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/module.h>
  18 #include <linux/user_namespace.h>
  19
  20 struct user_namespace init_user_ns = {
  21         .kref = {
  22                 .refcount       = ATOMIC_INIT(2),
  23         },
  24         .creator = &root_user,
  25 };
  26 EXPORT_SYMBOL_GPL(init_user_ns);
  27
  28 /*
  29  * UID task count cache, to get fast user lookup in "alloc_uid"
  30  * when changing user ID's (ie setuid() and friends).
  31  */
  32
  33 #define UIDHASH_MASK            (UIDHASH_SZ - 1)
  34 #define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
  35 #define uidhashentry(ns, uid)   ((ns)->uidhash_table + __uidhashfn((uid)))
  36
  37 static struct kmem_cache *uid_cachep;
  38
  39 /*
  40  * The uidhash_lock is mostly taken from process context, but it is
  41  * occasionally also taken from softirq/tasklet context, when
  42  * task-structs get RCU-freed. Hence all locking must be softirq-safe.
  43  * But free_uid() is also called with local interrupts disabled, and running
  44  * local_bh_enable() with local interrupts disabled is an error - we'll run
  45  * softirq callbacks, and they can unconditionally enable interrupts, and
  46  * the caller of free_uid() didn't expect that..
  47  */
  48 static DEFINE_SPINLOCK(uidhash_lock);
  49
  50 /* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->creator */
  51 struct user_struct root_user = {
  52         .__count        = ATOMIC_INIT(2),
  53         .processes      = ATOMIC_INIT(1),
  54         .files          = ATOMIC_INIT(0),
  55         .sigpending     = ATOMIC_INIT(0),
  56         .locked_shm     = 0,
  57         .user_ns        = &init_user_ns,
  58 };
  59
  60 /*
  61  * These routines must be called with the uidhash spinlock held!
  62  */
  63 static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
  64 {
  65         hlist_add_head(&up->uidhash_node, hashent);
  66 }
  67
  68 static void uid_hash_remove(struct user_struct *up)
  69 {
  70         hlist_del_init(&up->uidhash_node);
  71         put_user_ns(up->user_ns);
  72 }
  73
  74 static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
  75 {
  76         struct user_struct *user;
  77         struct hlist_node *h;
  78
  79         hlist_for_each_entry(user, h, hashent, uidhash_node) {
  80                 if (user->uid == uid) {
  81                         atomic_inc(&user->__count);
  82                         return user;
  83                 }
  84         }
  85
  86         return NULL;
  87 }
  88
  89 /* IRQs are disabled and uidhash_lock is held upon function entry.
  90  * IRQ state (as stored in flags) is restored and uidhash_lock released
  91  * upon function exit.
  92  */
  93 static void free_user(struct user_struct *up, unsigned long flags)
  94         __releases(&uidhash_lock)
  95 {
  96         uid_hash_remove(up);
  97         spin_unlock_irqrestore(&uidhash_lock, flags);
  98         key_put(up->uid_keyring);
  99         key_put(up->session_keyring);
 100         kmem_cache_free(uid_cachep, up);
 101 }
 102
 103 /*
 104  * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 105  * caller must undo that ref with free_uid().
 106  *
 107  * If the user_struct could not be found, return NULL.
 108  */
 109 struct user_struct *find_user(uid_t uid)
 110 {
 111         struct user_struct *ret;
 112         unsigned long flags;
 113         struct user_namespace *ns = current_user_ns();
 114
 115         spin_lock_irqsave(&uidhash_lock, flags);
 116         ret = uid_hash_find(uid, uidhashentry(ns, uid));
 117         spin_unlock_irqrestore(&uidhash_lock, flags);
 118         return ret;
 119 }
 120
 121 void free_uid(struct user_struct *up)
 122 {
 123         unsigned long flags;
 124
 125         if (!up)
 126                 return;
 127
 128         local_irq_save(flags);
 129         if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
 130                 free_user(up, flags);
 131         else
 132                 local_irq_restore(flags);
 133 }
 134
 135 struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
 136 {
 137         struct hlist_head *hashent = uidhashentry(ns, uid);
 138         struct user_struct *up, *new;
 139
 140         /* Make uid_hash_find() + uid_hash_insert() atomic. */
 141         spin_lock_irq(&uidhash_lock);
 142         up = uid_hash_find(uid, hashent);
 143         spin_unlock_irq(&uidhash_lock);
 144
 145         if (!up) {
 146                 new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
 147                 if (!new)
 148                         goto out_unlock;
 149
 150                 new->uid = uid;
 151                 atomic_set(&new->__count, 1);
 152
 153                 new->user_ns = get_user_ns(ns);
 154
 155                 /*
 156                  * Before adding this, check whether we raced
 157                  * on adding the same user already..
 158                  */
 159                 spin_lock_irq(&uidhash_lock);
 160                 up = uid_hash_find(uid, hashent);
 161                 if (up) {
 162                         key_put(new->uid_keyring);
 163                         key_put(new->session_keyring);
 164                         kmem_cache_free(uid_cachep, new);
 165                 } else {
 166                         uid_hash_insert(new, hashent);
 167                         up = new;
 168                 }
 169                 spin_unlock_irq(&uidhash_lock);
 170         }
 171
 172         return up;
 173
 174 out_unlock:
 175         return NULL;
 176 }
 177
 178 static int __init uid_cache_init(void)
 179 {
 180         int n;
 181
 182         uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
 183                         0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
 184
 185         for(n = 0; n < UIDHASH_SZ; ++n)
 186                 INIT_HLIST_HEAD(init_user_ns.uidhash_table + n);
 187
 188         /* Insert the root user immediately (init already runs as root) */
 189         spin_lock_irq(&uidhash_lock);
 190         uid_hash_insert(&root_user, uidhashentry(&init_user_ns, 0));
 191         spin_unlock_irq(&uidhash_lock);
 192
 193         return 0;
 194 }
 195
 196 module_init(uid_cache_init);