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

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