net: sctp: cache auth_enable per endpoint

Currently, it is possible to create an SCTP socket, then switch
auth_enable via sysctl setting to 1 and crash the system on connect:

Oops[#1]:
CPU: 0 PID: 0 Comm: swapper Not tainted 3.14.1-mipsgit-20140415 #1
task: ffffffff8056ce80 ti: ffffffff8055c000 task.ti: ffffffff8055c000
[...]
Call Trace:
[<ffffffff8043c4e8>] sctp_auth_asoc_set_default_hmac+0x68/0x80
[<ffffffff8042b300>] sctp_process_init+0x5e0/0x8a4
[<ffffffff8042188c>] sctp_sf_do_5_1B_init+0x234/0x34c
[<ffffffff804228c8>] sctp_do_sm+0xb4/0x1e8
[<ffffffff80425a08>] sctp_endpoint_bh_rcv+0x1c4/0x214
[<ffffffff8043af68>] sctp_rcv+0x588/0x630
[<ffffffff8043e8e8>] sctp6_rcv+0x10/0x24
[<ffffffff803acb50>] ip6_input+0x2c0/0x440
[<ffffffff8030fc00>] __netif_receive_skb_core+0x4a8/0x564
[<ffffffff80310650>] process_backlog+0xb4/0x18c
[<ffffffff80313cbc>] net_rx_action+0x12c/0x210
[<ffffffff80034254>] __do_softirq+0x17c/0x2ac
[<ffffffff800345e0>] irq_exit+0x54/0xb0
[<ffffffff800075a4>] ret_from_irq+0x0/0x4
[<ffffffff800090ec>] rm7k_wait_irqoff+0x24/0x48
[<ffffffff8005e388>] cpu_startup_entry+0xc0/0x148
[<ffffffff805a88b0>] start_kernel+0x37c/0x398
Code: dd0900b8  000330f8  0126302d <dcc60000> 50c0fff1  0047182a  a48306a0
03e00008  00000000
---[ end trace b530b0551467f2fd ]---
Kernel panic - not syncing: Fatal exception in interrupt

What happens while auth_enable=0 in that case is, that
ep->auth_hmacs is initialized to NULL in sctp_auth_init_hmacs()
when endpoint is being created.

After that point, if an admin switches over to auth_enable=1,
the machine can crash due to NULL pointer dereference during
reception of an INIT chunk. When we enter sctp_process_init()
via sctp_sf_do_5_1B_init() in order to respond to an INIT chunk,
the INIT verification succeeds and while we walk and process
all INIT params via sctp_process_param() we find that
net->sctp.auth_enable is set, therefore do not fall through,
but invoke sctp_auth_asoc_set_default_hmac() instead, and thus,
dereference what we have set to NULL during endpoint
initialization phase.

The fix is to make auth_enable immutable by caching its value
during endpoint initialization, so that its original value is
being carried along until destruction. The bug seems to originate
from the very first days.

Fix in joint work with Daniel Borkmann.

Reported-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Tested-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 11 deletions

diff --git a/net/sctp/auth.c b/net/sctp/auth.c
index 683c7d1..0e85291 100644
--- a/net/sctp/auth.c
+++ b/net/sctp/auth.c
@@ -386,14 +386,13 @@ nomem:
  */
 int sctp_auth_asoc_init_active_key(struct sctp_association *asoc, gfp_t gfp)
 {
-	struct net *net = sock_net(asoc->base.sk);
 	struct sctp_auth_bytes	*secret;
 	struct sctp_shared_key *ep_key;
 
 	/* If we don't support AUTH, or peer is not capable
 	 * we don't need to do anything.
 	 */
-	if (!net->sctp.auth_enable || !asoc->peer.auth_capable)
+	if (!asoc->ep->auth_enable || !asoc->peer.auth_capable)
 		return 0;
 
 	/* If the key_id is non-zero and we couldn't find an
@@ -440,16 +439,16 @@ struct sctp_shared_key *sctp_auth_get_shkey(
  */
 int sctp_auth_init_hmacs(struct sctp_endpoint *ep, gfp_t gfp)
 {
-	struct net *net = sock_net(ep->base.sk);
 	struct crypto_hash *tfm = NULL;
 	__u16   id;
 
-	/* if the transforms are already allocted, we are done */
-	if (!net->sctp.auth_enable) {
+	/* If AUTH extension is disabled, we are done */
+	if (!ep->auth_enable) {
 		ep->auth_hmacs = NULL;
 		return 0;
 	}
 
+	/* If the transforms are already allocated, we are done */
 	if (ep->auth_hmacs)
 		return 0;
 
@@ -665,12 +664,10 @@ static int __sctp_auth_cid(sctp_cid_t chunk, struct sctp_chunks_param *param)
 /* Check if peer requested that this chunk is authenticated */
 int sctp_auth_send_cid(sctp_cid_t chunk, const struct sctp_association *asoc)
 {
-	struct net  *net;
 	if (!asoc)
 		return 0;
 
-	net = sock_net(asoc->base.sk);
-	if (!net->sctp.auth_enable || !asoc->peer.auth_capable)
+	if (!asoc->ep->auth_enable || !asoc->peer.auth_capable)
 		return 0;
 
 	return __sctp_auth_cid(chunk, asoc->peer.peer_chunks);
@@ -679,12 +676,10 @@ int sctp_auth_send_cid(sctp_cid_t chunk, const struct sctp_association *asoc)
 /* Check if we requested that peer authenticate this chunk. */
 int sctp_auth_recv_cid(sctp_cid_t chunk, const struct sctp_association *asoc)
 {
-	struct net *net;
 	if (!asoc)
 		return 0;
 
-	net = sock_net(asoc->base.sk);
-	if (!net->sctp.auth_enable)
+	if (!asoc->ep->auth_enable)
 		return 0;
 
 	return __sctp_auth_cid(chunk,