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/export.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        .uid_map = {
  26                .nr_extents = 1,
  27                .extent[0] = {
  28                        .first = 0,
  29                        .lower_first = 0,
  30                        .count = 4294967295U,
  31                },
  32        },
  33        .gid_map = {
  34                .nr_extents = 1,
  35                .extent[0] = {
  36                        .first = 0,
  37                        .lower_first = 0,
  38                        .count = 4294967295U,
  39                },
  40        },
  41        .projid_map = {
  42                .nr_extents = 1,
  43                .extent[0] = {
  44                        .first = 0,
  45                        .lower_first = 0,
  46                        .count = 4294967295U,
  47                },
  48        },
  49        .kref = {
  50                .refcount       = ATOMIC_INIT(3),
  51        },
  52        .owner = GLOBAL_ROOT_UID,
  53        .group = GLOBAL_ROOT_GID,
  54};
  55EXPORT_SYMBOL_GPL(init_user_ns);
  56
  57/*
  58 * UID task count cache, to get fast user lookup in "alloc_uid"
  59 * when changing user ID's (ie setuid() and friends).
  60 */
  61
  62#define UIDHASH_BITS    (CONFIG_BASE_SMALL ? 3 : 7)
  63#define UIDHASH_SZ      (1 << UIDHASH_BITS)
  64#define UIDHASH_MASK            (UIDHASH_SZ - 1)
  65#define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
  66#define uidhashentry(uid)       (uidhash_table + __uidhashfn((__kuid_val(uid))))
  67
  68static struct kmem_cache *uid_cachep;
  69struct hlist_head uidhash_table[UIDHASH_SZ];
  70
  71/*
  72 * The uidhash_lock is mostly taken from process context, but it is
  73 * occasionally also taken from softirq/tasklet context, when
  74 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
  75 * But free_uid() is also called with local interrupts disabled, and running
  76 * local_bh_enable() with local interrupts disabled is an error - we'll run
  77 * softirq callbacks, and they can unconditionally enable interrupts, and
  78 * the caller of free_uid() didn't expect that..
  79 */
  80static DEFINE_SPINLOCK(uidhash_lock);
  81
  82/* root_user.__count is 1, for init task cred */
  83struct user_struct root_user = {
  84        .__count        = ATOMIC_INIT(1),
  85        .processes      = ATOMIC_INIT(1),
  86        .files          = ATOMIC_INIT(0),
  87        .sigpending     = ATOMIC_INIT(0),
  88        .locked_shm     = 0,
  89        .uid            = GLOBAL_ROOT_UID,
  90};
  91
  92/*
  93 * These routines must be called with the uidhash spinlock held!
  94 */
  95static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
  96{
  97        hlist_add_head(&up->uidhash_node, hashent);
  98}
  99
 100static void uid_hash_remove(struct user_struct *up)
 101{
 102        hlist_del_init(&up->uidhash_node);
 103}
 104
 105static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
 106{
 107        struct user_struct *user;
 108        struct hlist_node *h;
 109
 110        hlist_for_each_entry(user, h, hashent, uidhash_node) {
 111                if (uid_eq(user->uid, uid)) {
 112                        atomic_inc(&user->__count);
 113                        return user;
 114                }
 115        }
 116
 117        return NULL;
 118}
 119
 120/* IRQs are disabled and uidhash_lock is held upon function entry.
 121 * IRQ state (as stored in flags) is restored and uidhash_lock released
 122 * upon function exit.
 123 */
 124static void free_user(struct user_struct *up, unsigned long flags)
 125        __releases(&uidhash_lock)
 126{
 127        uid_hash_remove(up);
 128        spin_unlock_irqrestore(&uidhash_lock, flags);
 129        key_put(up->uid_keyring);
 130        key_put(up->session_keyring);
 131        kmem_cache_free(uid_cachep, up);
 132}
 133
 134/*
 135 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 136 * caller must undo that ref with free_uid().
 137 *
 138 * If the user_struct could not be found, return NULL.
 139 */
 140struct user_struct *find_user(kuid_t uid)
 141{
 142        struct user_struct *ret;
 143        unsigned long flags;
 144
 145        spin_lock_irqsave(&uidhash_lock, flags);
 146        ret = uid_hash_find(uid, uidhashentry(uid));
 147        spin_unlock_irqrestore(&uidhash_lock, flags);
 148        return ret;
 149}
 150
 151void free_uid(struct user_struct *up)
 152{
 153        unsigned long flags;
 154
 155        if (!up)
 156                return;
 157
 158        local_irq_save(flags);
 159        if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
 160                free_user(up, flags);
 161        else
 162                local_irq_restore(flags);
 163}
 164
 165struct user_struct *alloc_uid(kuid_t uid)
 166{
 167        struct hlist_head *hashent = uidhashentry(uid);
 168        struct user_struct *up, *new;
 169
 170        spin_lock_irq(&uidhash_lock);
 171        up = uid_hash_find(uid, hashent);
 172        spin_unlock_irq(&uidhash_lock);
 173
 174        if (!up) {
 175                new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
 176                if (!new)
 177                        goto out_unlock;
 178
 179                new->uid = uid;
 180                atomic_set(&new->__count, 1);
 181
 182                /*
 183                 * Before adding this, check whether we raced
 184                 * on adding the same user already..
 185                 */
 186                spin_lock_irq(&uidhash_lock);
 187                up = uid_hash_find(uid, hashent);
 188                if (up) {
 189                        key_put(new->uid_keyring);
 190                        key_put(new->session_keyring);
 191                        kmem_cache_free(uid_cachep, new);
 192                } else {
 193                        uid_hash_insert(new, hashent);
 194                        up = new;
 195                }
 196                spin_unlock_irq(&uidhash_lock);
 197        }
 198
 199        return up;
 200
 201out_unlock:
 202        return NULL;
 203}
 204
 205static int __init uid_cache_init(void)
 206{
 207        int n;
 208
 209        uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
 210                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
 211
 212        for(n = 0; n < UIDHASH_SZ; ++n)
 213                INIT_HLIST_HEAD(uidhash_table + n);
 214
 215        /* Insert the root user immediately (init already runs as root) */
 216        spin_lock_irq(&uidhash_lock);
 217        uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
 218        spin_unlock_irq(&uidhash_lock);
 219
 220        return 0;
 221}
 222
 223module_init(uid_cache_init);
 224
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