[PATCH 2.6.32 04/42] RDS: fix race condition when sending a message on unbound socket

From: Willy Tarreau
Date: Sat Jan 23 2016 - 09:47:02 EST

2.6.32-longterm review patch. If anyone has any objections, please let me know.


From: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>

commit 8c7188b23474cca017b3ef354c4a58456f68303a upstream.

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@xxxxxxxxxxxxx>

Reviewed-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Reviewed-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
Acked-by: Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx>
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
net/rds/connection.c | 6 ------
net/rds/send.c | 4 +++-
2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/net/rds/connection.c b/net/rds/connection.c
index 8c3ddcd..cc8b568 100644
--- a/net/rds/connection.c
+++ b/net/rds/connection.c
@@ -187,12 +187,6 @@ static struct rds_connection *__rds_conn_create(__be32 laddr, __be32 faddr,

- 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);
diff --git a/net/rds/send.c b/net/rds/send.c
index 28c88ff..ad1a2d8 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -842,11 +842,13 @@ int rds_sendmsg(struct kiocb *iocb, struct socket *sock, struct msghdr *msg,

- /* 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);

rm = rds_message_copy_from_user(msg->msg_iov, payload_len);
if (IS_ERR(rm)) {