kernel/user.c

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