net/ieee802154: don't warn zero-sized raw_sendmsg()

In the Linux kernel, the following vulnerability has been resolved:

net/ieee802154: don't warn zero-sized raw_sendmsg()

syzbot is hitting skb_assert_len() warning at __dev_queue_xmit() [1],

for PF_IEEE802154 socket's zero-sized raw_sendmsg() request is hitting

__dev_queue_xmit() with skb->len == 0.

Since PF_IEEE802154 socket's zero-sized raw_sendmsg() request was

able to return 0, don't call __dev_queue_xmit() if packet length is 0.

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#include <sys/socket.h>

#include <netinet/in.h>

int main(int argc, char *argv[])

{

struct sockaddr_in addr = { .sin_family = AF_INET, .sin_addr.s_addr = htonl(INADDR_LOOPBACK) };

struct iovec iov = { };

struct msghdr hdr = { .msg_name = &addr, .msg_namelen = sizeof(addr), .msg_iov = &iov, .msg_iovlen = 1 };

sendmsg(socket(PF_IEEE802154, SOCK_RAW, 0), &hdr, 0);

return 0;

}

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Note that this might be a sign that commit fd1894224407c484 ("bpf: Don't

redirect packets with invalid pkt_len") should be reverted, for

skb->len == 0 was acceptable for at least PF_IEEE802154 socket.