netfilter: xt_hashlimit: fix race condition and simplify locking

As noticed by Shin Hong <hongshin@gmail.com>, there is a race between
htable_find_get() and htable_put():

htable_put():				htable_find_get():

					spin_lock_bh(&hashlimit_lock);
					<search entry>
atomic_dec_and_test(&hinfo->use)
					atomic_inc(&hinfo->use)
					spin_unlock_bh(&hashlimit_lock)
					return hinfo;
spin_lock_bh(&hashlimit_lock);
hlist_del(&hinfo->node);
spin_unlock_bh(&hashlimit_lock);
htable_destroy(hinfo);

The entire locking concept is overly complicated, tables are only
created/referenced and released in process context, so a single
mutex works just fine. Remove the hashinfo_spinlock and atomic
reference count and use the mutex to protect table lookups/creation
and reference count changes.

Signed-off-by: Patrick McHardy <kaber@trash.net>

diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
index fb7fcb7..017c959 100644 (file)
--- a/net/netfilter/xt_hashlimit.c
+++ b/net/netfilter/xt_hashlimit.c
@@ -90,7 +90,7 @@ struct dsthash_ent {
 
 struct xt_hashlimit_htable {
        struct hlist_node node;         /* global list of all htables */
-       atomic_t use;
+       int use;
        u_int8_t family;
        bool rnd_initialized;
 
@@ -109,8 +109,7 @@ struct xt_hashlimit_htable {
        struct hlist_head hash[0];      /* hashtable itself */
 };
 
-static DEFINE_SPINLOCK(hashlimit_lock);        /* protects htables list */
-static DEFINE_MUTEX(hlimit_mutex);     /* additional checkentry protection */
+static DEFINE_MUTEX(hashlimit_mutex);  /* protects htables list */
 static struct kmem_cache *hashlimit_cachep __read_mostly;
 
 static inline bool dst_cmp(const struct dsthash_ent *ent,
@@ -244,7 +243,7 @@ static int htable_create_v0(struct net *net, struct xt_hashlimit_info *minfo, u_
        for (i = 0; i < hinfo->cfg.size; i++)
                INIT_HLIST_HEAD(&hinfo->hash[i]);
 
-       atomic_set(&hinfo->use, 1);
+       hinfo->use = 1;
        hinfo->count = 0;
        hinfo->family = family;
        hinfo->rnd_initialized = false;
@@ -263,9 +262,9 @@ static int htable_create_v0(struct net *net, struct xt_hashlimit_info *minfo, u_
        hinfo->timer.expires = jiffies + msecs_to_jiffies(hinfo->cfg.gc_interval);
        add_timer(&hinfo->timer);
 
-       spin_lock_bh(&hashlimit_lock);
+       mutex_lock(&hashlimit_mutex);
        hlist_add_head(&hinfo->node, &hashlimit_net->htables);
-       spin_unlock_bh(&hashlimit_lock);
+       mutex_unlock(&hashlimit_mutex);
 
        return 0;
 }
@@ -308,7 +307,7 @@ static int htable_create(struct net *net, struct xt_hashlimit_mtinfo1 *minfo,
        for (i = 0; i < hinfo->cfg.size; i++)
                INIT_HLIST_HEAD(&hinfo->hash[i]);
 
-       atomic_set(&hinfo->use, 1);
+       hinfo->use = 1;
        hinfo->count = 0;
        hinfo->family = family;
        hinfo->rnd_initialized = false;
@@ -328,9 +327,9 @@ static int htable_create(struct net *net, struct xt_hashlimit_mtinfo1 *minfo,
        hinfo->timer.expires = jiffies + msecs_to_jiffies(hinfo->cfg.gc_interval);
        add_timer(&hinfo->timer);
 
-       spin_lock_bh(&hashlimit_lock);
+       mutex_lock(&hashlimit_mutex);
        hlist_add_head(&hinfo->node, &hashlimit_net->htables);
-       spin_unlock_bh(&hashlimit_lock);
+       mutex_unlock(&hashlimit_mutex);
 
        return 0;
 }
@@ -402,27 +401,24 @@ static struct xt_hashlimit_htable *htable_find_get(struct net *net,
        struct xt_hashlimit_htable *hinfo;
        struct hlist_node *pos;
 
-       spin_lock_bh(&hashlimit_lock);
        hlist_for_each_entry(hinfo, pos, &hashlimit_net->htables, node) {
                if (!strcmp(name, hinfo->pde->name) &&
                    hinfo->family == family) {
-                       atomic_inc(&hinfo->use);
-                       spin_unlock_bh(&hashlimit_lock);
+                       hinfo->use++;
                        return hinfo;
                }
        }
-       spin_unlock_bh(&hashlimit_lock);
        return NULL;
 }
 
 static void htable_put(struct xt_hashlimit_htable *hinfo)
 {
-       if (atomic_dec_and_test(&hinfo->use)) {
-               spin_lock_bh(&hashlimit_lock);
+       mutex_lock(&hashlimit_mutex);
+       if (--hinfo->use == 0) {
                hlist_del(&hinfo->node);
-               spin_unlock_bh(&hashlimit_lock);
                htable_destroy(hinfo);
        }
+       mutex_unlock(&hashlimit_mutex);
 }
 
 /* The algorithm used is the Simple Token Bucket Filter (TBF)
@@ -710,19 +706,13 @@ static bool hashlimit_mt_check_v0(const struct xt_mtchk_param *par)
        if (r->name[sizeof(r->name) - 1] != '\0')
                return false;
 
-       /* This is the best we've got: We cannot release and re-grab lock,
-        * since checkentry() is called before x_tables.c grabs xt_mutex.
-        * We also cannot grab the hashtable spinlock, since htable_create will
-        * call vmalloc, and that can sleep.  And we cannot just re-search
-        * the list of htable's in htable_create(), since then we would
-        * create duplicate proc files. -HW */
-       mutex_lock(&hlimit_mutex);
+       mutex_lock(&hashlimit_mutex);
        r->hinfo = htable_find_get(net, r->name, par->match->family);
        if (!r->hinfo && htable_create_v0(net, r, par->match->family) != 0) {
-               mutex_unlock(&hlimit_mutex);
+               mutex_unlock(&hashlimit_mutex);
                return false;
        }
-       mutex_unlock(&hlimit_mutex);
+       mutex_unlock(&hashlimit_mutex);
 
        return true;
 }
@@ -752,19 +742,13 @@ static bool hashlimit_mt_check(const struct xt_mtchk_param *par)
                        return false;
        }
 
-       /* This is the best we've got: We cannot release and re-grab lock,
-        * since checkentry() is called before x_tables.c grabs xt_mutex.
-        * We also cannot grab the hashtable spinlock, since htable_create will
-        * call vmalloc, and that can sleep.  And we cannot just re-search
-        * the list of htable's in htable_create(), since then we would
-        * create duplicate proc files. -HW */
-       mutex_lock(&hlimit_mutex);
+       mutex_lock(&hashlimit_mutex);
        info->hinfo = htable_find_get(net, info->name, par->match->family);
        if (!info->hinfo && htable_create(net, info, par->match->family) != 0) {
-               mutex_unlock(&hlimit_mutex);
+               mutex_unlock(&hashlimit_mutex);
                return false;
        }
-       mutex_unlock(&hlimit_mutex);
+       mutex_unlock(&hashlimit_mutex);
        return true;
 }