[3.16.y-ckt stable] Patch "bridge: fix br_stp_set_bridge_priority race conditions" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    bridge: fix br_stp_set_bridge_priority race conditions

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 1a47fb7dc365badaf3d41c6e6d356db0eaad04c5 Mon Sep 17 00:00:00 2001
From: Nikolay Aleksandrov <razor at blackwall.org>
Date: Mon, 15 Jun 2015 20:28:51 +0300
Subject: bridge: fix br_stp_set_bridge_priority race conditions

commit 2dab80a8b486f02222a69daca6859519e05781d9 upstream.

After the ->set() spinlocks were removed br_stp_set_bridge_priority
was left running without any protection when used via sysfs. It can
race with port add/del and could result in use-after-free cases and
corrupted lists. Tested by running port add/del in a loop with stp
enabled while setting priority in a loop, crashes are easily
reproducible.
The spinlocks around sysfs ->set() were removed in commit:
14f98f258f19 ("bridge: range check STP parameters")
There's also a race condition in the netlink priority support that is
fixed by this change, but it was introduced recently and the fixes tag
covers it, just in case it's needed the commit is:
af615762e972 ("bridge: add ageing_time, stp_state, priority over netlink")

Signed-off-by: Nikolay Aleksandrov <razor at blackwall.org>
Fixes: 14f98f258f19 ("bridge: range check STP parameters")
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/bridge/br_ioctl.c  | 2 --
 net/bridge/br_stp_if.c | 4 +++-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/bridge/br_ioctl.c b/net/bridge/br_ioctl.c
index a9a4a1b7863d..8d423bc649b9 100644
--- a/net/bridge/br_ioctl.c
+++ b/net/bridge/br_ioctl.c
@@ -247,9 +247,7 @@ static int old_dev_ioctl(struct net_device *dev, struct ifreq *rq, int cmd)
 		if (!ns_capable(dev_net(dev)->user_ns, CAP_NET_ADMIN))
 			return -EPERM;

-		spin_lock_bh(&br->lock);
 		br_stp_set_bridge_priority(br, args[1]);
-		spin_unlock_bh(&br->lock);
 		return 0;

 	case BRCTL_SET_PORT_PRIORITY:
diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
index 189ba1e7d851..9a0005aee9ad 100644
--- a/net/bridge/br_stp_if.c
+++ b/net/bridge/br_stp_if.c
@@ -243,12 +243,13 @@ bool br_stp_recalculate_bridge_id(struct net_bridge *br)
 	return true;
 }

-/* called under bridge lock */
+/* Acquires and releases bridge lock */
 void br_stp_set_bridge_priority(struct net_bridge *br, u16 newprio)
 {
 	struct net_bridge_port *p;
 	int wasroot;

+	spin_lock_bh(&br->lock);
 	wasroot = br_is_root_bridge(br);

 	list_for_each_entry(p, &br->port_list, list) {
@@ -266,6 +267,7 @@ void br_stp_set_bridge_priority(struct net_bridge *br, u16 newprio)
 	br_port_state_selection(br);
 	if (br_is_root_bridge(br) && !wasroot)
 		br_become_root_bridge(br);
+	spin_unlock_bh(&br->lock);
 }

 /* called under bridge lock */