[PROBLEM] a data race between tcp_set_default_congestion_control() and tcp_set_congestion_control()

[Gong, Sishuai]

Hi,

We found a data race between tcp_set_default_congestion_control() and 
tcp_set_congestion_control() in linux-5.12-rc3. 
In general, when tcp_set_congestion_control() is reading ca->flags with a lock 
grabbed, tcp_set_default_congestion_control() 
may be updating ca->flags at the same time, as shown below.

When the writer and reader are running parallel, tcp_set_congestion_control()’s 
control flow 
might be non-deterministic, either returning a -EPERM or calling 
tcp_reinit_congestion_control().

We also notice in tcp_set_allowed_congestion_control(), the write to ca->flags 
is protected by tcp_cong_list_lock,
so we want to point it out in case the data race is unexpected.

Thread 1                                                        Thread 2
//tcp_set_default_congestion_control()  //tcp_set_congestion_control()
                                                                // lock_sock() 
grabbed
                                                                if 
(!((ca->flags & TCP_CONG_NON_RESTRICTED) || cap_net_admin))
                                                                        err = 
-EPERM;
                                                                else if 
(!bpf_try_module_get(ca, ca->owner))
                                                                        err = 
-EBUSY;
                                                                else
                                                                        
tcp_reinit_congestion_control(sk, ca);
ca->flags |= TCP_CONG_NON_RESTRICTED;
        


Thanks,
Sishuai