[Trusty SRU] Fix CVE-2017-6346

[Stefan Bader]

>From 667526d7ccd320b438b81136af53fe2ab0f11013 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at google.com>
Date: Tue, 14 Feb 2017 09:03:51 -0800
Subject: [PATCH] packet: fix races in fanout_add()

Multiple threads can call fanout_add() at the same time.

We need to grab fanout_mutex earlier to avoid races that could
lead to one thread freeing po->rollover that was set by another thread.

Do the same in fanout_release(), for peace of mind, and to help us
finding lockdep issues earlier.

Fixes: dc99f600698d ("packet: Add fanout support.")
Fixes: 0648ab70afe6 ("packet: rollover prepare: per-socket state")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Cc: Willem de Bruijn <willemb at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>

CVE-2017-6346

(backported from commit d199fab63c11998a602205f7ee7ff7c05c97164b)
[smb: no po->rollover, verified against 3.2.y backport from bwh]
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 Notes:
 - was fixed in 4.10 and 4.4.52, so only Trusty left to fix
 - in Trusty there is no po->rollover, so context had to be adjusted
   to make up for that.
 - The resulting changes look like the possible race window is much
   smaller than for later kernels, but on the other hand it cannot
   hurt to extend the mutex coverage by a bit.

-Stefan

 net/packet/af_packet.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 146274a..59a1558 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1315,13 +1315,16 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		return -EINVAL;
 	}
 
+	mutex_lock(&fanout_mutex);
+
+	err = -EINVAL;
 	if (!po->running)
-		return -EINVAL;
+		goto out;
 
+	err = -EALREADY;
 	if (po->fanout)
-		return -EALREADY;
+		goto out;
 
-	mutex_lock(&fanout_mutex);
 	match = NULL;
 	list_for_each_entry(f, &fanout_list, list) {
 		if (f->id == id &&
@@ -1377,17 +1380,16 @@ static void fanout_release(struct sock *sk)
 	struct packet_sock *po = pkt_sk(sk);
 	struct packet_fanout *f;
 
-	f = po->fanout;
-	if (!f)
-		return;
-
 	mutex_lock(&fanout_mutex);
-	po->fanout = NULL;
+	f = po->fanout;
+	if (f) {
+		po->fanout = NULL;
 
-	if (atomic_dec_and_test(&f->sk_ref)) {
-		list_del(&f->list);
-		dev_remove_pack(&f->prot_hook);
-		kfree(f);
+		if (atomic_dec_and_test(&f->sk_ref)) {
+			list_del(&f->list);
+			dev_remove_pack(&f->prot_hook);
+			kfree(f);
+		}
 	}
 	mutex_unlock(&fanout_mutex);
 }
-- 
2.7.4