Linux Ethernet Bridging - Nov 2018 - [Bridge] [PATCH net] net: bridge: fix per-port vlan stats use-after-free on destruction

Syzbot reported a use-after-free of the global vlan context on port vlan
destruction. When I added per-port vlan stats I missed the fact that the
global vlan context can be freed before the per-port vlan rcu callback.
There're a few different ways to deal with this, I've chosen to add a
new private flag that is set only when per-port stats are allocated so
we can directly check it on destruction without dereferencing the global
context at all. The flag is internally controlled by the kernel and
user-space isn't allowed to set it.

Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan
stats")
Reported-by: syzbot+04681da557a0e49a52e5 at syzkaller.appspotmail.com
Signed-off-by: Nikolay Aleksandrov <nikolay at cumulusnetworks.com>
---
 include/uapi/linux/if_bridge.h | 3 +++
 net/bridge/br_netlink.c        | 7 ++++++-
 net/bridge/br_vlan.c           | 3 ++-
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/if_bridge.h b/include/uapi/linux/if_bridge.h
index e41eda3c71f1..fa1f72276712 100644
--- a/include/uapi/linux/if_bridge.h
+++ b/include/uapi/linux/if_bridge.h
@@ -130,6 +130,9 @@ enum {
 #define BRIDGE_VLAN_INFO_RANGE_BEGIN	(1<<3) /* VLAN is start of vlan
range */
 #define BRIDGE_VLAN_INFO_RANGE_END	(1<<4) /* VLAN is end of vlan range */
 #define BRIDGE_VLAN_INFO_BRENTRY	(1<<5) /* Global bridge VLAN entry */
+#define BRIDGE_VLAN_INFO_PORT_STATS	(1<<6) /* Per-port VLAN stats */
+
+#define BRIDGE_VLAN_INFO_PRIVATE_FLAGS BRIDGE_VLAN_INFO_PORT_STATS
 
 struct bridge_vlan_info {
 	__u16 flags;
diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
index 3345f1984542..c600fedcfb76 100644
--- a/net/bridge/br_netlink.c
+++ b/net/bridge/br_netlink.c
@@ -527,8 +527,12 @@ int br_getlink(struct sk_buff *skb, u32 pid, u32 seq,
 static int br_vlan_info(struct net_bridge *br, struct net_bridge_port *p,
 			int cmd, struct bridge_vlan_info *vinfo, bool *changed)
 {
+	int err = -EINVAL;
 	bool curr_change;
-	int err = 0;
+
+	/* don't allow user-space control over private flags */
+	if (vinfo->flags & BRIDGE_VLAN_INFO_PRIVATE_FLAGS)
+		return err;
 
 	switch (cmd) {
 	case RTM_SETLINK:
@@ -548,6 +552,7 @@ static int br_vlan_info(struct net_bridge *br, struct
net_bridge_port *p,
 		break;
 
 	case RTM_DELLINK:
+		err = 0;
 		if (p) {
 			if (!nbp_vlan_delete(p, vinfo->vid))
 				*changed = true;
diff --git a/net/bridge/br_vlan.c b/net/bridge/br_vlan.c
index 8c9297a01947..004e1f8c5040 100644
--- a/net/bridge/br_vlan.c
+++ b/net/bridge/br_vlan.c
@@ -197,7 +197,7 @@ static void nbp_vlan_rcu_free(struct rcu_head *rcu)
 	v = container_of(rcu, struct net_bridge_vlan, rcu);
 	WARN_ON(br_vlan_is_master(v));
 	/* if we had per-port stats configured then free them here */
-	if (v->brvlan->stats != v->stats)
+	if (v->flags & BRIDGE_VLAN_INFO_PORT_STATS)
 		free_percpu(v->stats);
 	v->stats = NULL;
 	kfree(v);
@@ -264,6 +264,7 @@ static int __vlan_add(struct net_bridge_vlan *v, u16 flags)
 				err = -ENOMEM;
 				goto out_filt;
 			}
+			v->flags |= BRIDGE_VLAN_INFO_PORT_STATS;
 		} else {
 			v->stats = masterv->stats;
 		}
-- 
2.17.2

On 11/13/18 3:01 AM, Nikolay Aleksandrov wrote:
> Syzbot reported a use-after-free of the global vlan context on port vlan
> destruction. When I added per-port vlan stats I missed the fact that the
> global vlan context can be freed before the per-port vlan rcu callback.
> There're a few different ways to deal with this, I've chosen to add
a
> new private flag that is set only when per-port stats are allocated so
> we can directly check it on destruction without dereferencing the global
> context at all. The flag is internally controlled by the kernel and
> user-space isn't allowed to set it.
> 
> Fixes: 9163a0fc1f0c ("net: bridge: add support for per-port vlan
stats")
> Reported-by: syzbot+04681da557a0e49a52e5 at syzkaller.appspotmail.com
> Signed-off-by: Nikolay Aleksandrov <nikolay at cumulusnetworks.com>
> ---
I'll post v2 with a cosmetic change - move the check up one level where
it's more logical to be and the rest of the checks are done.