2021-04-15 15:48:42

by Gong, Sishuai

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

2021-04-15 15:54:35

by Eric Dumazet

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

On Thu, Apr 15, 2021 at 5:47 PM Gong, Sishuai <[email protected]> wrote:
>
> 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
>

Yes, obviously reading ca->flags while another thread might set the bit is racy.

This is of no consequence, if you want to silence KCSAN please a patch.