user.c

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/*
 * The "user cache".
 *
 * (C) Copyright 1991-2000 Linus Torvalds
 *
 * We have a per-user structure to keep track of how many
 * processes, files etc the user has claimed, in order to be
 * able to have per-user limits for system resources. 
 */

#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/bitops.h>
#include <linux/key.h>
#include <linux/interrupt.h>
#include <linux/export.h>
#include <linux/user_namespace.h>

/*
 * userns count is 1 for root user, 1 for init_uts_ns,
 * and 1 for... ?
 */
struct user_namespace init_user_ns = {
    .uid_map = {
        .nr_extents = 1,
        .extent[0] = {
            .first = 0,
            .lower_first = 0,
            .count = 4294967295U,
        },
    },
    .gid_map = {
        .nr_extents = 1,
        .extent[0] = {
            .first = 0,
            .lower_first = 0,
            .count = 4294967295U,
        },
    },
    .projid_map = {
        .nr_extents = 1,
        .extent[0] = {
            .first = 0,
            .lower_first = 0,
            .count = 4294967295U,
        },
    },
    .kref = {
        .refcount   = ATOMIC_INIT(3),
    },
    .owner = GLOBAL_ROOT_UID,
    .group = GLOBAL_ROOT_GID,
};
EXPORT_SYMBOL_GPL(init_user_ns);

/*
 * UID task count cache, to get fast user lookup in "alloc_uid"
 * when changing user ID's (ie setuid() and friends).
 */

#define UIDHASH_BITS    (CONFIG_BASE_SMALL ? 3 : 7)
#define UIDHASH_SZ  (1 << UIDHASH_BITS)
#define UIDHASH_MASK        (UIDHASH_SZ - 1)
#define __uidhashfn(uid)    (((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
#define uidhashentry(uid)   (uidhash_table + __uidhashfn((__kuid_val(uid))))

static struct kmem_cache *uid_cachep;
struct hlist_head uidhash_table[UIDHASH_SZ];

/*
 * The uidhash_lock is mostly taken from process context, but it is
 * occasionally also taken from softirq/tasklet context, when
 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
 * But free_uid() is also called with local interrupts disabled, and running
 * local_bh_enable() with local interrupts disabled is an error - we'll run
 * softirq callbacks, and they can unconditionally enable interrupts, and
 * the caller of free_uid() didn't expect that..
 */
static DEFINE_SPINLOCK(uidhash_lock);

/* root_user.__count is 1, for init task cred */
struct user_struct root_user = {
    .__count    = ATOMIC_INIT(1),
    .processes  = ATOMIC_INIT(1),
    .files      = ATOMIC_INIT(0),
    .sigpending = ATOMIC_INIT(0),
    .locked_shm     = 0,
    .uid        = GLOBAL_ROOT_UID,
};

/*
 * These routines must be called with the uidhash spinlock held!
 */
static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
{
    hlist_add_head(&up->uidhash_node, hashent);
}

static void uid_hash_remove(struct user_struct *up)
{
    hlist_del_init(&up->uidhash_node);
}

static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
{
    struct user_struct *user;
    struct hlist_node *h;

    hlist_for_each_entry(user, h, hashent, uidhash_node) {
        if (uid_eq(user->uid, uid)) {
            atomic_inc(&user->__count);
            return user;
        }
    }

    return NULL;
}

/* IRQs are disabled and uidhash_lock is held upon function entry.
 * IRQ state (as stored in flags) is restored and uidhash_lock released
 * upon function exit.
 */
static void free_user(struct user_struct *up, unsigned long flags)
    __releases(&uidhash_lock)
{
    uid_hash_remove(up);
    spin_unlock_irqrestore(&uidhash_lock, flags);
    key_put(up->uid_keyring);
    key_put(up->session_keyring);
    kmem_cache_free(uid_cachep, up);
}

/*
 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 * caller must undo that ref with free_uid().
 *
 * If the user_struct could not be found, return NULL.
 */
struct user_struct *find_user(kuid_t uid)
{
    struct user_struct *ret;
    unsigned long flags;

    spin_lock_irqsave(&uidhash_lock, flags);
    ret = uid_hash_find(uid, uidhashentry(uid));
    spin_unlock_irqrestore(&uidhash_lock, flags);
    return ret;
}

void free_uid(struct user_struct *up)
{
    unsigned long flags;

    if (!up)
        return;

    local_irq_save(flags);
    if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
        free_user(up, flags);
    else
        local_irq_restore(flags);
}

struct user_struct *alloc_uid(kuid_t uid)
{
    struct hlist_head *hashent = uidhashentry(uid);
    struct user_struct *up, *new;

    spin_lock_irq(&uidhash_lock);
    up = uid_hash_find(uid, hashent);
    spin_unlock_irq(&uidhash_lock);

    if (!up) {
        new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
        if (!new)
            goto out_unlock;

        new->uid = uid;
        atomic_set(&new->__count, 1);

        /*
         * Before adding this, check whether we raced
         * on adding the same user already..
         */
        spin_lock_irq(&uidhash_lock);
        up = uid_hash_find(uid, hashent);
        if (up) {
            key_put(new->uid_keyring);
            key_put(new->session_keyring);
            kmem_cache_free(uid_cachep, new);
        } else {
            uid_hash_insert(new, hashent);
            up = new;
        }
        spin_unlock_irq(&uidhash_lock);
    }

    return up;

out_unlock:
    return NULL;
}

static int __init uid_cache_init(void)
{
    int n;

    uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
            0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);

    for(n = 0; n < UIDHASH_SZ; ++n)
        INIT_HLIST_HEAD(uidhash_table + n);

    /* Insert the root user immediately (init already runs as root) */
    spin_lock_irq(&uidhash_lock);
    uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
    spin_unlock_irq(&uidhash_lock);

    return 0;
}

module_init(uid_cache_init);