tipc: check minimum bearer MTU

Qian Zhang (张谦) reported a potential socket buffer overflow in
tipc_msg_build() which is also known as CVE-2016-8632: due to
insufficient checks, a buffer overflow can occur if MTU is too short for
even tipc headers. As anyone can set device MTU in a user/net namespace,
this issue can be abused by a regular user.

As agreed in the discussion on Ben Hutchings' original patch, we should
check the MTU at the moment a bearer is attached rather than for each
processed packet. We also need to repeat the check when bearer MTU is
adjusted to new device MTU. UDP case also needs a check to avoid
overflow when calculating bearer MTU.

Fixes: b97bf3fd8f6a ("[TIPC] Initial merge")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reported-by: Qian Zhang (张谦) <zhangqian-c@360.cn>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 27 insertions, 2 deletions

diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c
index 975dbeb60ab0..52d74760fb68 100644
--- a/net/tipc/bearer.c
+++ b/net/tipc/bearer.c
@@ -421,6 +421,10 @@ int tipc_enable_l2_media(struct net *net, struct tipc_bearer *b,
         dev = dev_get_by_name(net, driver_name);
         if (!dev)
                 return -ENODEV;
+        if (tipc_mtu_bad(dev, 0)) {
+                dev_put(dev);
+                return -EINVAL;
+        }
 
         /* Associate TIPC bearer with L2 bearer */
         rcu_assign_pointer(b->media_ptr, dev);
@@ -610,8 +614,6 @@ static int tipc_l2_device_event(struct notifier_block *nb, unsigned long evt,
         if (!b)
                 return NOTIFY_DONE;
 
-        b->mtu = dev->mtu;
-
         switch (evt) {
         case NETDEV_CHANGE:
                 if (netif_carrier_ok(dev))
@@ -624,6 +626,11 @@ static int tipc_l2_device_event(struct notifier_block *nb, unsigned long evt,
                 tipc_reset_bearer(net, b);
                 break;
         case NETDEV_CHANGEMTU:
+                if (tipc_mtu_bad(dev, 0)) {
+                        bearer_disable(net, b);
+                        break;
+                }
+                b->mtu = dev->mtu;
                 tipc_reset_bearer(net, b);
                 break;
         case NETDEV_CHANGEADDR:
diff --git a/net/tipc/bearer.h b/net/tipc/bearer.h
index 78892e2f53e3..278ff7f616f9 100644
--- a/net/tipc/bearer.h
+++ b/net/tipc/bearer.h
@@ -39,6 +39,7 @@
 
 #include "netlink.h"
 #include "core.h"
+#include "msg.h"
 #include <net/genetlink.h>
 
 #define MAX_MEDIA       3
@@ -59,6 +60,9 @@
 #define TIPC_MEDIA_TYPE_IB      2
 #define TIPC_MEDIA_TYPE_UDP     3
 
+/* minimum bearer MTU */
+#define TIPC_MIN_BEARER_MTU     (MAX_H_SIZE + INT_H_SIZE)
+
 /**
  * struct tipc_media_addr - destination address used by TIPC bearers
  * @value: address info (format defined by media)
@@ -215,4 +219,13 @@ void tipc_bearer_xmit(struct net *net, u32 bearer_id,
 void tipc_bearer_bc_xmit(struct net *net, u32 bearer_id,
                          struct sk_buff_head *xmitq);
 
+/* check if device MTU is too low for tipc headers */
+static inline bool tipc_mtu_bad(struct net_device *dev, unsigned int reserve)
+{
+        if (dev->mtu >= TIPC_MIN_BEARER_MTU + reserve)
+                return false;
+        netdev_warn(dev, "MTU too low for tipc bearer\n");
+        return true;
+}
+
 #endif  /* _TIPC_BEARER_H */
diff --git a/net/tipc/udp_media.c b/net/tipc/udp_media.c
index 78cab9c5a445..b58dc95f3d35 100644
--- a/net/tipc/udp_media.c
+++ b/net/tipc/udp_media.c
@@ -697,6 +697,11 @@ static int tipc_udp_enable(struct net *net, struct tipc_bearer *b,
                 udp_conf.local_ip.s_addr = htonl(INADDR_ANY);
                 udp_conf.use_udp_checksums = false;
                 ub->ifindex = dev->ifindex;
+                if (tipc_mtu_bad(dev, sizeof(struct iphdr) +
+                                      sizeof(struct udphdr))) {
+                        err = -EINVAL;
+                        goto err;
+                }
                 b->mtu = dev->mtu - sizeof(struct iphdr)
                         - sizeof(struct udphdr);
 #if IS_ENABLED(CONFIG_IPV6)