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         .creator = &root_user,
  29 };
  30 EXPORT_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
  41 static 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  */
  52 static DEFINE_SPINLOCK(uidhash_lock);
  53
  54 /* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->user_ns */
  55 struct 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  */
  67 static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
  68 {
  69         hlist_add_head(&up->uidhash_node, hashent);
  70 }
  71
  72 static 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
  78 static 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  */
  97 static 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  */
 113 struct 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
 125 void 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
 139 struct 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
 178 out_unlock:
 179         return NULL;
 180 }
 181
 182 static 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
 200 module_init(uid_cache_init);