RDS: fix race condition when sending a message on unbound socket

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.

I cannot reproduce the NULL pointer dereference using Vegard's reproducer
with this patch, whereas I could without.

Complete earlier incomplete fix to CVE-2015-6937:

  74e98eb08588 ("RDS: verify the underlying transport exists before creating a connection")

Cc: David S. Miller <davem@davemloft.net>
Cc: stable@vger.kernel.org
Change-Id: I697622374cf9a4b8d805fb5a58987cfe8646afed
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Junghoon Kim <jhoon20.kim@samsung.com>

diff --git a/net/rds/send.c b/net/rds/send.c
index 88eace5..31c9fa4 100644 (file)
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -955,11 +955,13 @@ int rds_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,
                release_sock(sk);
        }
 
-       /* 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);
 
        /* size of rm including all sgs */
        ret = rds_rm_size(msg, payload_len);