user.c 5.25 KB
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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>
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#include <linux/interrupt.h>
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#include <linux/export.h>
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#include <linux/user_namespace.h>
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#include <linux/proc_ns.h>
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/*
 * userns count is 1 for root user, 1 for init_uts_ns,
 * and 1 for... ?
 */
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struct user_namespace init_user_ns = {
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	.uid_map = {
		.nr_extents = 1,
		.extent[0] = {
			.first = 0,
			.lower_first = 0,
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			.count = 4294967295U,
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		},
	},
	.gid_map = {
		.nr_extents = 1,
		.extent[0] = {
			.first = 0,
			.lower_first = 0,
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			.count = 4294967295U,
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		},
	},
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	.projid_map = {
		.nr_extents = 1,
		.extent[0] = {
			.first = 0,
			.lower_first = 0,
			.count = 4294967295U,
		},
	},
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	.count = ATOMIC_INIT(3),
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	.owner = GLOBAL_ROOT_UID,
	.group = GLOBAL_ROOT_GID,
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	.proc_inum = PROC_USER_INIT_INO,
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	.flags = USERNS_INIT_FLAGS,
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	.may_mount_sysfs = true,
	.may_mount_proc = true,
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};
EXPORT_SYMBOL_GPL(init_user_ns);

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

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#define UIDHASH_BITS	(CONFIG_BASE_SMALL ? 3 : 7)
#define UIDHASH_SZ	(1 << UIDHASH_BITS)
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#define UIDHASH_MASK		(UIDHASH_SZ - 1)
#define __uidhashfn(uid)	(((uid >> UIDHASH_BITS) + uid) & UIDHASH_MASK)
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#define uidhashentry(uid)	(uidhash_table + __uidhashfn((__kuid_val(uid))))
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static struct kmem_cache *uid_cachep;
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struct hlist_head uidhash_table[UIDHASH_SZ];
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/*
 * 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.
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 * 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..
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 */
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static DEFINE_SPINLOCK(uidhash_lock);

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

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

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

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

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

	return NULL;
}

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/* 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.
 */
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static void free_user(struct user_struct *up, unsigned long flags)
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	__releases(&uidhash_lock)
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{
	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);
}

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/*
 * 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.
 */
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struct user_struct *find_user(kuid_t uid)
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{
	struct user_struct *ret;
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	unsigned long flags;
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	spin_lock_irqsave(&uidhash_lock, flags);
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	ret = uid_hash_find(uid, uidhashentry(uid));
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	spin_unlock_irqrestore(&uidhash_lock, flags);
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	return ret;
}

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

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	if (!up)
		return;

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

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struct user_struct *alloc_uid(kuid_t uid)
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{
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	struct hlist_head *hashent = uidhashentry(uid);
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	struct user_struct *up, *new;
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	spin_lock_irq(&uidhash_lock);
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	up = uid_hash_find(uid, hashent);
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	spin_unlock_irq(&uidhash_lock);
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	if (!up) {
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		new = kmem_cache_zalloc(uid_cachep, GFP_KERNEL);
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		if (!new)
			goto out_unlock;
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		new->uid = uid;
		atomic_set(&new->__count, 1);

		/*
		 * Before adding this, check whether we raced
		 * on adding the same user already..
		 */
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		spin_lock_irq(&uidhash_lock);
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		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;
		}
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		spin_unlock_irq(&uidhash_lock);
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	}
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	return up;
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out_unlock:
	return NULL;
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}

static int __init uid_cache_init(void)
{
	int n;

	uid_cachep = kmem_cache_create("uid_cache", sizeof(struct user_struct),
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			0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
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	for(n = 0; n < UIDHASH_SZ; ++n)
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		INIT_HLIST_HEAD(uidhash_table + n);
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	/* Insert the root user immediately (init already runs as root) */
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	spin_lock_irq(&uidhash_lock);
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	uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));
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	spin_unlock_irq(&uidhash_lock);
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	return 0;
}

module_init(uid_cache_init);