Root/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#include "cred-internals.h"
20
21struct user_namespace init_user_ns = {
22    .kref = {
23        .refcount = ATOMIC_INIT(2),
24    },
25    .creator = &root_user,
26};
27EXPORT_SYMBOL_GPL(init_user_ns);
28
29/*
30 * UID task count cache, to get fast user lookup in "alloc_uid"
31 * when changing user ID's (ie setuid() and friends).
32 */
33
34#define UIDHASH_MASK (UIDHASH_SZ - 1)
35#define __uidhashfn(uid) (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
36#define uidhashentry(ns, uid) ((ns)->uidhash_table + __uidhashfn((uid)))
37
38static struct kmem_cache *uid_cachep;
39
40/*
41 * The uidhash_lock is mostly taken from process context, but it is
42 * occasionally also taken from softirq/tasklet context, when
43 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
44 * But free_uid() is also called with local interrupts disabled, and running
45 * local_bh_enable() with local interrupts disabled is an error - we'll run
46 * softirq callbacks, and they can unconditionally enable interrupts, and
47 * the caller of free_uid() didn't expect that..
48 */
49static DEFINE_SPINLOCK(uidhash_lock);
50
51/* root_user.__count is 2, 1 for init task cred, 1 for init_user_ns->creator */
52struct user_struct root_user = {
53    .__count = ATOMIC_INIT(2),
54    .processes = ATOMIC_INIT(1),
55    .files = ATOMIC_INIT(0),
56    .sigpending = ATOMIC_INIT(0),
57    .locked_shm = 0,
58    .user_ns = &init_user_ns,
59};
60
61/*
62 * These routines must be called with the uidhash spinlock held!
63 */
64static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
65{
66    hlist_add_head(&up->uidhash_node, hashent);
67}
68
69static void uid_hash_remove(struct user_struct *up)
70{
71    hlist_del_init(&up->uidhash_node);
72    put_user_ns(up->user_ns);
73}
74
75static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
76{
77    struct user_struct *user;
78    struct hlist_node *h;
79
80    hlist_for_each_entry(user, h, hashent, uidhash_node) {
81        if (user->uid == uid) {
82            atomic_inc(&user->__count);
83            return user;
84        }
85    }
86
87    return NULL;
88}
89
90/* IRQs are disabled and uidhash_lock is held upon function entry.
91 * IRQ state (as stored in flags) is restored and uidhash_lock released
92 * upon function exit.
93 */
94static void free_user(struct user_struct *up, unsigned long flags)
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 */
109struct 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
121void 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
135struct 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() + uids_user_create() + uid_hash_insert()
141     * atomic.
142     */
143    spin_lock_irq(&uidhash_lock);
144    up = uid_hash_find(uid, hashent);
145    spin_unlock_irq(&uidhash_lock);
146
147    if (!up) {
148        new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
149        if (!new)
150            goto out_unlock;
151
152        new->uid = uid;
153        atomic_set(&new->__count, 1);
154
155        new->user_ns = get_user_ns(ns);
156
157        /*
158         * Before adding this, check whether we raced
159         * on adding the same user already..
160         */
161        spin_lock_irq(&uidhash_lock);
162        up = uid_hash_find(uid, hashent);
163        if (up) {
164            /* This case is not possible when CONFIG_USER_SCHED
165             * is defined, since we serialize alloc_uid() using
166             * uids_mutex. Hence no need to call
167             * sched_destroy_user() or remove_user_sysfs_dir().
168             */
169            key_put(new->uid_keyring);
170            key_put(new->session_keyring);
171            kmem_cache_free(uid_cachep, new);
172        } else {
173            uid_hash_insert(new, hashent);
174            up = new;
175        }
176        spin_unlock_irq(&uidhash_lock);
177    }
178
179    return up;
180
181    put_user_ns(new->user_ns);
182    kmem_cache_free(uid_cachep, new);
183out_unlock:
184    return NULL;
185}
186
187static int __init uid_cache_init(void)
188{
189    int n;
190
191    uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
192            0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
193
194    for(n = 0; n < UIDHASH_SZ; ++n)
195        INIT_HLIST_HEAD(init_user_ns.uidhash_table + n);
196
197    /* Insert the root user immediately (init already runs as root) */
198    spin_lock_irq(&uidhash_lock);
199    uid_hash_insert(&root_user, uidhashentry(&init_user_ns, 0));
200    spin_unlock_irq(&uidhash_lock);
201
202    return 0;
203}
204
205module_init(uid_cache_init);
206

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