inetpeer: fix a race in inetpeer_gc_worker()

[linux.git] / net / ipv4 / inetpeer.c

/*
 *		INETPEER - A storage for permanent information about peers
 *
 *  This source is covered by the GNU GPL, the same as all kernel sources.
 *
 *  Authors:	Andrey V. Savochkin <[email protected]>
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/slab.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
#include <linux/random.h>
#include <linux/timer.h>
#include <linux/time.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/net.h>
#include <linux/workqueue.h>
#include <net/ip.h>
#include <net/inetpeer.h>
#include <net/secure_seq.h>

/*
 *  Theory of operations.
 *  We keep one entry for each peer IP address.  The nodes contains long-living
 *  information about the peer which doesn't depend on routes.
 *  At this moment this information consists only of ID field for the next
 *  outgoing IP packet.  This field is incremented with each packet as encoded
 *  in inet_getid() function (include/net/inetpeer.h).
 *  At the moment of writing this notes identifier of IP packets is generated
 *  to be unpredictable using this code only for packets subjected
 *  (actually or potentially) to defragmentation.  I.e. DF packets less than
 *  PMTU in size uses a constant ID and do not use this code (see
 *  ip_select_ident() in include/net/ip.h).
 *
 *  Route cache entries hold references to our nodes.
 *  New cache entries get references via lookup by destination IP address in
 *  the avl tree.  The reference is grabbed only when it's needed i.e. only
 *  when we try to output IP packet which needs an unpredictable ID (see
 *  __ip_select_ident() in net/ipv4/route.c).
 *  Nodes are removed only when reference counter goes to 0.
 *  When it's happened the node may be removed when a sufficient amount of
 *  time has been passed since its last use.  The less-recently-used entry can
 *  also be removed if the pool is overloaded i.e. if the total amount of
 *  entries is greater-or-equal than the threshold.
 *
 *  Node pool is organised as an AVL tree.
 *  Such an implementation has been chosen not just for fun.  It's a way to
 *  prevent easy and efficient DoS attacks by creating hash collisions.  A huge
 *  amount of long living nodes in a single hash slot would significantly delay
 *  lookups performed with disabled BHs.
 *
 *  Serialisation issues.
 *  1.  Nodes may appear in the tree only with the pool lock held.
 *  2.  Nodes may disappear from the tree only with the pool lock held
 *      AND reference count being 0.
 *  3.  Global variable peer_total is modified under the pool lock.
 *  4.  struct inet_peer fields modification:
 *		avl_left, avl_right, avl_parent, avl_height: pool lock
 *		refcnt: atomically against modifications on other CPU;
 *		   usually under some other lock to prevent node disappearing
 *		daddr: unchangeable
 *		ip_id_count: atomic value (no lock needed)
 */

static struct kmem_cache *peer_cachep __read_mostly;

static LIST_HEAD(gc_list);
static const int gc_delay = 60 * HZ;
static struct delayed_work gc_work;
static DEFINE_SPINLOCK(gc_lock);

#define node_height(x) x->avl_height

#define peer_avl_empty ((struct inet_peer *)&peer_fake_node)
#define peer_avl_empty_rcu ((struct inet_peer __rcu __force *)&peer_fake_node)
static const struct inet_peer peer_fake_node = {
	.avl_left	= peer_avl_empty_rcu,
	.avl_right	= peer_avl_empty_rcu,
	.avl_height	= 0
};

struct inet_peer_base {
	struct inet_peer __rcu *root;
	seqlock_t	lock;
	int		total;
};

static struct inet_peer_base v4_peers = {
	.root		= peer_avl_empty_rcu,
	.lock		= __SEQLOCK_UNLOCKED(v4_peers.lock),
	.total		= 0,
};

static struct inet_peer_base v6_peers = {
	.root		= peer_avl_empty_rcu,
	.lock		= __SEQLOCK_UNLOCKED(v6_peers.lock),
	.total		= 0,
};

#define PEER_MAXDEPTH 40 /* sufficient for about 2^27 nodes */

/* Exported for sysctl_net_ipv4.  */
int inet_peer_threshold __read_mostly = 65536 + 128;	/* start to throw entries more
					 * aggressively at this stage */
int inet_peer_minttl __read_mostly = 120 * HZ;	/* TTL under high load: 120 sec */
int inet_peer_maxttl __read_mostly = 10 * 60 * HZ;	/* usual time to live: 10 min */

static void inetpeer_gc_worker(struct work_struct *work)
{
	struct inet_peer *p, *n;
	LIST_HEAD(list);

	spin_lock_bh(&gc_lock);
	list_replace_init(&gc_list, &list);
	spin_unlock_bh(&gc_lock);

	if (list_empty(&list))
		return;

	list_for_each_entry_safe(p, n, &list, gc_list) {

		if(need_resched())
			cond_resched();

		if (p->avl_left != peer_avl_empty) {
			list_add_tail(&p->avl_left->gc_list, &list);
			p->avl_left = peer_avl_empty;
		}

		if (p->avl_right != peer_avl_empty) {
			list_add_tail(&p->avl_right->gc_list, &list);
			p->avl_right = peer_avl_empty;
		}

		n = list_entry(p->gc_list.next, struct inet_peer, gc_list);

		if (!atomic_read(&p->refcnt)) {
			list_del(&p->gc_list);
			kmem_cache_free(peer_cachep, p);
		}
	}

	if (list_empty(&list))
		return;

	spin_lock_bh(&gc_lock);
	list_splice(&list, &gc_list);
	spin_unlock_bh(&gc_lock);

	schedule_delayed_work(&gc_work, gc_delay);
}

/* Called from ip_output.c:ip_init  */
void __init inet_initpeers(void)
{
	struct sysinfo si;

	/* Use the straight interface to information about memory. */
	si_meminfo(&si);
	/* The values below were suggested by Alexey Kuznetsov
	 * <[email protected]>.  I don't have any opinion about the values
	 * myself.  --SAW
	 */
	if (si.totalram <= (32768*1024)/PAGE_SIZE)
		inet_peer_threshold >>= 1; /* max pool size about 1MB on IA32 */
	if (si.totalram <= (16384*1024)/PAGE_SIZE)
		inet_peer_threshold >>= 1; /* about 512KB */
	if (si.totalram <= (8192*1024)/PAGE_SIZE)
		inet_peer_threshold >>= 2; /* about 128KB */

	peer_cachep = kmem_cache_create("inet_peer_cache",
			sizeof(struct inet_peer),
			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC,
			NULL);

	INIT_DELAYED_WORK_DEFERRABLE(&gc_work, inetpeer_gc_worker);
}

static int addr_compare(const struct inetpeer_addr *a,
			const struct inetpeer_addr *b)
{
	int i, n = (a->family == AF_INET ? 1 : 4);

	for (i = 0; i < n; i++) {
		if (a->addr.a6[i] == b->addr.a6[i])
			continue;
		if ((__force u32)a->addr.a6[i] < (__force u32)b->addr.a6[i])
			return -1;
		return 1;
	}

	return 0;
}

#define rcu_deref_locked(X, BASE)				\
	rcu_dereference_protected(X, lockdep_is_held(&(BASE)->lock.lock))

/*
 * Called with local BH disabled and the pool lock held.
 */
#define lookup(_daddr, _stack, _base)				\
({								\
	struct inet_peer *u;					\
	struct inet_peer __rcu **v;				\
								\
	stackptr = _stack;					\
	*stackptr++ = &_base->root;				\
	for (u = rcu_deref_locked(_base->root, _base);		\
	     u != peer_avl_empty; ) {				\
		int cmp = addr_compare(_daddr, &u->daddr);	\
		if (cmp == 0)					\
			break;					\
		if (cmp == -1)					\
			v = &u->avl_left;			\
		else						\
			v = &u->avl_right;			\
		*stackptr++ = v;				\
		u = rcu_deref_locked(*v, _base);		\
	}							\
	u;							\
})

/*
 * Called with rcu_read_lock()
 * Because we hold no lock against a writer, its quite possible we fall
 * in an endless loop.
 * But every pointer we follow is guaranteed to be valid thanks to RCU.
 * We exit from this function if number of links exceeds PEER_MAXDEPTH
 */
static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
				    struct inet_peer_base *base)
{
	struct inet_peer *u = rcu_dereference(base->root);
	int count = 0;

	while (u != peer_avl_empty) {
		int cmp = addr_compare(daddr, &u->daddr);
		if (cmp == 0) {
			/* Before taking a reference, check if this entry was
			 * deleted (refcnt=-1)
			 */
			if (!atomic_add_unless(&u->refcnt, 1, -1))
				u = NULL;
			return u;
		}
		if (cmp == -1)
			u = rcu_dereference(u->avl_left);
		else
			u = rcu_dereference(u->avl_right);
		if (unlikely(++count == PEER_MAXDEPTH))
			break;
	}
	return NULL;
}

/* Called with local BH disabled and the pool lock held. */
#define lookup_rightempty(start, base)				\
({								\
	struct inet_peer *u;					\
	struct inet_peer __rcu **v;				\
	*stackptr++ = &start->avl_left;				\
	v = &start->avl_left;					\
	for (u = rcu_deref_locked(*v, base);			\
	     u->avl_right != peer_avl_empty_rcu; ) {		\
		v = &u->avl_right;				\
		*stackptr++ = v;				\
		u = rcu_deref_locked(*v, base);			\
	}							\
	u;							\
})

/* Called with local BH disabled and the pool lock held.
 * Variable names are the proof of operation correctness.
 * Look into mm/map_avl.c for more detail description of the ideas.
 */
static void peer_avl_rebalance(struct inet_peer __rcu **stack[],
			       struct inet_peer __rcu ***stackend,
			       struct inet_peer_base *base)
{
	struct inet_peer __rcu **nodep;
	struct inet_peer *node, *l, *r;
	int lh, rh;

	while (stackend > stack) {
		nodep = *--stackend;
		node = rcu_deref_locked(*nodep, base);
		l = rcu_deref_locked(node->avl_left, base);
		r = rcu_deref_locked(node->avl_right, base);
		lh = node_height(l);
		rh = node_height(r);
		if (lh > rh + 1) { /* l: RH+2 */
			struct inet_peer *ll, *lr, *lrl, *lrr;
			int lrh;
			ll = rcu_deref_locked(l->avl_left, base);
			lr = rcu_deref_locked(l->avl_right, base);
			lrh = node_height(lr);
			if (lrh <= node_height(ll)) {	/* ll: RH+1 */
				RCU_INIT_POINTER(node->avl_left, lr);	/* lr: RH or RH+1 */
				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
				node->avl_height = lrh + 1; /* RH+1 or RH+2 */
				RCU_INIT_POINTER(l->avl_left, ll);       /* ll: RH+1 */
				RCU_INIT_POINTER(l->avl_right, node);	/* node: RH+1 or RH+2 */
				l->avl_height = node->avl_height + 1;
				RCU_INIT_POINTER(*nodep, l);
			} else { /* ll: RH, lr: RH+1 */
				lrl = rcu_deref_locked(lr->avl_left, base);/* lrl: RH or RH-1 */
				lrr = rcu_deref_locked(lr->avl_right, base);/* lrr: RH or RH-1 */
				RCU_INIT_POINTER(node->avl_left, lrr);	/* lrr: RH or RH-1 */
				RCU_INIT_POINTER(node->avl_right, r);	/* r: RH */
				node->avl_height = rh + 1; /* node: RH+1 */
				RCU_INIT_POINTER(l->avl_left, ll);	/* ll: RH */
				RCU_INIT_POINTER(l->avl_right, lrl);	/* lrl: RH or RH-1 */
				l->avl_height = rh + 1;	/* l: RH+1 */
				RCU_INIT_POINTER(lr->avl_left, l);	/* l: RH+1 */
				RCU_INIT_POINTER(lr->avl_right, node);	/* node: RH+1 */
				lr->avl_height = rh + 2;
				RCU_INIT_POINTER(*nodep, lr);
			}
		} else if (rh > lh + 1) { /* r: LH+2 */
			struct inet_peer *rr, *rl, *rlr, *rll;
			int rlh;
			rr = rcu_deref_locked(r->avl_right, base);
			rl = rcu_deref_locked(r->avl_left, base);
			rlh = node_height(rl);
			if (rlh <= node_height(rr)) {	/* rr: LH+1 */
				RCU_INIT_POINTER(node->avl_right, rl);	/* rl: LH or LH+1 */
				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
				node->avl_height = rlh + 1; /* LH+1 or LH+2 */
				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH+1 */
				RCU_INIT_POINTER(r->avl_left, node);	/* node: LH+1 or LH+2 */
				r->avl_height = node->avl_height + 1;
				RCU_INIT_POINTER(*nodep, r);
			} else { /* rr: RH, rl: RH+1 */
				rlr = rcu_deref_locked(rl->avl_right, base);/* rlr: LH or LH-1 */
				rll = rcu_deref_locked(rl->avl_left, base);/* rll: LH or LH-1 */
				RCU_INIT_POINTER(node->avl_right, rll);	/* rll: LH or LH-1 */
				RCU_INIT_POINTER(node->avl_left, l);	/* l: LH */
				node->avl_height = lh + 1; /* node: LH+1 */
				RCU_INIT_POINTER(r->avl_right, rr);	/* rr: LH */
				RCU_INIT_POINTER(r->avl_left, rlr);	/* rlr: LH or LH-1 */
				r->avl_height = lh + 1;	/* r: LH+1 */
				RCU_INIT_POINTER(rl->avl_right, r);	/* r: LH+1 */
				RCU_INIT_POINTER(rl->avl_left, node);	/* node: LH+1 */
				rl->avl_height = lh + 2;
				RCU_INIT_POINTER(*nodep, rl);
			}
		} else {
			node->avl_height = (lh > rh ? lh : rh) + 1;
		}
	}
}

/* Called with local BH disabled and the pool lock held. */
#define link_to_pool(n, base)					\
do {								\
	n->avl_height = 1;					\
	n->avl_left = peer_avl_empty_rcu;			\
	n->avl_right = peer_avl_empty_rcu;			\
	/* lockless readers can catch us now */			\
	rcu_assign_pointer(**--stackptr, n);			\
	peer_avl_rebalance(stack, stackptr, base);		\
} while (0)

static void inetpeer_free_rcu(struct rcu_head *head)
{
	kmem_cache_free(peer_cachep, container_of(head, struct inet_peer, rcu));
}

static void unlink_from_pool(struct inet_peer *p, struct inet_peer_base *base,
			     struct inet_peer __rcu **stack[PEER_MAXDEPTH])
{
	struct inet_peer __rcu ***stackptr, ***delp;

	if (lookup(&p->daddr, stack, base) != p)
		BUG();
	delp = stackptr - 1; /* *delp[0] == p */
	if (p->avl_left == peer_avl_empty_rcu) {
		*delp[0] = p->avl_right;
		--stackptr;
	} else {
		/* look for a node to insert instead of p */
		struct inet_peer *t;
		t = lookup_rightempty(p, base);
		BUG_ON(rcu_deref_locked(*stackptr[-1], base) != t);
		**--stackptr = t->avl_left;
		/* t is removed, t->daddr > x->daddr for any
		 * x in p->avl_left subtree.
		 * Put t in the old place of p. */
		RCU_INIT_POINTER(*delp[0], t);
		t->avl_left = p->avl_left;
		t->avl_right = p->avl_right;
		t->avl_height = p->avl_height;
		BUG_ON(delp[1] != &p->avl_left);
		delp[1] = &t->avl_left; /* was &p->avl_left */
	}
	peer_avl_rebalance(stack, stackptr, base);
	base->total--;
	call_rcu(&p->rcu, inetpeer_free_rcu);
}

static struct inet_peer_base *family_to_base(int family)
{
	return family == AF_INET ? &v4_peers : &v6_peers;
}

/* perform garbage collect on all items stacked during a lookup */
static int inet_peer_gc(struct inet_peer_base *base,
			struct inet_peer __rcu **stack[PEER_MAXDEPTH],
			struct inet_peer __rcu ***stackptr)
{
	struct inet_peer *p, *gchead = NULL;
	__u32 delta, ttl;
	int cnt = 0;

	if (base->total >= inet_peer_threshold)
		ttl = 0; /* be aggressive */
	else
		ttl = inet_peer_maxttl
				- (inet_peer_maxttl - inet_peer_minttl) / HZ *
					base->total / inet_peer_threshold * HZ;
	stackptr--; /* last stack slot is peer_avl_empty */
	while (stackptr > stack) {
		stackptr--;
		p = rcu_deref_locked(**stackptr, base);
		if (atomic_read(&p->refcnt) == 0) {
			smp_rmb();
			delta = (__u32)jiffies - p->dtime;
			if (delta >= ttl &&
			    atomic_cmpxchg(&p->refcnt, 0, -1) == 0) {
				p->gc_next = gchead;
				gchead = p;
			}
		}
	}
	while ((p = gchead) != NULL) {
		gchead = p->gc_next;
		cnt++;
		unlink_from_pool(p, base, stack);
	}
	return cnt;
}

struct inet_peer *inet_getpeer(const struct inetpeer_addr *daddr, int create)
{
	struct inet_peer __rcu **stack[PEER_MAXDEPTH], ***stackptr;
	struct inet_peer_base *base = family_to_base(daddr->family);
	struct inet_peer *p;
	unsigned int sequence;
	int invalidated, gccnt = 0;

	/* Attempt a lockless lookup first.
	 * Because of a concurrent writer, we might not find an existing entry.
	 */
	rcu_read_lock();
	sequence = read_seqbegin(&base->lock);
	p = lookup_rcu(daddr, base);
	invalidated = read_seqretry(&base->lock, sequence);
	rcu_read_unlock();

	if (p)
		return p;

	/* If no writer did a change during our lookup, we can return early. */
	if (!create && !invalidated)
		return NULL;

	/* retry an exact lookup, taking the lock before.
	 * At least, nodes should be hot in our cache.
	 */
	write_seqlock_bh(&base->lock);
relookup:
	p = lookup(daddr, stack, base);
	if (p != peer_avl_empty) {
		atomic_inc(&p->refcnt);
		write_sequnlock_bh(&base->lock);
		return p;
	}
	if (!gccnt) {
		gccnt = inet_peer_gc(base, stack, stackptr);
		if (gccnt && create)
			goto relookup;
	}
	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
	if (p) {
		p->daddr = *daddr;
		atomic_set(&p->refcnt, 1);
		atomic_set(&p->rid, 0);
		atomic_set(&p->ip_id_count,
				(daddr->family == AF_INET) ?
					secure_ip_id(daddr->addr.a4) :
					secure_ipv6_id(daddr->addr.a6));
		p->tcp_ts_stamp = 0;
		p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
		p->rate_tokens = 0;
		p->rate_last = 0;
		p->pmtu_expires = 0;
		p->pmtu_orig = 0;
		memset(&p->redirect_learned, 0, sizeof(p->redirect_learned));
		INIT_LIST_HEAD(&p->gc_list);

		/* Link the node. */
		link_to_pool(p, base);
		base->total++;
	}
	write_sequnlock_bh(&base->lock);

	return p;
}
EXPORT_SYMBOL_GPL(inet_getpeer);

void inet_putpeer(struct inet_peer *p)
{
	p->dtime = (__u32)jiffies;
	smp_mb__before_atomic_dec();
	atomic_dec(&p->refcnt);
}
EXPORT_SYMBOL_GPL(inet_putpeer);

/*
 *	Check transmit rate limitation for given message.
 *	The rate information is held in the inet_peer entries now.
 *	This function is generic and could be used for other purposes
 *	too. It uses a Token bucket filter as suggested by Alexey Kuznetsov.
 *
 *	Note that the same inet_peer fields are modified by functions in
 *	route.c too, but these work for packet destinations while xrlim_allow
 *	works for icmp destinations. This means the rate limiting information
 *	for one "ip object" is shared - and these ICMPs are twice limited:
 *	by source and by destination.
 *
 *	RFC 1812: 4.3.2.8 SHOULD be able to limit error message rate
 *			  SHOULD allow setting of rate limits
 *
 * 	Shared between ICMPv4 and ICMPv6.
 */
#define XRLIM_BURST_FACTOR 6
bool inet_peer_xrlim_allow(struct inet_peer *peer, int timeout)
{
	unsigned long now, token;
	bool rc = false;

	if (!peer)
		return true;

	token = peer->rate_tokens;
	now = jiffies;
	token += now - peer->rate_last;
	peer->rate_last = now;
	if (token > XRLIM_BURST_FACTOR * timeout)
		token = XRLIM_BURST_FACTOR * timeout;
	if (token >= timeout) {
		token -= timeout;
		rc = true;
	}
	peer->rate_tokens = token;
	return rc;
}
EXPORT_SYMBOL(inet_peer_xrlim_allow);

static void inetpeer_inval_rcu(struct rcu_head *head)
{
	struct inet_peer *p = container_of(head, struct inet_peer, gc_rcu);

	spin_lock_bh(&gc_lock);
	list_add_tail(&p->gc_list, &gc_list);
	spin_unlock_bh(&gc_lock);

	schedule_delayed_work(&gc_work, gc_delay);
}

void inetpeer_invalidate_tree(int family)
{
	struct inet_peer *old, *new, *prev;
	struct inet_peer_base *base = family_to_base(family);

	write_seqlock_bh(&base->lock);

	old = base->root;
	if (old == peer_avl_empty_rcu)
		goto out;

	new = peer_avl_empty_rcu;

	prev = cmpxchg(&base->root, old, new);
	if (prev == old) {
		base->total = 0;
		call_rcu(&prev->gc_rcu, inetpeer_inval_rcu);
	}

out:
	write_sequnlock_bh(&base->lock);
}
EXPORT_SYMBOL(inetpeer_invalidate_tree);