Re: [PATCH] SCTP: fix race between sctp_bind_addr_free() and sctp_bind_addr_conflict()

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 18 mai 2011 à 09:01 +0200, Jacek Luczak a écrit :
> During the sctp_close() call, we do not use rcu primitives to
> destroy the address list attached to the endpoint.  At the same
> time, we do the removal of addresses from this list before
> attempting to remove the socket from the port hash
> 
> As a result, it is possible for another process to find the socket
> in the port hash that is in the process of being closed.  It then
> proceeds to traverse the address list to find the conflict, only
> to have that address list suddenly disappear without rcu() critical
> section.
> 
> This can result in a kernel crash with general protection fault or
> kernel NULL pointer dereference.
> 
> Fix issue by closing address list removal inside RCU critical
> section.
> 
> Signed-off-by: Jacek Luczak <luczak.jacek@gmail.com>
> Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
> 
> ---
>  bind_addr.c |   12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sctp/bind_addr.c b/net/sctp/bind_addr.c
> index faf71d1..19d1329 100644
> --- a/net/sctp/bind_addr.c
> +++ b/net/sctp/bind_addr.c
> @@ -155,8 +155,16 @@ static void sctp_bind_addr_clean(struct sctp_bind_addr *bp)
>  /* Dispose of an SCTP_bind_addr structure  */
>  void sctp_bind_addr_free(struct sctp_bind_addr *bp)
>  {
> -       /* Empty the bind address list. */
> -       sctp_bind_addr_clean(bp);
> +       struct sctp_sockaddr_entry *addr;
> +
> +       /* Empty the bind address list inside RCU section. */
> +       rcu_read_lock();
> +       list_for_each_entry_rcu(addr, &bp->address_list, list) {
> +               list_del_rcu(&addr->list);
> +               call_rcu(&addr->rcu, sctp_local_addr_free);
> +               SCTP_DBG_OBJCNT_DEC(addr);
> +       }
> +       rcu_read_unlock();
> 

Sorry this looks odd.

If you're removing items from this list, you must be a writer here, with
exclusive access. So rcu_read_lock()/rcu_read_unlock() is not necessary.

Therefore, I guess following code is better :

list_for_each_entry(addr, &bp->address_list, list) {
	list_del_rcu(&addr->list);
	call_rcu(&addr->rcu, sctp_local_addr_free);
	SCTP_DBG_OBJCNT_DEC(addr);
}

Then, why dont you fix sctp_bind_addr_clean() instead ?

if 'struct sctp_sockaddr_entry' is recu protected, then all frees should
be protected as well.