CAPABILITY:
 407                if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 408                        rc = -EPERM;
 409                break;
 410        case YAMA_SCOPE_NO_ATTACH:
 411                rc = -EPERM;
 412                break;
 413        }
 414
 415        if (rc) {
 416                task_lock(current);
 417                report_access("traceme", current, parent);
 418                task_unlock(current);
 419        }
 420
 421        return rc;
 422}
 423
 424static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
 425        LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
 426        LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
 427        LSM_HOOK_INIT(task_prctl, yama_task_prctl),
 428        LSM_HOOK_INIT(task_free, yama_task_free),
 429};
 430
 431#ifdef CONFIG_SYSCTL
 432static int yama_dointvec_minmax(struct ctl_table *table, int write,
 433                                void __user *buffer, size_t *lenp, loff_t *ppos)
 434{
 435        struct ctl_table table_copy;
 436
 437        if (write && !capable(CAP_SYS_PTRACE))
 438                return -EPERM;
 439
 440        /* Lock the max value if it ever gets set. */
 441        table_copy = *table;
 442        if (*(int *)table_copy.data == *(int *)table_copy.extra2)
 443                table_copy.extra1 = table_copy.extra2;
 444
 445        return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
 446}
 447
 448static int max_scope = YAMA_SCOPE_NO_ATTACH;
 449
 450static struct ctl_path yama_sysctl_path[] = {
 451        { .procname = "kernel", },
 452        { .procname = "yama", },
 453        { }
 454};
 455
 456static struct ctl_table yama_sysctl_table[] = {
 457        {
 458                .procname       = "ptrace_scope",
 459                .data           = &ptrace_scope,
 460                .maxlen         = sizeof(int),
 461                .mode           = 0644,
 462                .proc_handler   = yama_dointvec_minmax,
 463                .extra1         = SYSCTL_ZERO,
 464                .extra2         = &max_scope,
 465        },
 466        { }
 467};
 468static void __init yama_init_sysctl(void)
 469{
 470        if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
 471                panic("Yama: sysctl registration failed.\n");
 472}
 473#else
 474static inline void yama_init_sysctl(void) { }
 475#endif /* CONFIG_SYSCTL */
 476
 477static int __init yama_init(void)
 478{
 479        pr_info("Yama: becoming mindful.\n");
 480        security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), "yama");
 481        yama_init_sysctl();
 482        return 0;
 483}
 484
 485DEFINE_LSM(yama) = {
 486        .name = "yama",
 487        .init = yama_init,
 488};
 489