CVE-2017-15649: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') (CWE-362)
value: net/packet/af_packet.c in the Linux kernel before 4.13.6 allows local users to gain privileges via crafted system calls that trigger mishandling of packet_fanout data structures, because of a race condition (involving fanout_add and packet_do_bind) that leads to a use-after-free, a different vulnerability than CVE-2017-6346.
CWE 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.
CVSS v3 base score: 7.8
Published: 2017-10-19T22:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/packet/af_packet.c
Commit date: 2017-09-20 13:57:19 -0700
Commit message: packet: hold bind lock when rebinding to fanout hook

Packet socket bind operations must hold the po->bind_lock. This keeps
po->running consistent with whether the socket is actually on a ptype
list to receive packets.

fanout_add unbinds a socket and its packet_rcv/tpacket_rcv call, then
binds the fanout object to receive through packet_rcv_fanout.

Make it hold the po->bind_lock when testing po->running and rebinding.
Else, it can race with other rebind operations, such as that in
packet_set_ring from packet_rcv to tpacket_rcv. Concurrent updates
can result in a socket being added to a fanout group twice, causing
use-after-free KASAN bug reports, among others.

Reported independently by both trinity and syzkaller.
Verified that the syzkaller reproducer passes after this patch.

Fixes: dc99f600698d ("packet: Add fanout support.")
Reported-by: nixioaming <nixiaoming@huawei.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index c26172995511f7..d288f52c53f7c1 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1682,14 +1682,10 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		return -EINVAL;
 	}
 
 	mutex_lock(&fanout_mutex);
 
-	err = -EINVAL;
-	if (!po->running)
-		goto out;
-
 	err = -EALREADY;
 	if (po->fanout)
 		goto out;
 
 	if (type == PACKET_FANOUT_ROLLOVER ||
@@ -1747,11 +1743,14 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		match->prot_hook.af_packet_priv = match;
 		match->prot_hook.id_match = match_fanout_group;
 		list_add(&match->list, &fanout_list);
 	}
 	err = -EINVAL;
-	if (match->type == type &&
+
+	spin_lock(&po->bind_lock);
+	if (po->running &&
+	    match->type == type &&
 	    match->prot_hook.type == po->prot_hook.type &&
 	    match->prot_hook.dev == po->prot_hook.dev) {
 		err = -ENOSPC;
 		if (refcount_read(&match->sk_ref) < PACKET_FANOUT_MAX) {
 			__dev_remove_pack(&po->prot_hook);
@@ -1759,10 +1758,17 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 			refcount_set(&match->sk_ref, refcount_read(&match->sk_ref) + 1);
 			__fanout_link(sk, po);
 			err = 0;
 		}
 	}
+	spin_unlock(&po->bind_lock);
+
+	if (err && !refcount_read(&match->sk_ref)) {
+		list_del(&match->list);
+		kfree(match);
+	}
+
 out:
 	if (err && rollover) {
 		kfree(rollover);
 		po->rollover = NULL;
 	}

Vulnerable code: 	err = -EINVAL;
	if (!po->running)
		goto out;

	if (match->type == type &&
Fixed code: 
	spin_lock(&po->bind_lock);
	if (po->running &&
	    match->type == type &&
	spin_unlock(&po->bind_lock);

	if (err && !refcount_read(&match->sk_ref)) {
		list_del(&match->list);
		kfree(match);
	}