From: Santosh Shilimkar
> Sent: 24 November 2015 22:13
...
> Sasha's found a NULL pointer dereference in the RDS connection code when
> sending a message to an apparently unbound socket. The problem is caused
> by the code checking if the socket is bound in rds_sendmsg(), which checks
> the rs_bound_addr field without taking a lock on the socket. This opens a
> race where rs_bound_addr is temporarily set but where the transport is not
> in rds_bind(), leading to a NULL pointer dereference when trying to
> dereference 'trans' in __rds_conn_create().
>
> Vegard wrote a reproducer for this issue, so kindly ask him to share if
> you're interested.
...
> diff --git a/net/rds/send.c b/net/rds/send.c
> index 827155c..c9cdb35 100644
> --- a/net/rds/send.c
> +++ b/net/rds/send.c
> @@ -1013,11 +1013,13 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
> release_sock(sk);
This is falling though into an unconditional lock_sock().
No need to unlock and relock immediately.
> }
>
> - /* racing with another thread binding seems ok here */
> + lock_sock(sk);
> if (daddr == 0 || rs->rs_bound_addr == 0) {
> + release_sock(sk);
> ret = -ENOTCONN; /* XXX not a great errno */
> goto out;
> }
> + release_sock(sk);
>
On the face of it the above looks somewhat dubious.
Locks usually tie together two action (eg a test and use of a value),
In this case you only have a test inside the lock.
That either means that the state can change after you release the lock
(ie rs->rs_bound_addr = 0 is executed somewhere), or you don't
really need the lock.
David
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