packet: tpacket_snd(): fix signed/unsigned comparison

commit dbd46ab412b8fb395f2b0ff6f6a7eec9df311550 upstream.

tpacket_fill_skb() can return a negative value (-errno) which
is stored in tp_len variable. In that case the following
condition will be (but shouldn't be) true:

tp_len > dev->mtu + dev->hard_header_len

as dev->mtu and dev->hard_header_len are both unsigned.

That may lead to just returning an incorrect EMSGSIZE errno
to the user.

Fixes: 52f1454f629fa ("packet: allow to transmit +4 byte in TX_RING slot for VLAN case")
Signed-off-by: Alexander Drozdov <al.drozdov@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 7dc6a05f09c82cac096aec7d17e357961b64177b..1f2e7d3417817d87b648aaa48c5b4cc5f5e767ad 100644 (file)
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2284,7 +2284,8 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 
                tp_len = tpacket_fill_skb(po, skb, ph, dev, size_max, proto,
                                          addr, hlen);
-               if (tp_len > dev->mtu + dev->hard_header_len) {
+               if (likely(tp_len >= 0) &&
+                   tp_len > dev->mtu + dev->hard_header_len) {
                        struct ethhdr *ehdr;
                        /* Earlier code assumed this would be a VLAN pkt,
                         * double-check this now that we have the actual