kernel-ark/kernel/user.c
Linus Torvalds 45c18b0bb5 Fix unlikely (but possible) race condition on task->user access
There's a possible race condition when doing a "switch_uid()" from one
user to another, which could race with another thread doing a signal
allocation and looking at the old thread ->user pointer as it is freed.

This explains an oops reported by Lukasz Trabinski:
	http://permalink.gmane.org/gmane.linux.kernel/462241

We fix this by delaying the (reference-counted) freeing of the user
structure until the thread signal handler lock has been released, so
that we know that the signal allocation has either seen the new value or
has properly incremented the reference count of the old one.

Race identified by Oleg Nesterov.

Cc: Lukasz Trabinski <lukasz@wsisiz.edu.pl>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@osdl.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-11-04 10:06:02 -08:00

225 lines
5.4 KiB
C

/*
* 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>
/*
* 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 : 8)
#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((uid)))
static kmem_cache_t *uid_cachep;
static struct list_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);
struct user_struct root_user = {
.__count = ATOMIC_INIT(1),
.processes = ATOMIC_INIT(1),
.files = ATOMIC_INIT(0),
.sigpending = ATOMIC_INIT(0),
.mq_bytes = 0,
.locked_shm = 0,
#ifdef CONFIG_KEYS
.uid_keyring = &root_user_keyring,
.session_keyring = &root_session_keyring,
#endif
};
/*
* These routines must be called with the uidhash spinlock held!
*/
static inline void uid_hash_insert(struct user_struct *up, struct list_head *hashent)
{
list_add(&up->uidhash_list, hashent);
}
static inline void uid_hash_remove(struct user_struct *up)
{
list_del(&up->uidhash_list);
}
static inline struct user_struct *uid_hash_find(uid_t uid, struct list_head *hashent)
{
struct list_head *up;
list_for_each(up, hashent) {
struct user_struct *user;
user = list_entry(up, struct user_struct, uidhash_list);
if(user->uid == uid) {
atomic_inc(&user->__count);
return user;
}
}
return NULL;
}
/*
* 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(uid_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)) {
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);
} else {
local_irq_restore(flags);
}
}
struct user_struct * alloc_uid(uid_t uid)
{
struct list_head *hashent = uidhashentry(uid);
struct user_struct *up;
spin_lock_irq(&uidhash_lock);
up = uid_hash_find(uid, hashent);
spin_unlock_irq(&uidhash_lock);
if (!up) {
struct user_struct *new;
new = kmem_cache_alloc(uid_cachep, SLAB_KERNEL);
if (!new)
return NULL;
new->uid = uid;
atomic_set(&new->__count, 1);
atomic_set(&new->processes, 0);
atomic_set(&new->files, 0);
atomic_set(&new->sigpending, 0);
#ifdef CONFIG_INOTIFY_USER
atomic_set(&new->inotify_watches, 0);
atomic_set(&new->inotify_devs, 0);
#endif
new->mq_bytes = 0;
new->locked_shm = 0;
if (alloc_uid_keyring(new, current) < 0) {
kmem_cache_free(uid_cachep, new);
return NULL;
}
/*
* 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;
}
void switch_uid(struct user_struct *new_user)
{
struct user_struct *old_user;
/* What if a process setreuid()'s and this brings the
* new uid over his NPROC rlimit? We can check this now
* cheaply with the new uid cache, so if it matters
* we should be checking for it. -DaveM
*/
old_user = current->user;
atomic_inc(&new_user->processes);
atomic_dec(&old_user->processes);
switch_uid_keyring(new_user);
current->user = new_user;
/*
* We need to synchronize with __sigqueue_alloc()
* doing a get_uid(p->user).. If that saw the old
* user value, we need to wait until it has exited
* its critical region before we can free the old
* structure.
*/
smp_mb();
spin_unlock_wait(&current->sighand->siglock);
free_uid(old_user);
suid_keys(current);
}
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, NULL);
for(n = 0; n < UIDHASH_SZ; ++n)
INIT_LIST_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(0));
spin_unlock_irq(&uidhash_lock);
return 0;
}
module_init(uid_cache_init);