From: Aldo Ariel Panzardo <redacted>

rds_find_bound() looks up the destination socket using a global
rhashtable keyed solely on (addr, port, scope_id).  Network namespaces
are not part of the key, so a sender in netns A can deliver an incoming
message (inc) to a socket that lives in a different netns B.

When this happens, inc->i_conn points to an rds_connection whose c_net
is netns A, but the receiving rs lives in netns B.  Once the child
process that created netns A exits, cleanup_net() calls
rds_loop_exit_net() -> rds_loop_kill_conns() -> rds_conn_destroy(),
freeing that connection.  If the survivor socket in netns B still holds
the inc, any subsequent dereference of inc->i_conn is a use-after-free.

There are two dangerous sites in rds_clear_recv_queue():
  1. inc->i_conn->c_lcong (offset 88 of freed rds_connection, size 200)
     read via rds_recv_rcvbuf_delta() -- confirmed by KASAN.
  2. inc->i_conn->c_trans->inc_free(inc) (function pointer at offset 80)
     called via rds_inc_put() when the inc refcount reaches zero -- same
     race window, potential call-through-freed-object primitive.

The bug is reachable from unprivileged user namespaces
(CLONE_NEWUSER + CLONE_NEWNET), available since Linux 3.8.

Fix this by rejecting the delivery in rds_recv_incoming() when the
socket returned by rds_find_bound() belongs to a different network
namespace than the connection that carried the message.  Use the
existing rds_conn_net() / sock_net() helpers and net_eq() for the
comparison.

Fixes: c809195f5523 ("rds: clean up loopback rds_connections on netns deletion")
Signed-off-by: Aldo Ariel Panzardo <redacted>
Reviewed-by: Allison Henderson <achender@kernel.org>
Tested-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Allison Henderson <achender@kernel.org>
---
 net/rds/recv.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/net/rds/recv.c b/net/rds/recv.c
index 4b3f9e4a8bfd..cf3884d87931 100644
--- a/net/rds/recv.c
+++ b/net/rds/recv.c

@@ -399,6 +399,21 @@ void rds_recv_incoming(struct rds_connection *conn, struct in6_addr *saddr,
 		goto out;
 	}

+	/*
+	 * rds_find_bound() uses a global (netns-agnostic) hash table.
+	 * An RDS connection created in netns A can match a socket bound
+	 * in the init netns, delivering inc cross-netns with inc->i_conn
+	 * pointing into netns A.  When cleanup_net() then frees that conn,
+	 * any subsequent dereference of inc->i_conn is a use-after-free.
+	 * Drop the inc if the receiving socket lives in a different netns.
+	 */
+	if (!net_eq(sock_net(rds_rs_to_sk(rs)), rds_conn_net(conn))) {
+		rds_stats_inc(s_recv_drop_no_sock);
+		rds_sock_put(rs);
+		rs = NULL;
+		goto out;
+	}
+
 	/* Process extension headers */
 	rds_recv_incoming_exthdrs(inc, rs);

--

2.53.0