[PATCH net-next v3 2/3] virtio_net: add optional flow dissection in virtio_net_hdr_to_skb

[Tanner Love <tannerlove.kernel@gmail.com>]

From: Tanner Love <tannerlove@google.com>

Syzkaller bugs have resulted from loose specification of
virtio_net_hdr[1]. Enable execution of a BPF flow dissector program
in virtio_net_hdr_to_skb to validate the vnet header and drop bad
input.

The existing behavior of accepting these vnet headers is part of the
ABI. But individual admins may want to enforce restrictions. For
example, verifying that a GSO_TCPV4 gso_type matches packet contents:
unencapsulated TCP/IPV4 packet with payload exceeding gso_size and
hdr_len at payload offset.

Introduce a new sysctl net.core.flow_dissect_vnet_hdr controlling a
static key to decide whether to perform flow dissection. When the key
is false, virtio_net_hdr_to_skb computes as before.

[1] https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0

[ um build error ]
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tanner Love <tannerlove@google.com>
Suggested-by: Willem de Bruijn <willemb@google.com>
---
 include/linux/virtio_net.h | 25 +++++++++++++++++++++----
 net/core/flow_dissector.c  |  3 +++
 net/core/sysctl_net_core.c |  9 +++++++++
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b465f8f3e554..cdc6152b40c6 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -25,10 +25,13 @@ static inline int virtio_net_hdr_set_proto(struct sk_buff *skb,
 	return 0;
 }
 
+DECLARE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
+
 static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 					const struct virtio_net_hdr *hdr,
 					bool little_endian)
 {
+	struct flow_keys_basic keys;
 	unsigned int gso_type = 0;
 	unsigned int thlen = 0;
 	unsigned int p_off = 0;
@@ -78,13 +81,24 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 		p_off = skb_transport_offset(skb) + thlen;
 		if (!pskb_may_pull(skb, p_off))
 			return -EINVAL;
-	} else {
+	}
+
+	/* BPF flow dissection for optional strict validation.
+	 *
+	 * Admins can define permitted packets more strictly, such as dropping
+	 * deprecated UDP_UFO packets and requiring skb->protocol to be non-zero
+	 * and matching packet headers.
+	 */
+	if (static_branch_unlikely(&sysctl_flow_dissect_vnet_hdr_key) &&
+	    !__skb_flow_dissect_flow_keys_basic(NULL, skb, &keys, NULL, 0, 0, 0,
+						0, hdr))
+		return -EINVAL;
+
+	if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM)) {
 		/* gso packets without NEEDS_CSUM do not set transport_offset.
 		 * probe and drop if does not match one of the above types.
 		 */
 		if (gso_type && skb->network_header) {
-			struct flow_keys_basic keys;
-
 			if (!skb->protocol) {
 				__be16 protocol = dev_parse_header_protocol(skb);
 
@@ -92,8 +106,11 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 				if (protocol && protocol != skb->protocol)
 					return -EINVAL;
 			}
+
 retry:
-			if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
+			/* only if flow dissection not already done */
+			if (!static_branch_unlikely(&sysctl_flow_dissect_vnet_hdr_key) &&
+			    !skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
 							      NULL, 0, 0, 0,
 							      0)) {
 				/* UFO does not specify ipv4 or 6: try both */
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 4b171ebec084..ae2ce382f4f4 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -35,6 +35,9 @@
 #endif
 #include <linux/bpf-netns.h>
 
+DEFINE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
+EXPORT_SYMBOL(sysctl_flow_dissect_vnet_hdr_key);
+
 static void dissector_set_key(struct flow_dissector *flow_dissector,
 			      enum flow_dissector_key_id key_id)
 {
diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
index c8496c1142c9..c01b9366bb75 100644
--- a/net/core/sysctl_net_core.c
+++ b/net/core/sysctl_net_core.c
@@ -36,6 +36,8 @@ static int net_msg_warn;	/* Unused, but still a sysctl */
 int sysctl_fb_tunnels_only_for_init_net __read_mostly = 0;
 EXPORT_SYMBOL(sysctl_fb_tunnels_only_for_init_net);
 
+DECLARE_STATIC_KEY_FALSE(sysctl_flow_dissect_vnet_hdr_key);
+
 /* 0 - Keep current behavior:
  *     IPv4: inherit all current settings from init_net
  *     IPv6: reset all settings to default
@@ -580,6 +582,13 @@ static struct ctl_table net_core_table[] = {
 		.extra1		= SYSCTL_ONE,
 		.extra2		= &int_3600,
 	},
+	{
+		.procname       = "flow_dissect_vnet_hdr",
+		.data           = &sysctl_flow_dissect_vnet_hdr_key.key,
+		.maxlen         = sizeof(sysctl_flow_dissect_vnet_hdr_key),
+		.mode           = 0644,
+		.proc_handler   = proc_do_static_key,
+	},
 	{ }
 };
 
-- 
2.32.0.rc0.204.g9fa02ecfa5-goog