netfilter 24/62: xt_hashlimit: fix race condition and simplify locking

Patrick McHardy
2010-02-16T14:55:53Z

commit 2eff25c18c3d332d3c4dd98f2ac9b7114e9771b0
Author: Patrick McHardy <kaber@trash.net>
Date: Wed Feb 3 13:24:54 2010 +0100

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
--- 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;
}