CVE-2015-8767 (published 2016-02-08T03:59Z)

net/sctp/sm_sideeffect.c in the Linux kernel before 4.3 does not properly manage the relationship between a lock and a socket, which allows local users to cause a denial of service (deadlock) via a crafted sctp_accept call.

Severity: MEDIUM
CVSS v2 base score: 4.9 (AV:L/AC:L/Au:N/C:N/I:N/A:C)
CVSS v3 base score: 6.2, MEDIUM (CVSS:3.0/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)

Weakness information:
cwe_id: CWE-362
cwe_name: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
extended_description: 
            This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider.
            A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc.
            A race condition violates these properties, which are closely related:
               
                  Exclusivity - the code sequence is given exclusive access to the shared resource, i.e., no other code sequence can modify properties of the shared resource before the original sequence has completed execution.
                  Atomicity - the code sequence is behaviorally atomic, i.e., no other thread or process can concurrently execute the same sequence of instructions (or a subset) against the same resource.
               
            A race condition exists when an "interfering code sequence" can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single "x++" statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x).
            The interfering code sequence could be "trusted" or "untrusted." A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.
         
         

Fix information:
repo_url: https://github.com/torvalds/linux
commit_details: repo_url: https://github.com/torvalds/linux
author: Karl Heiss
author_date: 2015-09-24 12:15:07-04:00
msg: sctp: Prevent soft lockup when sctp_accept() is called during a timeout event

A case can occur when sctp_accept() is called by the user during
a heartbeat timeout event after the 4-way handshake.  Since
sctp_assoc_migrate() changes both assoc->base.sk and assoc->ep, the
bh_sock_lock in sctp_generate_heartbeat_event() will be taken with
the listening socket but released with the new association socket.
The result is a deadlock on any future attempts to take the listening
socket lock.

Note that this race can occur with other SCTP timeouts that take
the bh_lock_sock() in the event sctp_accept() is called.

 BUG: soft lockup - CPU#9 stuck for 67s! [swapper:0]
 ...
 RIP: 0010:[<ffffffff8152d48e>]  [<ffffffff8152d48e>] _spin_lock+0x1e/0x30
 RSP: 0018:ffff880028323b20  EFLAGS: 00000206
 RAX: 0000000000000002 RBX: ffff880028323b20 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: ffff880028323be0 RDI: ffff8804632c4b48
 RBP: ffffffff8100bb93 R08: 0000000000000000 R09: 0000000000000000
 R10: ffff880610662280 R11: 0000000000000100 R12: ffff880028323aa0
 R13: ffff8804383c3880 R14: ffff880028323a90 R15: ffffffff81534225
 FS:  0000000000000000(0000) GS:ffff880028320000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
 CR2: 00000000006df528 CR3: 0000000001a85000 CR4: 00000000000006e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
 Process swapper (pid: 0, threadinfo ffff880616b70000, task ffff880616b6cab0)
 Stack:
 ffff880028323c40 ffffffffa01c2582 ffff880614cfb020 0000000000000000
 <d> 0100000000000000 00000014383a6c44 ffff8804383c3880 ffff880614e93c00
 <d> ffff880614e93c00 0000000000000000 ffff8804632c4b00 ffff8804383c38b8
 Call Trace:
 <IRQ>
 [<ffffffffa01c2582>] ? sctp_rcv+0x492/0xa10 [sctp]
 [<ffffffff8148c559>] ? nf_iterate+0x69/0xb0
 [<ffffffff814974a0>] ? ip_local_deliver_finish+0x0/0x2d0
 [<ffffffff8148c716>] ? nf_hook_slow+0x76/0x120
 [<ffffffff814974a0>] ? ip_local_deliver_finish+0x0/0x2d0
 [<ffffffff8149757d>] ? ip_local_deliver_finish+0xdd/0x2d0
 [<ffffffff81497808>] ? ip_local_deliver+0x98/0xa0
 [<ffffffff81496ccd>] ? ip_rcv_finish+0x12d/0x440
 [<ffffffff81497255>] ? ip_rcv+0x275/0x350
 [<ffffffff8145cfeb>] ? __netif_receive_skb+0x4ab/0x750
 ...

With lockdep debugging:

 =====================================
 [ BUG: bad unlock balance detected! ]
 -------------------------------------
 CslRx/12087 is trying to release lock (slock-AF_INET) at:
 [<ffffffffa01bcae0>] sctp_generate_timeout_event+0x40/0xe0 [sctp]
 but there are no more locks to release!

 other info that might help us debug this:
 2 locks held by CslRx/12087:
 #0:  (&asoc->timers[i]){+.-...}, at: [<ffffffff8108ce1f>] run_timer_softirq+0x16f/0x3e0
 #1:  (slock-AF_INET){+.-...}, at: [<ffffffffa01bcac3>] sctp_generate_timeout_event+0x23/0xe0 [sctp]

Ensure the socket taken is also the same one that is released by
saving a copy of the socket before entering the timeout event
critical section.

Signed-off-by: Karl Heiss <kheiss@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
repository: repo_url: https://github.com/torvalds/linux
repo_name: torvalds/linux
description: Linux kernel source tree
repo_language: C
owner: torvalds
file_changes: filename: sm_sideeffect.c
change_type: ModificationType.MODIFY
diff: @@ -244,12 +244,13 @@ void sctp_generate_t3_rtx_event(unsigned long peer)
 	int error;
 	struct sctp_transport *transport = (struct sctp_transport *) peer;
 	struct sctp_association *asoc = transport->asoc;
-	struct net *net = sock_net(asoc->base.sk);
+	struct sock *sk = asoc->base.sk;
+	struct net *net = sock_net(sk);
 
 	/* Check whether a task is in the sock.  */
 
-	bh_lock_sock(asoc->base.sk);
-	if (sock_owned_by_user(asoc->base.sk)) {
+	bh_lock_sock(sk);
+	if (sock_owned_by_user(sk)) {
 		pr_debug("%s: sock is busy\n", __func__);
 
 		/* Try again later.  */
@@ -272,10 +273,10 @@ void sctp_generate_t3_rtx_event(unsigned long peer)
 			   transport, GFP_ATOMIC);
 
 	if (error)
-		asoc->base.sk->sk_err = -error;
+		sk->sk_err = -error;
 
 out_unlock:
-	bh_unlock_sock(asoc->base.sk);
+	bh_unlock_sock(sk);
 	sctp_transport_put(transport);
 }
 
@@ -285,11 +286,12 @@ void sctp_generate_t3_rtx_event(unsigned long peer)
 static void sctp_generate_timeout_event(struct sctp_association *asoc,
 					sctp_event_timeout_t timeout_type)
 {
-	struct net *net = sock_net(asoc->base.sk);
+	struct sock *sk = asoc->base.sk;
+	struct net *net = sock_net(sk);
 	int error = 0;
 
-	bh_lock_sock(asoc->base.sk);
-	if (sock_owned_by_user(asoc->base.sk)) {
+	bh_lock_sock(sk);
+	if (sock_owned_by_user(sk)) {
 		pr_debug("%s: sock is busy: timer %d\n", __func__,
 			 timeout_type);
 
@@ -312,10 +314,10 @@ static void sctp_generate_timeout_event(struct sctp_association *asoc,
 			   (void *)timeout_type, GFP_ATOMIC);
 
 	if (error)
-		asoc->base.sk->sk_err = -error;
+		sk->sk_err = -error;
 
 out_unlock:
-	bh_unlock_sock(asoc->base.sk);
+	bh_unlock_sock(sk);
 	sctp_association_put(asoc);
 }
 
@@ -365,10 +367,11 @@ void sctp_generate_heartbeat_event(unsigned long data)
 	int error = 0;
 	struct sctp_transport *transport = (struct sctp_transport *) data;
 	struct sctp_association *asoc = transport->asoc;
-	struct net *net = sock_net(asoc->base.sk);
+	struct sock *sk = asoc->base.sk;
+	struct net *net = sock_net(sk);
 
-	bh_lock_sock(asoc->base.sk);
-	if (sock_owned_by_user(asoc->base.sk)) {
+	bh_lock_sock(sk);
+	if (sock_owned_by_user(sk)) {
 		pr_debug("%s: sock is busy\n", __func__);
 
 		/* Try again later.  */
@@ -389,10 +392,10 @@ void sctp_generate_heartbeat_event(unsigned long data)
 			   transport, GFP_ATOMIC);
 
 	if (error)
-		asoc->base.sk->sk_err = -error;
+		sk->sk_err = -error;
 
 out_unlock:
-	bh_unlock_sock(asoc->base.sk);
+	bh_unlock_sock(sk);
 	sctp_transport_put(transport);
 }
 
@@ -403,10 +406,11 @@ void sctp_generate_proto_unreach_event(unsigned long data)
 {
 	struct sctp_transport *transport = (struct sctp_transport *) data;
 	struct sctp_association *asoc = transport->asoc;
-	struct net *net = sock_net(asoc->base.sk);
+	struct sock *sk = asoc->base.sk;
+	struct net *net = sock_net(sk);
 
-	bh_lock_sock(asoc->base.sk);
-	if (sock_owned_by_user(asoc->base.sk)) {
+	bh_lock_sock(sk);
+	if (sock_owned_by_user(sk)) {
 		pr_debug("%s: sock is busy\n", __func__);
 
 		/* Try again later.  */
@@ -427,7 +431,7 @@ void sctp_generate_proto_unreach_event(unsigned long data)
 		   asoc->state, asoc->ep, asoc, transport, GFP_ATOMIC);
 
 out_unlock:
-	bh_unlock_sock(asoc->base.sk);
+	bh_unlock_sock(sk);
 	sctp_association_put(asoc);
 }
 

programming_language: C