netlink: fix netlink_change_ngroups()

commit 6c04bb18ddd633 (netlink: use call_rcu for netlink_change_ngroups)
used a somewhat convoluted and racy way to perform call_rcu().

The old block of memory is freed after a grace period, but the rcu_head
used to track it is located in new block.

This can clash if we call two times or more netlink_change_ngroups(),
and a block is freed before another. call_rcu() called on different cpus
makes no guarantee in order of callbacks.

Fix this using a more standard way of handling this : Each block of
memory contains its own rcu_head, so that no 'use after free' can
happens.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Johannes Berg <johannes@sipsolutions.net>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 24 insertions, 41 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index cd96ed3ccee4..478181d53c55 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -83,9 +83,9 @@ struct netlink_sock {
         struct module           *module;
 };
 
-struct listeners_rcu_head {
-        struct rcu_head rcu_head;
-        void *ptr;
+struct listeners {
+        struct rcu_head         rcu;
+        unsigned long           masks[0];
 };
 
 #define NETLINK_KERNEL_SOCKET   0x1
@@ -119,7 +119,7 @@ struct nl_pid_hash {
 struct netlink_table {
         struct nl_pid_hash hash;
         struct hlist_head mc_list;
-        unsigned long *listeners;
+        struct listeners __rcu *listeners;
         unsigned int nl_nonroot;
         unsigned int groups;
         struct mutex *cb_mutex;
@@ -338,7 +338,7 @@ netlink_update_listeners(struct sock *sk)
                         if (i < NLGRPLONGS(nlk_sk(sk)->ngroups))
                                 mask |= nlk_sk(sk)->groups[i];
                 }
-                tbl->listeners[i] = mask;
+                tbl->listeners->masks[i] = mask;
         }
         /* this function is only called with the netlink table "grabbed", which
          * makes sure updates are visible before bind or setsockopt return. */
@@ -936,7 +936,7 @@ EXPORT_SYMBOL(netlink_unicast);
 int netlink_has_listeners(struct sock *sk, unsigned int group)
 {
         int res = 0;
-        unsigned long *listeners;
+        struct listeners *listeners;
 
         BUG_ON(!netlink_is_kernel(sk));
 
@@ -944,7 +944,7 @@ int netlink_has_listeners(struct sock *sk, unsigned int group)
         listeners = rcu_dereference(nl_table[sk->sk_protocol].listeners);
 
         if (group - 1 < nl_table[sk->sk_protocol].groups)
-                res = test_bit(group - 1, listeners);
+                res = test_bit(group - 1, listeners->masks);
 
         rcu_read_unlock();
 
@@ -1498,7 +1498,7 @@ netlink_kernel_create(struct net *net, int unit, unsigned int groups,
         struct socket *sock;
         struct sock *sk;
         struct netlink_sock *nlk;
-        unsigned long *listeners = NULL;
+        struct listeners *listeners = NULL;
 
         BUG_ON(!nl_table);
 
@@ -1523,8 +1523,7 @@ netlink_kernel_create(struct net *net, int unit, unsigned int groups,
         if (groups < 32)
                 groups = 32;
 
-        listeners = kzalloc(NLGRPSZ(groups) + sizeof(struct listeners_rcu_head),
-                            GFP_KERNEL);
+        listeners = kzalloc(sizeof(*listeners) + NLGRPSZ(groups), GFP_KERNEL);
         if (!listeners)
                 goto out_sock_release;
 
@@ -1541,7 +1540,7 @@ netlink_kernel_create(struct net *net, int unit, unsigned int groups,
         netlink_table_grab();
         if (!nl_table[unit].registered) {
                 nl_table[unit].groups = groups;
-                nl_table[unit].listeners = listeners;
+                rcu_assign_pointer(nl_table[unit].listeners, listeners);
                 nl_table[unit].cb_mutex = cb_mutex;
                 nl_table[unit].module = module;
                 nl_table[unit].registered = 1;
@@ -1572,43 +1571,28 @@ netlink_kernel_release(struct sock *sk)
 EXPORT_SYMBOL(netlink_kernel_release);
 
 
-static void netlink_free_old_listeners(struct rcu_head *rcu_head)
+static void listeners_free_rcu(struct rcu_head *head)
 {
-        struct listeners_rcu_head *lrh;
-
-        lrh = container_of(rcu_head, struct listeners_rcu_head, rcu_head);
-        kfree(lrh->ptr);
+        kfree(container_of(head, struct listeners, rcu));
 }
 
 int __netlink_change_ngroups(struct sock *sk, unsigned int groups)
 {
-        unsigned long *listeners, *old = NULL;
-        struct listeners_rcu_head *old_rcu_head;
+        struct listeners *new, *old;
         struct netlink_table *tbl = &nl_table[sk->sk_protocol];
 
         if (groups < 32)
                 groups = 32;
 
         if (NLGRPSZ(tbl->groups) < NLGRPSZ(groups)) {
-                listeners = kzalloc(NLGRPSZ(groups) +
-                                    sizeof(struct listeners_rcu_head),
-                                    GFP_ATOMIC);
-                if (!listeners)
+                new = kzalloc(sizeof(*new) + NLGRPSZ(groups), GFP_ATOMIC);
+                if (!new)
                         return -ENOMEM;
-                old = tbl->listeners;
-                memcpy(listeners, old, NLGRPSZ(tbl->groups));
-                rcu_assign_pointer(tbl->listeners, listeners);
-                /*
-                 * Free the old memory after an RCU grace period so we
-                 * don't leak it. We use call_rcu() here in order to be
-                 * able to call this function from atomic contexts. The
-                 * allocation of this memory will have reserved enough
-                 * space for struct listeners_rcu_head at the end.
-                 */
-                old_rcu_head = (void *)(tbl->listeners +
-                                        NLGRPLONGS(tbl->groups));
-                old_rcu_head->ptr = old;
-                call_rcu(&old_rcu_head->rcu_head, netlink_free_old_listeners);
+                old = rcu_dereference_raw(tbl->listeners);
+                memcpy(new->masks, old->masks, NLGRPSZ(tbl->groups));
+                rcu_assign_pointer(tbl->listeners, new);
+
+                call_rcu(&old->rcu, listeners_free_rcu);
         }
         tbl->groups = groups;
 
@@ -2104,18 +2088,17 @@ static void __net_exit netlink_net_exit(struct net *net)
 
 static void __init netlink_add_usersock_entry(void)
 {
-        unsigned long *listeners;
+        struct listeners *listeners;
         int groups = 32;
 
-        listeners = kzalloc(NLGRPSZ(groups) + sizeof(struct listeners_rcu_head),
-                            GFP_KERNEL);
+        listeners = kzalloc(sizeof(*listeners) + NLGRPSZ(groups), GFP_KERNEL);
         if (!listeners)
-                panic("netlink_add_usersock_entry: Cannot allocate listneres\n");
+                panic("netlink_add_usersock_entry: Cannot allocate listeners\n");
 
         netlink_table_grab();
 
         nl_table[NETLINK_USERSOCK].groups = groups;
-        nl_table[NETLINK_USERSOCK].listeners = listeners;
+        rcu_assign_pointer(nl_table[NETLINK_USERSOCK].listeners, listeners);
         nl_table[NETLINK_USERSOCK].module = THIS_MODULE;
         nl_table[NETLINK_USERSOCK].registered = 1;