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Date: Sun, 24 Oct 2010 21:14:56 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: David Miller <davem@...emloft.net>,
netdev <netdev@...r.kernel.org>,
Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH] netlink: fix netlink_change_ngroups()
On Sun, Oct 24, 2010 at 04:27:10PM +0200, Eric Dumazet wrote:
> 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.
Good catch, Eric!!! I believe this needs to be mentioned in the RCU
docs, will get it in there.
Thanx, Paul
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> CC: Johannes Berg <johannes@...solutions.net>
> CC: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
> net/netlink/af_netlink.c | 65 +++++++++++++------------------------
> 1 files changed, 24 insertions(+), 41 deletions(-)
>
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index cd96ed3..478181d 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;
>
>
>
--
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