net: prevent infinite loop caused by incorrect proto from virtio_net_hdr_set_proto

These patches prevent an infinite loop for gso packets with a protocol from virtio net hdr that doesn't match the protocol in the packet. Note that packets coming from a device without header_ops->parse_protocol being implemented will not be caught by the check in virtio_net_hdr_to_skb, but the infinite loop will still be prevented by the check in the gso layer.

Changes from v2 to v3:
- Remove unused *eth.
- Use MPLS_HLEN to also check if the MPLS header length is a multiple of four.

Balazs Nemeth (2):
net: check if protocol extracted by virtio_net_hdr_set_proto is
correct
net: avoid infinite loop in mpls_gso_segment when mpls_hlen == 0

 include/linux/virtio_net.h | 7 ++++++-
 net/mpls/mpls_gso.c        | 3 +++
 2 files changed, 9 insertions(+), 1 deletion(-)

--
2.29.2

For gso packets, virtio_net_hdr_set_proto sets the protocol (if it isn't set) based on the type in the virtio net hdr, but the skb could contain anything since it could come from packet_snd through a raw socket. If there is a mismatch between what virtio_net_hdr_set_proto sets and the actual protocol, then the skb could be handled incorrectly later on.

An example where this poses an issue is with the subsequent call to skb_flow_dissect_flow_keys_basic which relies on skb->protocol being set correctly. A specially crafted packet could fool skb_flow_dissect_flow_keys_basic preventing EINVAL to be returned.

Avoid blindly trusting the information provided by the virtio net header by checking that the protocol in the packet actually matches the protocol set by virtio_net_hdr_set_proto. Note that since the protocol is only checked if skb->dev implements header_ops->parse_protocol, packets from devices without the implementation are not checked at this stage.

---
 include/linux/virtio_net.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index e8a924eeea3d..6b5fcfa1e555 100644
@@ -79,8 +79,13 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		if (gso_type && skb->network_header) {
 			struct flow_keys_basic keys;
 
-			if (!skb->protocol)
+			if (!skb->protocol) {
+				__be16 protocol = dev_parse_header_protocol(skb);
+
 				virtio_net_hdr_set_proto(skb, hdr);
+				if (protocol && protocol != skb->protocol)
+					return -EINVAL;
+			}
 retry:
 			if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
 							      NULL, 0, 0, 0,

2.29.2

A packet with skb_inner_network_header(skb) == skb_network_header(skb) and ETH_P_MPLS_UC will prevent mpls_gso_segment from pulling any headers from the packet. Subsequently, the call to skb_mac_gso_segment will again call mpls_gso_segment with the same packet leading to an infinite loop. In addition, ensure that the header length is a multiple of four, which should hold irrespective of the number of stacked labels.

---
 net/mpls/mpls_gso.c | 3 +++
 1 file changed, 3 insertions(+)

index b1690149b6fa..1482259de9b5 100644
@@ -14,6 +14,7 @@
 #include <linux/netdev_features.h>
 #include <linux/netdevice.h>
 #include <linux/skbuff.h>
+#include <net/mpls.h>
 
 static struct sk_buff *mpls_gso_segment(struct sk_buff *skb,
 				       netdev_features_t features)

@@ -27,6 +28,8 @@ static struct sk_buff *mpls_gso_segment(struct sk_buff *skb,
 
 	skb_reset_network_header(skb);
 	mpls_hlen = skb_inner_network_header(skb) - skb_network_header(skb);
+	if (unlikely(!mpls_hlen || mpls_hlen % MPLS_HLEN))
+		goto out;
 	if (unlikely(!pskb_may_pull(skb, mpls_hlen)))
 		goto out;
 

2.29.2

The compiler will convert that modulo into a cheap & (ETH_HLEN - 1) test for this constant.
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This still relies entirely on data from the untrusted process. But it adds the constraint that the otherwise untrusted data at least has to be consistent, closing one loophole.

As responded in v2, we may want to look at the (few) callers and make sure that they initialize skb->protocol before the call to virtio_net_hdr_to_skb where possible. That will avoid this entire branch.
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[email protected]
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