[PATCH 1/1] sctp: Protect cached endpoints to prevent possible UAF

[Lee Jones]

The cause of the resultant dump_stack() reported below is a
dereference of a freed pointer to 'struct sctp_endpoint' in
sctp_sock_dump().

This race condition occurs when a transport is cached into its
associated hash table then freed prior to its subsequent use in
sctp_diag_dump() which uses sctp_for_each_transport() to walk the
(now out of date) hash table calling into sctp_sock_dump() where the
dereference occurs.

To prevent this from happening we need to take a reference on the
to-be-used/dereferenced 'struct sctp_endpoint' until such a time when
we know it can be safely released.

When KASAN is not enabled, a similar, but slightly different NULL
pointer derefernce crash occurs later along the thread of execution in
inet_sctp_diag_fill() this time.

  BUG: KASAN: use-after-free in sctp_sock_dump+0xa8/0x438 [sctp_diag]
  Call trace:
   dump_backtrace+0x0/0x2dc
   show_stack+0x20/0x2c
   dump_stack+0x120/0x144
   print_address_description+0x80/0x2f4
   __kasan_report+0x174/0x194
   kasan_report+0x10/0x18
   __asan_load8+0x84/0x8c
   sctp_sock_dump+0xa8/0x438 [sctp_diag]
   sctp_for_each_transport+0x1e0/0x26c [sctp]
   sctp_diag_dump+0x180/0x1f0 [sctp_diag]
   inet_diag_dump+0x12c/0x168
   netlink_dump+0x24c/0x5b8
   __netlink_dump_start+0x274/0x2a8
   inet_diag_handler_cmd+0x224/0x274
   sock_diag_rcv_msg+0x21c/0x230
   netlink_rcv_skb+0xe0/0x1bc
   sock_diag_rcv+0x34/0x48
   netlink_unicast+0x3b4/0x430
   netlink_sendmsg+0x4f0/0x574
   sock_write_iter+0x18c/0x1f0
   do_iter_readv_writev+0x230/0x2a8
   do_iter_write+0xc8/0x2b4
   vfs_writev+0xf8/0x184
   do_writev+0xb0/0x1a8
   __arm64_sys_writev+0x4c/0x5c
   el0_svc_common+0x118/0x250
   el0_svc_handler+0x3c/0x9c
   el0_svc+0x8/0xc

Cc: Vlad Yasevich <vyasevich@xxxxxxxxx>
Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: lksctp developers <linux-sctp@xxxxxxxxxxxxxxx>
Cc: "H.P. Yarroll" <piggy@xxxxxxx>
Cc: Karl Knutson <karl@xxxxxxxxxxxxxxxxxxxx>
Cc: Jon Grimm <jgrimm@xxxxxxxxxx>
Cc: Xingang Guo <xingang.guo@xxxxxxxxx>
Cc: Hui Huang <hui.huang@xxxxxxxxx>
Cc: Sridhar Samudrala <sri@xxxxxxxxxx>
Cc: Daisy Chang <daisyc@xxxxxxxxxx>
Cc: Ryan Layer <rmlayer@xxxxxxxxxx>
Cc: Kevin Gao <kevin.gao@xxxxxxxxx>
Cc: linux-sctp@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Lee Jones <lee.jones@xxxxxxxxxx>
---
 net/sctp/associola.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sctp/associola.c b/net/sctp/associola.c
index be29da09cc7ab..df171a297d095 100644
--- a/net/sctp/associola.c
+++ b/net/sctp/associola.c
@@ -499,8 +499,9 @@ void sctp_assoc_rm_peer(struct sctp_association *asoc,
 
 	/* Remove this peer from the list. */
 	list_del_rcu(&peer->transports);
-	/* Remove this peer from the transport hashtable */
+	/* Remove this peer from the transport hashtable and remove its reference */
 	sctp_unhash_transport(peer);
+	sctp_endpoint_put(asoc->ep);
 
 	/* Get the first transport of asoc. */
 	pos = asoc->peer.transport_addr_list.next;
@@ -710,11 +711,12 @@ struct sctp_transport *sctp_assoc_add_peer(struct sctp_association *asoc,
 	/* Set the peer's active state. */
 	peer->state = peer_state;
 
-	/* Add this peer into the transport hashtable */
+	/* Add this peer into the transport hashtable and take a reference */
 	if (sctp_hash_transport(peer)) {
 		sctp_transport_free(peer);
 		return NULL;
 	}
+	sctp_endpoint_hold(asoc->ep);
 
 	sctp_transport_pl_reset(peer);
 
-- 
2.34.1.400.ga245620fadb-goog