[SRU Focal] UBUNTU: SAUCE: net/packet: fix overflow in tpacket_rcv

[Thadeu Lima de Souza Cascardo]

From: Or Cohen <orcohen at paloaltonetworks.com>

Using tp_reserve to calculate netoff can overflow as
tp_reserve is unsigned int and netoff is unsigned short.

This may lead to macoff receving a smaller value then
sizeof(struct virtio_net_hdr), and if po->has_vnet_hdr
is set, an out-of-bounds write will occur when
calling virtio_net_hdr_from_skb.

The bug is fixed by converting netoff to unsigned int
and checking if it exceeds USHRT_MAX.

This addresses CVE-2020-14386

Fixes: 8913336a7e8d ("packet: add PACKET_RESERVE sockopt")
Signed-off-by: Or Cohen <orcohen at paloaltonetworks.com>
Signed-off-by: Eric Dumazet <edumazet at google.com>
CVE-2020-14386
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 net/packet/af_packet.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 7735340c892e..fbc2d4dfddf0 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2169,7 +2169,8 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	int skb_len = skb->len;
 	unsigned int snaplen, res;
 	unsigned long status = TP_STATUS_USER;
-	unsigned short macoff, netoff, hdrlen;
+	unsigned short macoff, hdrlen;
+	unsigned int netoff;
 	struct sk_buff *copy_skb = NULL;
 	struct timespec ts;
 	__u32 ts_status;
@@ -2238,6 +2239,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 		}
 		macoff = netoff - maclen;
 	}
+	if (netoff > USHRT_MAX) {
+		atomic_inc(&po->tp_drops);
+		goto drop_n_restore;
+	}
 	if (po->tp_version <= TPACKET_V2) {
 		if (macoff + snaplen > po->rx_ring.frame_size) {
 			if (po->copy_thresh &&
-- 
2.25.1