[PATCH v2 2/2] sctp: hold 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 followed by an endpoint/sock migration to a new
association in sctp_assoc_migrate() prior to their subsequent use in
sctp_diag_dump() which uses sctp_for_each_transport() to walk the hash
table calling into sctp_sock_dump() where the dereference occurs.

  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

To prevent this from happening we need to take a reference to the
to-be-used/dereferenced 'struct sctp_endpoint' (which inherently
holds a reference to the problematic 'struct sock') until such a time
when we know they 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.
This time in inet_sctp_diag_fill().

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
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8f840e47f190c ("sctp: add the sctp_diag.c file")
Signed-off-by: Lee Jones <lee.jones@xxxxxxxxxx>
---
 net/sctp/diag.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/sctp/diag.c b/net/sctp/diag.c
index 760b367644c12..998488a56ce2b 100644
--- a/net/sctp/diag.c
+++ b/net/sctp/diag.c
@@ -292,15 +292,17 @@ static int sctp_tsp_dump_one(struct sctp_transport *tsp, void *p)
 
 static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
 {
-	struct sctp_endpoint *ep = tsp->asoc->ep;
+	struct sctp_endpoint *ep;
 	struct sctp_comm_param *commp = p;
-	struct sock *sk = ep->base.sk;
+	struct sock *sk;
 	struct sk_buff *skb = commp->skb;
 	struct netlink_callback *cb = commp->cb;
 	const struct inet_diag_req_v2 *r = commp->r;
 	struct sctp_association *assoc;
 	int err = 0;
 
+	ep = sctp_endpoint_hold(tsp->asoc->ep);
+	sk = ep->base.sk;
 	lock_sock(sk);
 	list_for_each_entry(assoc, &ep->asocs, asocs) {
 		if (cb->args[4] < cb->args[1])
@@ -341,6 +343,7 @@ static int sctp_sock_dump(struct sctp_transport *tsp, void *p)
 	cb->args[4] = 0;
 release:
 	release_sock(sk);
+	sctp_endpoint_put(ep);
 	return err;
 }
 
-- 
2.34.1.173.g76aa8bc2d0-goog