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#include <linux/proc_fs.h>
  20
  21/*
  22 * userns count is 1 for root user, 1 for init_uts_ns,
  23 * and 1 for... ?
  24 */
  25struct user_namespace init_user_ns = {
  26        .uid_map = {
  27                .nr_extents = 1,
  28                .extent[0] = {
  29                        .first = 0,
  30                        .lower_first = 0,
  31                        .count = 4294967295U,
  32                },
  33        },
  34        .gid_map = {
  35                .nr_extents = 1,
  36                .extent[0] = {
  37                        .first = 0,
  38                        .lower_first = 0,
  39                        .count = 4294967295U,
  40                },
  41        },
  42        .projid_map = {
  43                .nr_extents = 1,
  44                .extent[0] = {
  45                        .first = 0,
  46                        .lower_first = 0,
  47                        .count = 4294967295U,
  48                },
  49        },
  50        .count = ATOMIC_INIT(3),
  51        .owner = GLOBAL_ROOT_UID,
  52        .group = GLOBAL_ROOT_GID,
  53        .proc_inum = PROC_USER_INIT_INO,
  54        .may_mount_sysfs = true,
  55        .may_mount_proc = true,
  56};
  57EXPORT_SYMBOL_GPL(init_user_ns);
  58
  59/*
  60 * UID task count cache, to get fast user lookup in "alloc_uid"
  61 * when changing user ID's (ie setuid() and friends).
  62 */
  63
  64#define UIDHASH_BITS    (CONFIG_BASE_SMALL ? 3 : 7)
  65#define UIDHASH_SZ      (1 << UIDHASH_BITS)
  66#define UIDHASH_MASK            (UIDHASH_SZ - 1)
  67#define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
  68#define uidhashentry(uid)       (uidhash_table + __uidhashfn((__kuid_val(uid))))
  69
  70static struct kmem_cache *uid_cachep;
  71struct hlist_head uidhash_table[UIDHASH_SZ];
  72
  73/*
  74 * The uidhash_lock is mostly taken from process context, but it is
  75 * occasionally also taken from softirq/tasklet context, when
  76 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
  77 * But free_uid() is also called with local interrupts disabled, and running
  78 * local_bh_enable() with local interrupts disabled is an error - we'll run
  79 * softirq callbacks, and they can unconditionally enable interrupts, and
  80 * the caller of free_uid() didn't expect that..
  81 */
  82static DEFINE_SPINLOCK(uidhash_lock);
  83
  84/* root_user.__count is 1, for init task cred */
  85struct user_struct root_user = {
  86        .__count        = ATOMIC_INIT(1),
  87        .processes      = ATOMIC_INIT(1),
  88        .files          = ATOMIC_INIT(0),
  89        .sigpending     = ATOMIC_INIT(0),
  90        .locked_shm     = 0,
  91        .uid            = GLOBAL_ROOT_UID,
  92};
  93
  94/*
  95 * These routines must be called with the uidhash spinlock held!
  96 */
  97static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
  98{
  99        hlist_add_head(&up->uidhash_node, hashent);
 100}
 101
 102static void uid_hash_remove(struct user_struct *up)
 103{
 104        hlist_del_init(&up->uidhash_node);
 105}
 106
 107static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
 108{
 109        struct user_struct *user;
 110
 111        hlist_for_each_entry(user, hashent, uidhash_node) {
 112                if (uid_eq(user->uid, uid)) {
 113                        atomic_inc(&user->__count);
 114                        return user;
 115                }
 116        }
 117
 118        return NULL;
 119}
 120
 121/* IRQs are disabled and uidhash_lock is held upon function entry.
 122 * IRQ state (as stored in flags) is restored and uidhash_lock released
 123 * upon function exit.
 124 */
 125static void free_user(struct user_struct *up, unsigned long flags)
 126        __releases(&uidhash_lock)
 127{
 128        uid_hash_remove(up);
 129        spin_unlock_irqrestore(&uidhash_lock, flags);
 130        key_put(up->uid_keyring);
 131        key_put(up->session_keyring);
 132        kmem_cache_free(uid_cachep, up);
 133}
 134
 135/*
 136 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 137 * caller must undo that ref with free_uid().
 138 *
 139 * If the user_struct could not be found, return NULL.
 140 */
 141struct user_struct *find_user(kuid_t uid)
 142{
 143        struct user_struct *ret;
 144        unsigned long flags;
 145
 146        spin_lock_irqsave(&uidhash_lock, flags);
 147        ret = uid_hash_find(uid, uidhashentry(uid));
 148        spin_unlock_irqrestore(&uidhash_lock, flags);
 149        return ret;
 150}
 151
 152void free_uid(struct user_struct *up)
 153{
 154        unsigned long flags;
 155
 156        if (!up)
 157                return;
 158
 159        local_irq_save(flags);
 160        if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
 161                free_user(up, flags);
 162        else
 163                local_irq_restore(flags);
 164}
 165
 166struct user_struct *alloc_uid(kuid_t uid)
 167{
 168        struct hlist_head *hashent = uidhashentry(uid);
 169        struct user_struct *up, *new;
 170
 171        spin_lock_irq(&uidhash_lock);
 172        up = uid_hash_find(uid, hashent);
 173        spin_unlock_irq(&uidhash_lock);
 174
 175        if (!up) {
 176                new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
 177                if (!new)
 178                        goto out_unlock;
 179
 180                new->uid = uid;
 181                atomic_set(&new->__count, 1);
 182
 183                /*
 184                 * Before adding this, check whether we raced
 185                 * on adding the same user already..
 186                 */
 187                spin_lock_irq(&uidhash_lock);
 188                up = uid_hash_find(uid, hashent);
 189                if (up) {
 190                        key_put(new->uid_keyring);
 191                        key_put(new->session_keyring);
 192                        kmem_cache_free(uid_cachep, new);
 193                } else {
 194                        uid_hash_insert(new, hashent);
 195                        up = new;
 196                }
 197                spin_unlock_irq(&uidhash_lock);
 198        }
 199
 200        return up;
 201
 202out_unlock:
 203        return NULL;
 204}
 205
 206static int __init uid_cache_init(void)
 207{
 208        int n;
 209
 210        uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
 211                        0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
 212
 213        for(n = 0; n < UIDHASH_SZ; ++n)
 214                INIT_HLIST_HEAD(uidhash_table + n);
 215
 216        /* Insert the root user immediately (init already runs as root) */
 217        spin_lock_irq(&uidhash_lock);
 218        uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
 219        spin_unlock_irq(&uidhash_lock);
 220
 221        return 0;
 222}
 223
 224module_init(uid_cache_init);
 225
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