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
  18 /*
  19  * UID task count cache, to get fast user lookup in "alloc_uid"
  20  * when changing user ID's (ie setuid() and friends).
  21  */
  22
  23 #define UIDHASH_BITS (CONFIG_BASE_SMALL ? 3 : 8)
  24 #define UIDHASH_SZ              (1 << UIDHASH_BITS)
  25 #define UIDHASH_MASK            (UIDHASH_SZ - 1)
  26 #define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
  27 #define uidhashentry(uid)       (uidhash_table + __uidhashfn((uid)))
  28
  29 static kmem_cache_t *uid_cachep;
  30 static struct list_head uidhash_table[UIDHASH_SZ];
  31
  32 /*
  33  * The uidhash_lock is mostly taken from process context, but it is
  34  * occasionally also taken from softirq/tasklet context, when
  35  * task-structs get RCU-freed. Hence all locking must be softirq-safe.
  36  * But free_uid() is also called with local interrupts disabled, and running
  37  * local_bh_enable() with local interrupts disabled is an error - we'll run
  38  * softirq callbacks, and they can unconditionally enable interrupts, and
  39  * the caller of free_uid() didn't expect that..
  40  */
  41 static DEFINE_SPINLOCK(uidhash_lock);
  42
  43 struct user_struct root_user = {
  44         .__count        = ATOMIC_INIT(1),
  45         .processes      = ATOMIC_INIT(1),
  46         .files          = ATOMIC_INIT(0),
  47         .sigpending     = ATOMIC_INIT(0),
  48         .mq_bytes       = 0,
  49         .locked_shm     = 0,
  50 #ifdef CONFIG_KEYS
  51         .uid_keyring    = &root_user_keyring,
  52         .session_keyring = &root_session_keyring,
  53 #endif
  54 };
  55
  56 /*
  57  * These routines must be called with the uidhash spinlock held!
  58  */
  59 static inline void uid_hash_insert(struct user_struct *up, struct list_head *hashent)
  60 {
  61         list_add(&up->uidhash_list, hashent);
  62 }
  63
  64 static inline void uid_hash_remove(struct user_struct *up)
  65 {
  66         list_del(&up->uidhash_list);
  67 }
  68
  69 static inline struct user_struct *uid_hash_find(uid_t uid, struct list_head *hashent)
  70 {
  71         struct list_head *up;
  72
  73         list_for_each(up, hashent) {
  74                 struct user_struct *user;
  75
  76                 user = list_entry(up, struct user_struct, uidhash_list);
  77
  78                 if(user->uid == uid) {
  79                         atomic_inc(&user->__count);
  80                         return user;
  81                 }
  82         }
  83
  84         return NULL;
  85 }
  86
  87 /*
  88  * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
  89  * caller must undo that ref with free_uid().
  90  *
  91  * If the user_struct could not be found, return NULL.
  92  */
  93 struct user_struct *find_user(uid_t uid)
  94 {
  95         struct user_struct *ret;
  96         unsigned long flags;
  97
  98         spin_lock_irqsave(&uidhash_lock, flags);
  99         ret = uid_hash_find(uid, uidhashentry(uid));
 100         spin_unlock_irqrestore(&uidhash_lock, flags);
 101         return ret;
 102 }
 103
 104 void free_uid(struct user_struct *up)
 105 {
 106         unsigned long flags;
 107
 108         if (!up)
 109                 return;
 110
 111         local_irq_save(flags);
 112         if (atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
 113                 uid_hash_remove(up);
 114                 spin_unlock_irqrestore(&uidhash_lock, flags);
 115                 key_put(up->uid_keyring);
 116                 key_put(up->session_keyring);
 117                 kmem_cache_free(uid_cachep, up);
 118         } else {
 119                 local_irq_restore(flags);
 120         }
 121 }
 122
 123 struct user_struct * alloc_uid(uid_t uid)
 124 {
 125         struct list_head *hashent = uidhashentry(uid);
 126         struct user_struct *up;
 127
 128         spin_lock_irq(&uidhash_lock);
 129         up = uid_hash_find(uid, hashent);
 130         spin_unlock_irq(&uidhash_lock);
 131
 132         if (!up) {
 133                 struct user_struct *new;
 134
 135                 new = kmem_cache_alloc(uid_cachep, SLAB_KERNEL);
 136                 if (!new)
 137                         return NULL;
 138                 new->uid = uid;
 139                 atomic_set(&new->__count, 1);
 140                 atomic_set(&new->processes, 0);
 141                 atomic_set(&new->files, 0);
 142                 atomic_set(&new->sigpending, 0);
 143 #ifdef CONFIG_INOTIFY_USER
 144                 atomic_set(&new->inotify_watches, 0);
 145                 atomic_set(&new->inotify_devs, 0);
 146 #endif
 147
 148                 new->mq_bytes = 0;
 149                 new->locked_shm = 0;
 150
 151                 if (alloc_uid_keyring(new, current) < 0) {
 152                         kmem_cache_free(uid_cachep, new);
 153                         return NULL;
 154                 }
 155
 156                 /*
 157                  * Before adding this, check whether we raced
 158                  * on adding the same user already..
 159                  */
 160                 spin_lock_irq(&uidhash_lock);
 161                 up = uid_hash_find(uid, hashent);
 162                 if (up) {
 163                         key_put(new->uid_keyring);
 164                         key_put(new->session_keyring);
 165                         kmem_cache_free(uid_cachep, new);
 166                 } else {
 167                         uid_hash_insert(new, hashent);
 168                         up = new;
 169                 }
 170                 spin_unlock_irq(&uidhash_lock);
 171
 172         }
 173         return up;
 174 }
 175
 176 void switch_uid(struct user_struct *new_user)
 177 {
 178         struct user_struct *old_user;
 179
 180         /* What if a process setreuid()'s and this brings the
 181          * new uid over his NPROC rlimit?  We can check this now
 182          * cheaply with the new uid cache, so if it matters
 183          * we should be checking for it.  -DaveM
 184          */
 185         old_user = current->user;
 186         atomic_inc(&new_user->processes);
 187         atomic_dec(&old_user->processes);
 188         switch_uid_keyring(new_user);
 189         current->user = new_user;
 190
 191         /*
 192          * We need to synchronize with __sigqueue_alloc()
 193          * doing a get_uid(p->user).. If that saw the old
 194          * user value, we need to wait until it has exited
 195          * its critical region before we can free the old
 196          * structure.
 197          */
 198         smp_mb();
 199         spin_unlock_wait(&current->sighand->siglock);
 200
 201         free_uid(old_user);
 202         suid_keys(current);
 203 }
 204
 205
 206 static 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, NULL);
 212
 213         for(n = 0; n < UIDHASH_SZ; ++n)
 214                 INIT_LIST_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(0));
 219         spin_unlock_irq(&uidhash_lock);
 220
 221         return 0;
 222 }
 223
 224 module_init(uid_cache_init);