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Message-ID: <46E642AA.7090703@us.ibm.com>
Date: Tue, 11 Sep 2007 00:24:26 -0700
From: Sridhar Samudrala <sri@...ibm.com>
To: paulmck@...ux.vnet.ibm.com
CC: Vlad Yasevich <vladislav.yasevich@...com>, netdev@...r.kernel.org,
lksctp-developers@...ts.sourceforge.net
Subject: Re: [RFC PATCH 1/2] SCTP: Add RCU synchronization around sctp_localaddr_list
Paul E. McKenney wrote:
> On Mon, Sep 10, 2007 at 03:46:29PM -0400, Vlad Yasevich wrote:
>> sctp_localaddr_list is modified dynamically via NETDEV_UP
>> and NETDEV_DOWN events, but there is not synchronization
>> between writer (even handler) and readers. As a result,
>> the readers can access an entry that has been freed and
>> crash the sytem.
>
> Good start, but few questions interspersed below...
>
> Thanx, Paul
>
>> Signed-off-by: Vlad Yasevich <vladislav.yasevich@...com>
>> ---
>> include/net/sctp/sctp.h | 1 +
>> include/net/sctp/structs.h | 2 +
>> net/sctp/bind_addr.c | 2 +
>> net/sctp/ipv6.c | 33 ++++++++++++++++++++--------
>> net/sctp/protocol.c | 50 ++++++++++++++++++++++++++++++-------------
>> net/sctp/socket.c | 38 ++++++++++++++++++++++-----------
>> 6 files changed, 88 insertions(+), 38 deletions(-)
>>
>> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
>> index d529045..c9cc00c 100644
>> --- a/include/net/sctp/sctp.h
>> +++ b/include/net/sctp/sctp.h
>> @@ -123,6 +123,7 @@
>> * sctp/protocol.c
>> */
>> extern struct sock *sctp_get_ctl_sock(void);
>> +extern void sctp_local_addr_free(struct rcu_head *head);
>> extern int sctp_copy_local_addr_list(struct sctp_bind_addr *,
>> sctp_scope_t, gfp_t gfp,
>> int flags);
>> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
>> index c0d5848..2591c49 100644
>> --- a/include/net/sctp/structs.h
>> +++ b/include/net/sctp/structs.h
>> @@ -737,8 +737,10 @@ const union sctp_addr *sctp_source(const struct sctp_chunk *chunk);
>> /* This is a structure for holding either an IPv6 or an IPv4 address. */
>> struct sctp_sockaddr_entry {
>> struct list_head list;
>> + struct rcu_head rcu;
>> union sctp_addr a;
>> __u8 use_as_src;
>> + __u8 valid;
>> };
>>
>> typedef struct sctp_chunk *(sctp_packet_phandler_t)(struct sctp_association *);
>> diff --git a/net/sctp/bind_addr.c b/net/sctp/bind_addr.c
>> index fdb287a..7fc369f 100644
>> --- a/net/sctp/bind_addr.c
>> +++ b/net/sctp/bind_addr.c
>> @@ -163,8 +163,10 @@ int sctp_add_bind_addr(struct sctp_bind_addr *bp, union sctp_addr *new,
>> addr->a.v4.sin_port = htons(bp->port);
>>
>> addr->use_as_src = use_as_src;
>> + addr->valid = 1;
>>
>> INIT_LIST_HEAD(&addr->list);
>> + INIT_RCU_HEAD(&addr->rcu);
>> list_add_tail(&addr->list, &bp->address_list);
>> SCTP_DBG_OBJCNT_INC(addr);
>>
>> diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
>> index f8aa23d..fc2e4e2 100644
>> --- a/net/sctp/ipv6.c
>> +++ b/net/sctp/ipv6.c
>> @@ -77,13 +77,18 @@
>>
>> #include <asm/uaccess.h>
>>
>> -/* Event handler for inet6 address addition/deletion events. */
>> +/* Event handler for inet6 address addition/deletion events.
>> + * This even is part of the atomic notifier call chain
>> + * and thus happens atomically and can NOT sleep. As a result
>> + * we can't and really don't need to add any locks to guard the
>> + * RCU.
>> + */
>> static int sctp_inet6addr_event(struct notifier_block *this, unsigned long ev,
>> void *ptr)
>> {
>> struct inet6_ifaddr *ifa = (struct inet6_ifaddr *)ptr;
>> - struct sctp_sockaddr_entry *addr;
>> - struct list_head *pos, *temp;
>> + struct sctp_sockaddr_entry *addr = NULL;
>> + struct sctp_sockaddr_entry *temp;
>>
>> switch (ev) {
>> case NETDEV_UP:
>> @@ -94,19 +99,26 @@ static int sctp_inet6addr_event(struct notifier_block *this, unsigned long ev,
>> memcpy(&addr->a.v6.sin6_addr, &ifa->addr,
>> sizeof(struct in6_addr));
>> addr->a.v6.sin6_scope_id = ifa->idev->dev->ifindex;
>> - list_add_tail(&addr->list, &sctp_local_addr_list);
>> + addr->valid = 1;
>> + rcu_read_lock();
>> + list_add_tail_rcu(&addr->list, &sctp_local_addr_list);
>> + rcu_read_unlock();
>
> If we are under the protection of the update-side mutex, the rcu_read_lock()
> and rcu_read_unlock() are (harmlessly) redundant. If we are not under
> the protection of some mutex, what prevents concurrent list_add_tail_rcu()
> calls from messing up the sctp_sockaddr_entry list?
This is an atomic notifier call chain event and as such can be called from a
softirq. So i think we need a spin_lock_bh here.
>
>> }
>> break;
>> case NETDEV_DOWN:
>> - list_for_each_safe(pos, temp, &sctp_local_addr_list) {
>> - addr = list_entry(pos, struct sctp_sockaddr_entry, list);
>> - if (ipv6_addr_equal(&addr->a.v6.sin6_addr, &ifa->addr)) {
>> - list_del(pos);
>> - kfree(addr);
>> + rcu_read_lock();
>> + list_for_each_entry_safe(addr, temp,
>> + &sctp_local_addr_list, list) {
>> + if (ipv6_addr_equal(&addr->a.v6.sin6_addr,
>> + &ifa->addr)) {
>> + addr->valid = 0;
>> + list_del_rcu(&addr->list);
>> break;
>> }
>> }
>> -
>> + rcu_read_unlock();
>> + if (addr && !addr->valid)
>> + call_rcu(&addr->rcu, sctp_local_addr_free);
>
> Are we under the protection of the update-side lock here? If not,
> what prevents two different tasks from executing this in parallel,
> potentially tangling both the list that the sctp_sockaddr_entry list and
> the internal RCU lists? (It is forbidden to call_rcu() a given element
> twice concurrently.)
>
> If we are in fact under the protection of the update-side lock, the
> rcu_read_lock() and rcu_read_unlock() pairs are redundant (though this
> is harmless, aside from the (small) potential for confusion).
There is no update-side lock protection here. We need a spin_lock_bh().
>
>> break;
>> }
>>
>> @@ -368,6 +380,7 @@ static void sctp_v6_copy_addrlist(struct list_head *addrlist,
>> addr->a.v6.sin6_addr = ifp->addr;
>> addr->a.v6.sin6_scope_id = dev->ifindex;
>> INIT_LIST_HEAD(&addr->list);
>> + INIT_RCU_HEAD(&addr->rcu);
>> list_add_tail(&addr->list, addrlist);
>> }
>> }
>> diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
>> index e98579b..ac52f9e 100644
>> --- a/net/sctp/protocol.c
>> +++ b/net/sctp/protocol.c
>> @@ -153,6 +153,7 @@ static void sctp_v4_copy_addrlist(struct list_head *addrlist,
>> addr->a.v4.sin_family = AF_INET;
>> addr->a.v4.sin_port = 0;
>> addr->a.v4.sin_addr.s_addr = ifa->ifa_local;
>> + INIT_RCU_HEAD(&addr->rcu);
>> list_add_tail(&addr->list, addrlist);
>> }
>> }
>> @@ -192,16 +193,24 @@ static void sctp_free_local_addr_list(void)
>> }
>> }
>>
>> +void sctp_local_addr_free(struct rcu_head *head)
>> +{
>> + struct sctp_sockaddr_entry *e = container_of(head,
>> + struct sctp_sockaddr_entry, rcu);
>> + kfree(e);
>> +}
>> +
>> /* Copy the local addresses which are valid for 'scope' into 'bp'. */
>> int sctp_copy_local_addr_list(struct sctp_bind_addr *bp, sctp_scope_t scope,
>> gfp_t gfp, int copy_flags)
>> {
>> struct sctp_sockaddr_entry *addr;
>> int error = 0;
>> - struct list_head *pos, *temp;
>>
>> - list_for_each_safe(pos, temp, &sctp_local_addr_list) {
>> - addr = list_entry(pos, struct sctp_sockaddr_entry, list);
>> + rcu_read_lock();
>> + list_for_each_entry_rcu(addr, &sctp_local_addr_list, list) {
>> + if (!addr->valid)
>> + continue;
>
> What happens if the update-side code removes the element from the list
> and marks it !->valid right here?
>
> If this turns out to be harmless, why not just dispense with the ->valid
> flag entirely?
It should be OK if an address gets removed from the list. So i agree that
->valid flag is not really useful.
>
>> if (sctp_in_scope(&addr->a, scope)) {
>> /* Now that the address is in scope, check to see if
>> * the address type is really supported by the local
>> @@ -221,6 +230,7 @@ int sctp_copy_local_addr_list(struct sctp_bind_addr *bp, sctp_scope_t scope,
>> }
>>
>> end_copy:
>> + rcu_read_unlock();
>> return error;
>> }
>>
>> @@ -600,13 +610,17 @@ static void sctp_v4_seq_dump_addr(struct seq_file *seq, union sctp_addr *addr)
>> seq_printf(seq, "%d.%d.%d.%d ", NIPQUAD(addr->v4.sin_addr));
>> }
>>
>> -/* Event handler for inet address addition/deletion events. */
>> +/* Event handler for inet address addition/deletion events.
>> + * This is part of the blocking notifier call chain that is
>> + * guarted by a mutex. As a result we don't need to add
>> + * any additional guards for the RCU
>> + */
>> static int sctp_inetaddr_event(struct notifier_block *this, unsigned long ev,
>> void *ptr)
>> {
>> struct in_ifaddr *ifa = (struct in_ifaddr *)ptr;
>> - struct sctp_sockaddr_entry *addr;
>> - struct list_head *pos, *temp;
>> + struct sctp_sockaddr_entry *addr = NULL;
>> + struct sctp_sockaddr_entry *temp;
>>
>> switch (ev) {
>> case NETDEV_UP:
>> @@ -615,19 +629,25 @@ static int sctp_inetaddr_event(struct notifier_block *this, unsigned long ev,
>> addr->a.v4.sin_family = AF_INET;
>> addr->a.v4.sin_port = 0;
>> addr->a.v4.sin_addr.s_addr = ifa->ifa_local;
>> - list_add_tail(&addr->list, &sctp_local_addr_list);
>> + addr->valid = 1;
>> + rcu_read_lock();
>> + list_add_tail_rcu(&addr->list, &sctp_local_addr_list);
>> + rcu_read_unlock();
>
> Based on the additions to the header comment, I am assuming that we
> hold an update-side mutex. This means that the rcu_read_lock() and
> rcu_read_unlock() are (harmlessly) redundant.
This is called via a blocking notifier call chain and hence we could
protect using an update-side mutex. But considering that sctp_inet6addr_event
requires a spin_lock_bh(), may be we should use it here also to make it
simple.
>
>> }
>> break;
>> case NETDEV_DOWN:
>> - list_for_each_safe(pos, temp, &sctp_local_addr_list) {
>> - addr = list_entry(pos, struct sctp_sockaddr_entry, list);
>> + rcu_read_lock();
>> + list_for_each_entry_safe(addr, temp,
>> + &sctp_local_addr_list, list) {
>> if (addr->a.v4.sin_addr.s_addr == ifa->ifa_local) {
>> - list_del(pos);
>> - kfree(addr);
>> + addr->valid = 0;
>> + list_del_rcu(&addr->list);
>> break;
>> }
>> }
>> -
>> + rcu_read_unlock();
>
> Ditto.
>
>> + if (addr && !addr->valid)
>> + call_rcu(&addr->rcu, sctp_local_addr_free);
>
> This one is OK, since we hold the update-side mutex.
>
>> break;
>> }
>>
>> @@ -1227,6 +1247,9 @@ SCTP_STATIC __exit void sctp_exit(void)
>> sctp_v6_del_protocol();
>> inet_del_protocol(&sctp_protocol, IPPROTO_SCTP);
>>
>> + /* Unregister notifier for inet address additions/deletions. */
>> + unregister_inetaddr_notifier(&sctp_inetaddr_notifier);
>> +
>> /* Free the local address list. */
>> sctp_free_local_addr_list();
>>
>> @@ -1240,9 +1263,6 @@ SCTP_STATIC __exit void sctp_exit(void)
>> inet_unregister_protosw(&sctp_stream_protosw);
>> inet_unregister_protosw(&sctp_seqpacket_protosw);
>>
>> - /* Unregister notifier for inet address additions/deletions. */
>> - unregister_inetaddr_notifier(&sctp_inetaddr_notifier);
>> -
>> sctp_sysctl_unregister();
>> list_del(&sctp_ipv4_specific.list);
>>
>> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
>> index 3335460..a3acf78 100644
>> --- a/net/sctp/socket.c
>> +++ b/net/sctp/socket.c
>> @@ -4057,9 +4057,9 @@ static int sctp_getsockopt_local_addrs_num_old(struct sock *sk, int len,
>> int __user *optlen)
>> {
>> sctp_assoc_t id;
>> + struct list_head *pos;
>> struct sctp_bind_addr *bp;
>> struct sctp_association *asoc;
>> - struct list_head *pos, *temp;
>> struct sctp_sockaddr_entry *addr;
>> rwlock_t *addr_lock;
>> int cnt = 0;
>> @@ -4096,15 +4096,19 @@ static int sctp_getsockopt_local_addrs_num_old(struct sock *sk, int len,
>> addr = list_entry(bp->address_list.next,
>> struct sctp_sockaddr_entry, list);
>> if (sctp_is_any(&addr->a)) {
>> - list_for_each_safe(pos, temp, &sctp_local_addr_list) {
>> - addr = list_entry(pos,
>> - struct sctp_sockaddr_entry,
>> - list);
>> + rcu_read_lock();
>> + list_for_each_entry_rcu(addr,
>> + &sctp_local_addr_list, list) {
>> + if (!addr->valid)
>> + continue;
>> +
>
> Again, what happens if the element is deleted just at this point?
> If harmless, might be good to get rid of ->valid.
>
>> if ((PF_INET == sk->sk_family) &&
>> (AF_INET6 == addr->a.sa.sa_family))
>> continue;
>> +
>> cnt++;
>> }
>> + rcu_read_unlock();
>
> We are just counting these things, right? If on the other hand we are
> keeping a reference outside of rcu_read_lock() protection, then there
> needs to be some explicit mechanism preventing the corresponding entry
> from being freed.
>
>> } else {
>> cnt = 1;
>> }
>> @@ -4127,14 +4131,16 @@ static int sctp_copy_laddrs_old(struct sock *sk, __u16 port,
>> int max_addrs, void *to,
>> int *bytes_copied)
>> {
>> - struct list_head *pos, *next;
>> struct sctp_sockaddr_entry *addr;
>> union sctp_addr temp;
>> int cnt = 0;
>> int addrlen;
>>
>> - list_for_each_safe(pos, next, &sctp_local_addr_list) {
>> - addr = list_entry(pos, struct sctp_sockaddr_entry, list);
>> + rcu_read_lock();
>> + list_for_each_entry_rcu(addr, &sctp_local_addr_list, list) {
>> + if (!addr->valid)
>> + continue;
>> +
>
> Same question as before -- what happens if the element is deleted by some
> other CPU (thus clearing ->valid) just at this point?
>
>> if ((PF_INET == sk->sk_family) &&
>> (AF_INET6 == addr->a.sa.sa_family))
>> continue;
>> @@ -4149,6 +4155,7 @@ static int sctp_copy_laddrs_old(struct sock *sk, __u16 port,
>> cnt ++;
>> if (cnt >= max_addrs) break;
>> }
>> + rcu_read_unlock();
>>
>> return cnt;
>> }
>> @@ -4156,14 +4163,16 @@ static int sctp_copy_laddrs_old(struct sock *sk, __u16 port,
>> static int sctp_copy_laddrs(struct sock *sk, __u16 port, void *to,
>> size_t space_left, int *bytes_copied)
>> {
>> - struct list_head *pos, *next;
>> struct sctp_sockaddr_entry *addr;
>> union sctp_addr temp;
>> int cnt = 0;
>> int addrlen;
>>
>> - list_for_each_safe(pos, next, &sctp_local_addr_list) {
>> - addr = list_entry(pos, struct sctp_sockaddr_entry, list);
>> + rcu_read_lock();
>> + list_for_each_entry_rcu(addr, &sctp_local_addr_list, list) {
>> + if (!addr->valid)
>> + continue;
>> +
>
> And the same question here as well...
>
>> if ((PF_INET == sk->sk_family) &&
>> (AF_INET6 == addr->a.sa.sa_family))
>> continue;
>> @@ -4171,8 +4180,10 @@ static int sctp_copy_laddrs(struct sock *sk, __u16 port, void *to,
>> sctp_get_pf_specific(sk->sk_family)->addr_v4map(sctp_sk(sk),
>> &temp);
>> addrlen = sctp_get_af_specific(temp.sa.sa_family)->sockaddr_len;
>> - if (space_left < addrlen)
>> - return -ENOMEM;
>> + if (space_left < addrlen) {
>> + cnt = -ENOMEM;
>> + break;
>> + }
>> memcpy(to, &temp, addrlen);
>>
>> to += addrlen;
>> @@ -4180,6 +4191,7 @@ static int sctp_copy_laddrs(struct sock *sk, __u16 port, void *to,
>> space_left -= addrlen;
>> *bytes_copied += addrlen;
>> }
>> + rcu_read_unlock();
>>
>> return cnt;
>> }
>> --
>> 1.5.2.4
>>
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