Data race on dev->mtu betwen __dev_set_mtu() and rawv6_send_hdrinc()

[Gong, Sishuai]

Hello,

We found a data race on dev->mtu between function __dev_set_mtu() and 
rawv6_send_hdrinc(). It happens with the following interleaving. 

writer: __dev_set_mtu()                                                         
                reader: rawv6_send_hdrinc()
                                                                                
                                if (length > rt->dst.dev->mtu) {
        WRITE_ONCE(dev->mtu, new_mtu);
                                                                                
                                        ipv6_local_error(sk, EMSGSIZE, fl6, 
rt->dst.dev->mtu);

If the writer happens to change dev->mtu to a value that is bigger than the 
variable ‘length’, then ipv6_local_error will read a value that doesn’t satisfy 
this conditional statement. While there is no need to use lock to protect the 
read, it is probably better to only read dev->mtu once in rawv6_send_hdrinc().


Thanks,
Sishuai