Subject: Re: [RFC PATCH 1/2] SCTP: Add RCU synchronization around sctp_localaddr_list

From: Sridhar Samudrala <[email-masked]>
Date: Tue, 11 Sep 2007 00:24:26 -0700

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 <[email-masked]>
>> ---
>>  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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