CVE-2015-7990: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') (CWE-362)
value: Race condition in the rds_sendmsg function in net/rds/sendmsg.c in the Linux kernel before 4.3.3 allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact by using a socket that was not properly bound.  NOTE: this vulnerability exists because of an incomplete fix for CVE-2015-6937.
CWE description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
CVSS v3 base score: 5.8
Published: 2015-12-28T11:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/rds/connection.c, net/rds/send.c
Commit date: 2015-11-24 17:20:09 -0500
Commit message: 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

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>

Diff: diff --git a/net/rds/connection.c b/net/rds/connection.c
index d4564036a339a0..e3b118cae81d5e 100644
--- a/net/rds/connection.c
+++ b/net/rds/connection.c
@@ -184,16 +184,10 @@ static struct rds_connection *__rds_conn_create(struct net *net,
 			 */
 			trans = &rds_loop_transport;
 		}
 	}
 
-	if (trans == NULL) {
-		kmem_cache_free(rds_conn_slab, conn);
-		conn = ERR_PTR(-ENODEV);
-		goto out;
-	}
-
 	conn->c_trans = trans;
 
 	ret = trans->conn_alloc(conn, gfp);
 	if (ret) {
 		kmem_cache_free(rds_conn_slab, conn);
diff --git a/net/rds/send.c b/net/rds/send.c
index 827155c2ead103..c9cdb358ea885e 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -1011,15 +1011,17 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
 		daddr = rs->rs_conn_addr;
 		dport = rs->rs_conn_port;
 		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);
 
 	if (payload_len > rds_sk_sndbuf(rs)) {
 		ret = -EMSGSIZE;
 		goto out;
 	}

Vulnerable code: 	if (trans == NULL) {
		kmem_cache_free(rds_conn_slab, conn);
		conn = ERR_PTR(-ENODEV);
		goto out;
	}

	/* racing with another thread binding seems ok here */
Fixed code: 	lock_sock(sk);
		release_sock(sk);
	release_sock(sk);