linux/kernel/user.c
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   1/*
   2 * The "user cache".
   3 *
   4 * (C) Copyright 1991-2000 Linus Torvalds
   5 *
   6 * We have a per-user structure to keep track of how many
   7 * processes, files etc the user has claimed, in order to be
   8 * able to have per-user limits for system resources. 
   9 */
  10
  11#include <linux/init.h>
  12#include <linux/sched.h>
  13#include <linux/slab.h>
  14#include <linux/bitops.h>
  15#include <linux/key.h>
  16#include <linux/interrupt.h>
  17#include <linux/module.h>
  18#include <linux/user_namespace.h>
  19
  20/*
  21 * userns count is 1 for root user, 1 for init_uts_ns,
  22 * and 1 for... ?
  23 */
  24struct user_namespace init_user_ns = {
  25        .kref = {
  26                .refcount       = ATOMIC_INIT(3),
  27        },
  28        .creator = &root_user,
  29};
  30EXPORT_SYMBOL_GPL(init_user_ns);
  31
  32/*
  33 * UID task count cache, to get fast user lookup in "alloc_uid"
  34 * when changing user ID's (ie setuid() and friends).
  35 */
  36
  37#define UIDHASH_MASK            (UIDHASH_SZ - 1)
  38#define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
  39#define uidhashentry(ns, uid)   ((ns)->uidhash_table + __uidhashfn((uid)))
  40
  41static struct kmem_cache *uid_cachep;
  42
  43/*
  44 * The uidhash_lock is mostly taken from process context, but it is
  45 * occasionally also taken from softirq/tasklet context, when
  46 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
  47 * But free_uid() is also called with local interrupts disabled, and running
  48 * local_bh_enable() with local interrupts disabled is an error - we'll run
  49 * softirq callbacks, and they can unconditionally enable interrupts, and
  50 * the caller of free_uid() didn't expect that..
  51 */
  52static DEFINE_SPINLOCK(uidhash_lock);
  53
  54/* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->user_ns */
  55struct user_struct root_user = {
  56        .__count        = ATOMIC_INIT(2),
  57        .processes      = ATOMIC_INIT(1),
  58        .files          = ATOMIC_INIT(0),
  59        .sigpending     = ATOMIC_INIT(0),
  60        .locked_shm     = 0,
  61        .user_ns        = &init_user_ns,
  62};
  63
  64/*
  65 * These routines must be called with the uidhash spinlock held!
  66 */
  67static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
  68{
  69        hlist_add_head(&up->uidhash_node, hashent);
  70}
  71
  72static void uid_hash_remove(struct user_struct *up)
  73{
  74        hlist_del_init(&up->uidhash_node);
  75        put_user_ns(up->user_ns);
  76}
  77
  78static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
  79{
  80        struct user_struct *user;
  81        struct hlist_node *h;
  82
  83        hlist_for_each_entry(user, h, hashent, uidhash_node) {
  84                if (user->uid == uid) {
  85                        atomic_inc(&user->__count);
  86                        return user;
  87                }
  88        }
  89
  90        return NULL;
  91}
  92
  93/* IRQs are disabled and uidhash_lock is held upon function entry.
  94 * IRQ state (as stored in flags) is restored and uidhash_lock released
  95 * upon function exit.
  96 */
  97static void free_user(struct user_struct *up, unsigned long flags)
  98        __releases(&uidhash_lock)
  99{
 100        uid_hash_remove(up);
 101        spin_unlock_irqrestore(&uidhash_lock, flags);
 102        key_put(up->uid_keyring);
 103        key_put(up->session_keyring);
 104        kmem_cache_free(uid_cachep, up);
 105}
 106
 107/*
 108 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 109 * caller must undo that ref with free_uid().
 110 *
 111 * If the user_struct could not be found, return NULL.
 112 */
 113struct user_struct *find_user(uid_t uid)
 114{
 115        struct user_struct *ret;
 116        unsigned long flags;
 117        struct user_namespace *ns = current_user_ns();
 118
 119        spin_lock_irqsave(&uidhash_lock, flags);
 120        ret = uid_hash_find(uid, uidhashentry(ns, uid));
 121        spin_unlock_irqrestore(&uidhash_lock, flags);
 122        return ret;
 123}
 124
 125void free_uid(struct user_struct *up)
 126{
 127        unsigned long flags;
 128
 129        if (!up)
 130                return;
 131
 132        local_irq_save(flags);
 133        if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
 134                free_user(up, flags);
 135        else
 136                local_irq_restore(flags);
 137}
 138
 139struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
 140{
 141        struct hlist_head *hashent = uidhashentry(ns, uid);
 142        struct user_struct *up, *new;
 143
 144        spin_lock_irq(&uidhash_lock);
 145        up = uid_hash_find(uid, hashent);
 146        spin_unlock_irq(&uidhash_lock);
 147
 148        if (!up) {
 149                new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
 150                if (!new)
 151                        goto out_unlock;
 152
 153                new->uid = uid;
 154                atomic_set(&new->__count, 1);
 155
 156                new->user_ns = get_user_ns(ns);
 157
 158                /*
 159                 * Before adding this, check whether we raced
 160                 * on adding the same user already..
 161                 */
 162                spin_lock_irq(&uidhash_lock);
 163                up = uid_hash_find(uid, hashent);
 164                if (up) {
 165                        put_user_ns(ns);
 166                        key_put(new->uid_keyring);
 167                        key_put(new->session_keyring);
 168                        kmem_cache_free(uid_cachep, new);
 169                } else {
 170                        uid_hash_insert(new, hashent);
 171                        up = new;
 172                }
 173                spin_unlock_irq(&uidhash_lock);
 174        }
 175
 176        return up;
 177
 178out_unlock:
 179        return NULL;
 180}
 181
 182static int __init uid_cache_init(void)
 183{
 184        int n;
 185
 186        uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
 187                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
 188
 189        for(n = 0; n < UIDHASH_SZ; ++n)
 190                INIT_HLIST_HEAD(init_user_ns.uidhash_table + n);
 191
 192        /* Insert the root user immediately (init already runs as root) */
 193        spin_lock_irq(&uidhash_lock);
 194        uid_hash_insert(&root_user, uidhashentry(&init_user_ns, 0));
 195        spin_unlock_irq(&uidhash_lock);
 196
 197        return 0;
 198}
 199
 200module_init(uid_cache_init);
 201
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