linux-kernel - [PROBLEM] a data race between tcp_set_default_congestion_control() and tcp_set_congestion

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Date:   Thu, 15 Apr 2021 15:47:21 +0000
From:   "Gong, Sishuai" <sishuai@...due.edu>
To:     "edumazet@...gle.com" <edumazet@...gle.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "kuba@...nel.org" <kuba@...nel.org>
CC:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PROBLEM] a data race between tcp_set_default_congestion_control()
 and tcp_set_congestion_control()

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