[PATCH net-next v2 2/2] virtio-net: Ensure that TCP packets don't overflow gso_segs

[Alice Mikityanska <alice.kernel@fastmail.im>]

From: Alice Mikityanska <alice@isovalent.com>

The user can specify any gso_size in a packet crafted with an AF_PACKET
PACKET_VNET_HDR socket, even smaller than TCP_MIN_GSO_SIZE = 8. At the
same time, GSO_MAX_SIZE = 8 * GSO_MAX_SEGS = 8 * 65535. When the user
crafts a packet with gso_size < 8, there is a risk for partial GSO to
overflow the 16-bit gso_segs field when dividing the SKB length by
gso_size.

Adjust gso_size of TCP packets to be at least TCP_MIN_GSO_SIZE = 8. Keep
gso_size of UDP GSO packets, as gso_size=1 is valid and explicitly
tested at tools/testing/selftests/net/tun.c:649.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
---
 include/linux/virtio_net.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index f36d21b5bc19..c381b916c1b5 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -6,6 +6,7 @@
 #include <linux/ip.h>
 #include <linux/ipv6.h>
 #include <linux/udp.h>
+#include <net/tcp.h>
 #include <uapi/linux/tcp.h>
 #include <uapi/linux/virtio_net.h>
 
@@ -179,6 +180,9 @@ static inline int __virtio_net_hdr_to_skb(struct sk_buff *skb,
 			if (skb->ip_summed == CHECKSUM_PARTIAL &&
 			    skb->csum_offset != offsetof(struct tcphdr, check))
 				return -EINVAL;
+
+			BUILD_BUG_ON(TCP_MIN_GSO_SIZE * GSO_MAX_SEGS < GSO_MAX_SIZE);
+			gso_size = max(gso_size, TCP_MIN_GSO_SIZE);
 			break;
 		}
 
-- 
2.55.0