netdev - Re: [PATCH v2 net] net: Fix sk->sk_stamp race in sock_recv

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Message-ID: <CANn89iJAZ+OYVebm-x4pJgjYTdj8RiXc8iLnQC8X6JC3RUywuA@mail.gmail.com>
Date: Mon, 8 May 2023 19:31:12 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, kuba@...nel.org, kuni1840@...il.com, 
	netdev@...r.kernel.org, pabeni@...hat.com, syzkaller@...glegroups.com
Subject: Re: [PATCH v2 net] net: Fix sk->sk_stamp race in sock_recv_cmsgs().

On Mon, May 8, 2023 at 7:20 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Mon, 8 May 2023 19:08:58 +0200
> > On Mon, May 8, 2023 at 6:58 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > >
> > > KCSAN found a data race in sock_recv_cmsgs() [0] where the read access
> > > to sk->sk_stamp needs READ_ONCE().
> > >
> > > Also, there is another race like below.  If the torn load of the high
> > > 32-bits precedes WRITE_ONCE(sk, skb->tstamp) and later the written
> > > lower 32-bits happens to match with SK_DEFAULT_STAMP, the final result
> > > of sk->sk_stamp could be 0.
> > >
> > >   sock_recv_cmsgs()  ioctl(SIOCGSTAMP)      sock_recv_cmsgs()
> > >   |                  |                      |
> > >   |- if (sock_flag(sk, SOCK_TIMESTAMP))     |
> > >   |                  |                      |
> > >   |                  `- sock_set_flag(sk, SOCK_TIMESTAMP)
> > >   |                                         |
> > >   |                                          `- if (sock_flag(sk, SOCK_TIMESTAMP))
> > >   `- if (sk->sk_stamp == SK_DEFAULT_STAMP)      `- sock_write_timestamp(sk, skb->tstamp)
> > >       `- sock_write_timestamp(sk, 0)
> > >
> > > Even with READ_ONCE(), we could get the same result if READ_ONCE() precedes
> > > WRITE_ONCE() because the SK_DEFAULT_STAMP check and WRITE_ONCE(sk_stamp, 0)
> > > are not atomic.
> > >
> > > Let's avoid the race by cmpxchg() on 64-bits architecture or seqlock on
> > > 32-bits machines.
> > >
> >
> > I disagree. Please use WRITE_ONCE(), even if we know it is racy on 32bit.
> >
> > sock_read_timestamp() and sock_write_timestamp() already are racy, and
> > we do not care.
>
> I think it's not racy since commit 3a0ed3e96197 ("sock: Make sock->sk_stamp
> thread-safe"), which introduced seqlock in sock_read_timestamp() and
> sock_write_timestamp().

Please note I do not care of 32bit.

It is definitely racy on 64bit.

Please look at

commit f75359f3ac855940c5718af10ba089b8977bf339
Author: Eric Dumazet <edumazet@...gle.com>
Date:   Mon Nov 4 21:38:43 2019 -0800

    net: prevent load/store tearing on sk->sk_stamp


We can not use cmpxchg() only in one place and not the others.

cmpxchg() is expensive, we do not want it here on our fast path.

Thanks.