netdev - Re: [PATCH v1 net] udp: Set SOCK_RCU_FREE earlier in udp_lib_get

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Message-ID: <CANn89iKMDDzRox+KC4dGGBCL+VgBSR2S8NoKYcLHR3TS4r_XqQ@mail.gmail.com>
Date: Mon, 8 Jul 2024 12:07:56 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	kuni1840@...il.com, netdev@...r.kernel.org, pabeni@...hat.com, 
	syzkaller@...glegroups.com, willemdebruijn.kernel@...il.com
Subject: Re: [PATCH v1 net] udp: Set SOCK_RCU_FREE earlier in udp_lib_get_port().

On Mon, Jul 8, 2024 at 11:55 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Mon, 8 Jul 2024 11:38:41 -0700
> > On Mon, Jul 8, 2024 at 11:20 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > >
> > > syzkaller triggered the warning [0] in udp_v4_early_demux().
> > >
> > > In udp_v4_early_demux(), we do not touch the refcount of the looked-up
> > > sk and use sock_pfree() as skb->destructor, so we check SOCK_RCU_FREE
> > > to ensure that the sk is safe to access during the RCU grace period.
> > >
> > > Currently, SOCK_RCU_FREE is flagged for a bound socket after being put
> > > into the hash table.  Moreover, the SOCK_RCU_FREE check is done too
> > > early in udp_v4_early_demux(), so there could be a small race window:
> > >
> > >   CPU1                                 CPU2
> > >   ----                                 ----
> > >   udp_v4_early_demux()                 udp_lib_get_port()
> > >   |                                    |- hlist_add_head_rcu()
> > >   |- sk = __udp4_lib_demux_lookup()    |
> > >   |- DEBUG_NET_WARN_ON_ONCE(sk_is_refcounted(sk));
> > >                                        `- sock_set_flag(sk, SOCK_RCU_FREE)
> > >
> > > In practice, sock_pfree() is called much later, when SOCK_RCU_FREE
> > > is most likely propagated for other CPUs; otherwise, we will see
> > > another warning of sk refcount underflow, but at least I didn't.
> > >
> > > Technically, moving sock_set_flag(sk, SOCK_RCU_FREE) before
> > > hlist_add_{head,tail}_rcu() does not guarantee the order, and we
> > > must put smp_mb() between them, or smp_wmb() there and smp_rmb()
> > > in udp_v4_early_demux().
> > >
> > > But it's overkill in the real scenario, so I just put smp_mb() only under
> > > CONFIG_DEBUG_NET to silence the splat.  When we see the refcount underflow
> > > warning, we can remove the config guard.
> > >
> > > Another option would be to remove DEBUG_NET_WARN_ON_ONCE(), but this could
> > > make future debugging harder without the hints in udp_v4_early_demux() and
> > > udp_lib_get_port().
> > >
> > > [0]:
> > >
> > > Fixes: 08842c43d016 ("udp: no longer touch sk->sk_refcnt in early demux")
> > > Reported-by: syzkaller <syzkaller@...glegroups.com>
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > ---
> > >  net/ipv4/udp.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > > index 189c9113fe9a..1a05cc3d2b4f 100644
> > > --- a/net/ipv4/udp.c
> > > +++ b/net/ipv4/udp.c
> > > @@ -326,6 +326,12 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
> > >                         goto fail_unlock;
> > >                 }
> > >
> > > +               sock_set_flag(sk, SOCK_RCU_FREE);
> >
> > Nice catch.
> >
> > > +
> > > +               if (IS_ENABLED(CONFIG_DEBUG_NET))
> > > +                       /* for DEBUG_NET_WARN_ON_ONCE() in udp_v4_early_demux(). */
> > > +                       smp_mb();
> > > +
> >
> > I do not think this smp_mb() is needed. If this was, many other RCU
> > operations would need it,
> >
> > RCU rules mandate that all memory writes must be committed before the
> > object can be seen
> > by other cpus in the hash table.
> >
> > This includes the setting of the SOCK_RCU_FREE flag.
> >
> > For instance, hlist_add_head_rcu() does a
> > rcu_assign_pointer(hlist_first_rcu(h), n);
>
> Ah, I was thinking spinlock will not prevent reordering, but
> now I see, rcu_assign_pointer() had necessary barrier. :)
>
>   /**
>    * rcu_assign_pointer() - assign to RCU-protected pointer
>    ...
>    * Assigns the specified value to the specified RCU-protected
>    * pointer, ensuring that any concurrent RCU readers will see
>    * any prior initialization.
>
> will remove smp_mb() and update the changelog in v2.
>

A similar commit was

commit 871019b22d1bcc9fab2d1feba1b9a564acbb6e99
Author: Stanislav Fomichev <sdf@...ichev.me>
Date:   Wed Nov 8 13:13:25 2023 -0800

    net: set SOCK_RCU_FREE before inserting socket into hashtable

So I wonder if the bug could be older...