inetpeer.c

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/*
 *      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
};

void inet_peer_base_init(struct inet_peer_base *bp)
{
    bp->root = peer_avl_empty_rcu;
    seqlock_init(&bp->lock);
    bp->flush_seq = ~0U;
    bp->total = 0;
}
EXPORT_SYMBOL_GPL(inet_peer_base_init);

static atomic_t v4_seq = ATOMIC_INIT(0);
static atomic_t v6_seq = ATOMIC_INIT(0);

static atomic_t *inetpeer_seq_ptr(int family)
{
    return (family == AF_INET ? &v4_seq : &v6_seq);
}

static inline void flush_check(struct inet_peer_base *base, int family)
{
    atomic_t *fp = inetpeer_seq_ptr(family);

    if (unlikely(base->flush_seq != atomic_read(fp))) {
        inetpeer_invalidate_tree(base);
        base->flush_seq = atomic_read(fp);
    }
}

void inetpeer_invalidate_family(int family)
{
    atomic_t *fp = inetpeer_seq_ptr(family);

    atomic_inc(fp);
}

#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, *c;
    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();

        c = rcu_dereference_protected(p->avl_left, 1);
        if (c != peer_avl_empty) {
            list_add_tail(&c->gc_list, &list);
            p->avl_left = peer_avl_empty_rcu;
        }

        c = rcu_dereference_protected(p->avl_right, 1);
        if (c != peer_avl_empty) {
            list_add_tail(&c->gc_list, &list);
            p->avl_right = peer_avl_empty_rcu;
        }

        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_DEFERRABLE_WORK(&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);
}

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

    flush_check(base, daddr->family);

    /* 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->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
        p->rate_tokens = 0;
        /* 60*HZ is arbitrary, but chosen enough high so that the first
         * calculation of tokens is at its maximum.
         */
        p->rate_last = jiffies - 60*HZ;
        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(struct inet_peer_base *base)
{
    struct inet_peer *root;

    write_seqlock_bh(&base->lock);

    root = rcu_deref_locked(base->root, base);
    if (root != peer_avl_empty) {
        base->root = peer_avl_empty_rcu;
        base->total = 0;
        call_rcu(&root->gc_rcu, inetpeer_inval_rcu);
    }

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