packet: tpacket_snd(): fix signed/unsigned comparison

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>

1 files changed, 2 insertions, 1 deletions

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index c7c42eb617ef..ed458b315ef4 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2403,7 +2403,8 @@ 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