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Linux/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 #include <linux/proc_ns.h>
 20 
 21 /*
 22  * userns count is 1 for root user, 1 for init_uts_ns,
 23  * and 1 for... ?
 24  */
 25 struct user_namespace init_user_ns = {
 26         .uid_map = {
 27                 .nr_extents = 1,
 28                 .extent[0] = {
 29                         .first = 0,
 30                         .lower_first = 0,
 31                         .count = 4294967295U,
 32                 },
 33         },
 34         .gid_map = {
 35                 .nr_extents = 1,
 36                 .extent[0] = {
 37                         .first = 0,
 38                         .lower_first = 0,
 39                         .count = 4294967295U,
 40                 },
 41         },
 42         .projid_map = {
 43                 .nr_extents = 1,
 44                 .extent[0] = {
 45                         .first = 0,
 46                         .lower_first = 0,
 47                         .count = 4294967295U,
 48                 },
 49         },
 50         .count = ATOMIC_INIT(3),
 51         .owner = GLOBAL_ROOT_UID,
 52         .group = GLOBAL_ROOT_GID,
 53         .ns.inum = PROC_USER_INIT_INO,
 54 #ifdef CONFIG_USER_NS
 55         .ns.ops = &userns_operations,
 56 #endif
 57         .flags = USERNS_INIT_FLAGS,
 58 #ifdef CONFIG_PERSISTENT_KEYRINGS
 59         .persistent_keyring_register_sem =
 60         __RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
 61 #endif
 62 };
 63 EXPORT_SYMBOL_GPL(init_user_ns);
 64 
 65 /*
 66  * UID task count cache, to get fast user lookup in "alloc_uid"
 67  * when changing user ID's (ie setuid() and friends).
 68  */
 69 
 70 #define UIDHASH_BITS    (CONFIG_BASE_SMALL ? 3 : 7)
 71 #define UIDHASH_SZ      (1 << UIDHASH_BITS)
 72 #define UIDHASH_MASK            (UIDHASH_SZ - 1)
 73 #define __uidhashfn(uid)        (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
 74 #define uidhashentry(uid)       (uidhash_table + __uidhashfn((__kuid_val(uid))))
 75 
 76 static struct kmem_cache *uid_cachep;
 77 struct hlist_head uidhash_table[UIDHASH_SZ];
 78 
 79 /*
 80  * The uidhash_lock is mostly taken from process context, but it is
 81  * occasionally also taken from softirq/tasklet context, when
 82  * task-structs get RCU-freed. Hence all locking must be softirq-safe.
 83  * But free_uid() is also called with local interrupts disabled, and running
 84  * local_bh_enable() with local interrupts disabled is an error - we'll run
 85  * softirq callbacks, and they can unconditionally enable interrupts, and
 86  * the caller of free_uid() didn't expect that..
 87  */
 88 static DEFINE_SPINLOCK(uidhash_lock);
 89 
 90 /* root_user.__count is 1, for init task cred */
 91 struct user_struct root_user = {
 92         .__count        = ATOMIC_INIT(1),
 93         .processes      = ATOMIC_INIT(1),
 94         .sigpending     = ATOMIC_INIT(0),
 95         .locked_shm     = 0,
 96         .uid            = GLOBAL_ROOT_UID,
 97 };
 98 
 99 /*
100  * These routines must be called with the uidhash spinlock held!
101  */
102 static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
103 {
104         hlist_add_head(&up->uidhash_node, hashent);
105 }
106 
107 static void uid_hash_remove(struct user_struct *up)
108 {
109         hlist_del_init(&up->uidhash_node);
110 }
111 
112 static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
113 {
114         struct user_struct *user;
115 
116         hlist_for_each_entry(user, hashent, uidhash_node) {
117                 if (uid_eq(user->uid, uid)) {
118                         atomic_inc(&user->__count);
119                         return user;
120                 }
121         }
122 
123         return NULL;
124 }
125 
126 /* IRQs are disabled and uidhash_lock is held upon function entry.
127  * IRQ state (as stored in flags) is restored and uidhash_lock released
128  * upon function exit.
129  */
130 static void free_user(struct user_struct *up, unsigned long flags)
131         __releases(&uidhash_lock)
132 {
133         uid_hash_remove(up);
134         spin_unlock_irqrestore(&uidhash_lock, flags);
135         key_put(up->uid_keyring);
136         key_put(up->session_keyring);
137         kmem_cache_free(uid_cachep, up);
138 }
139 
140 /*
141  * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
142  * caller must undo that ref with free_uid().
143  *
144  * If the user_struct could not be found, return NULL.
145  */
146 struct user_struct *find_user(kuid_t uid)
147 {
148         struct user_struct *ret;
149         unsigned long flags;
150 
151         spin_lock_irqsave(&uidhash_lock, flags);
152         ret = uid_hash_find(uid, uidhashentry(uid));
153         spin_unlock_irqrestore(&uidhash_lock, flags);
154         return ret;
155 }
156 
157 void free_uid(struct user_struct *up)
158 {
159         unsigned long flags;
160 
161         if (!up)
162                 return;
163 
164         local_irq_save(flags);
165         if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
166                 free_user(up, flags);
167         else
168                 local_irq_restore(flags);
169 }
170 
171 struct user_struct *alloc_uid(kuid_t uid)
172 {
173         struct hlist_head *hashent = uidhashentry(uid);
174         struct user_struct *up, *new;
175 
176         spin_lock_irq(&uidhash_lock);
177         up = uid_hash_find(uid, hashent);
178         spin_unlock_irq(&uidhash_lock);
179 
180         if (!up) {
181                 new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
182                 if (!new)
183                         goto out_unlock;
184 
185                 new->uid = uid;
186                 atomic_set(&new->__count, 1);
187 
188                 /*
189                  * Before adding this, check whether we raced
190                  * on adding the same user already..
191                  */
192                 spin_lock_irq(&uidhash_lock);
193                 up = uid_hash_find(uid, hashent);
194                 if (up) {
195                         key_put(new->uid_keyring);
196                         key_put(new->session_keyring);
197                         kmem_cache_free(uid_cachep, new);
198                 } else {
199                         uid_hash_insert(new, hashent);
200                         up = new;
201                 }
202                 spin_unlock_irq(&uidhash_lock);
203         }
204 
205         return up;
206 
207 out_unlock:
208         return NULL;
209 }
210 
211 static int __init uid_cache_init(void)
212 {
213         int n;
214 
215         uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
216                         0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
217 
218         for(n = 0; n < UIDHASH_SZ; ++n)
219                 INIT_HLIST_HEAD(uidhash_table + n);
220 
221         /* Insert the root user immediately (init already runs as root) */
222         spin_lock_irq(&uidhash_lock);
223         uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
224         spin_unlock_irq(&uidhash_lock);
225 
226         return 0;
227 }
228 subsys_initcall(uid_cache_init);
229 

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