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