[PATCH net v2] tipc: check minimum bearer MTU

[PATCH net v2] tipc: check minimum bearer MTU

[Michal Kubecek]

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>
---
changes v1 to v2:
- add missing "static" to tipc_check_mtu() helper declaration
- rather than blocking device MTU change to too low value, disable tipc
  bearer if that happens (suggested by Ben Hutchings)
---
 net/tipc/bearer.c    | 11 +++++++++--
 net/tipc/bearer.h    | 13 +++++++++++++
 net/tipc/udp_media.c |  5 +++++
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c
index 975dbeb60ab0..1c004a46486f 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_check_mtu(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_check_mtu(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..6b8a1d9d313a 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 sufficient for tipc headers */
+static inline bool tipc_check_mtu(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..376ed3e3ed46 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_check_mtu(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)
-- 
2.10.2

Re: [PATCH net v2] tipc: check minimum bearer MTU

[Ben Hutchings]

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On Thu, 2016-12-01 at 12:02 +0100, Michal Kubecek wrote:
[...] 
> +/* check if device MTU is sufficient for tipc headers */
> +static inline bool tipc_check_mtu(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;
> +}
[...]

The comment says "check if ... sufficient" but the return value
indicates the opposite.  Could you make these consistent?

Other than that, this looks OK to me.  I haven't tested any version as
I don't know how to use TIPC.

Ben.

-- 
Ben Hutchings
A free society is one where it is safe to be unpopular. - Adlai
Stevenson


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Re: [PATCH net v2] tipc: check minimum bearer MTU

[Michal Kubecek]

On Thu, Dec 01, 2016 at 04:11:18PM +0000, Ben Hutchings wrote:
> On Thu, 2016-12-01 at 12:02 +0100, Michal Kubecek wrote:
> [...] 
> > +/* check if device MTU is sufficient for tipc headers */
> > +static inline bool tipc_check_mtu(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;
> > +}
> [...]
> 
> The comment says "check if ... sufficient" but the return value
> indicates the opposite.  Could you make these consistent?

Good point. I suppose renaming the function to e.g. tipc_mtu_bad() (and
rewording the commment accordingly) would also make the code more 
readable without looking at the definition.

I'll wait for other comments and send v3 tomorrow.

> Other than that, this looks OK to me.  I haven't tested any version as
> I don't know how to use TIPC.

I checked that the patch doesn't allow enabling a bearer on top of
device with insufficient MTU and it does for sufficient (100/128), both
in eth and udp case. I also checked that lowering MTU under 100 in eth
case disables attached bearer. I didn't run any deeper test like sending
an actual traffic but the patch shouldn't affect that.

Michal Kubecek