[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