netdev - Re: [PATCH net] sctp: use call

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Message-ID: <CADvbK_dxJ8_BTEJkDDX_M-FkLrjFnMfSDhBe_8ZseWrG+2agBw@mail.gmail.com>
Date:   Thu, 23 Dec 2021 11:57:19 -0500
From:   Xin Long <lucien.xin@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     network dev <netdev@...r.kernel.org>,
        "linux-sctp @ vger . kernel . org" <linux-sctp@...r.kernel.org>,
        davem <davem@...emloft.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH net] sctp: use call_rcu to free endpoint

On Wed, Dec 22, 2021 at 5:40 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Tue, 21 Dec 2021 14:51:52 -0500 Xin Long wrote:
> > This patch is to delay the endpoint free by calling call_rcu() to fix
> > another use-after-free issue in sctp_sock_dump():
> >
> >   BUG: KASAN: use-after-free in __lock_acquire+0x36d9/0x4c20
> >   Call Trace:
> >     __lock_acquire+0x36d9/0x4c20 kernel/locking/lockdep.c:3218
> >     lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
> >     __raw_spin_lock_bh include/linux/spinlock_api_smp.h:135 [inline]
> >     _raw_spin_lock_bh+0x31/0x40 kernel/locking/spinlock.c:168
> >     spin_lock_bh include/linux/spinlock.h:334 [inline]
> >     __lock_sock+0x203/0x350 net/core/sock.c:2253
> >     lock_sock_nested+0xfe/0x120 net/core/sock.c:2774
> >     lock_sock include/net/sock.h:1492 [inline]
> >     sctp_sock_dump+0x122/0xb20 net/sctp/diag.c:324
> >     sctp_for_each_transport+0x2b5/0x370 net/sctp/socket.c:5091
> >     sctp_diag_dump+0x3ac/0x660 net/sctp/diag.c:527
> >     __inet_diag_dump+0xa8/0x140 net/ipv4/inet_diag.c:1049
> >     inet_diag_dump+0x9b/0x110 net/ipv4/inet_diag.c:1065
> >     netlink_dump+0x606/0x1080 net/netlink/af_netlink.c:2244
> >     __netlink_dump_start+0x59a/0x7c0 net/netlink/af_netlink.c:2352
> >     netlink_dump_start include/linux/netlink.h:216 [inline]
> >     inet_diag_handler_cmd+0x2ce/0x3f0 net/ipv4/inet_diag.c:1170
> >     __sock_diag_cmd net/core/sock_diag.c:232 [inline]
> >     sock_diag_rcv_msg+0x31d/0x410 net/core/sock_diag.c:263
> >     netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2477
> >     sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:274
> >
> > This issue occurs when asoc is peeled off and the old sk is freed after
> > getting sk by asoc->base.sk and before calling lock_sock(sk).
> >
> > To prevent the ep/sk free, this patch is to call call_rcu to free the ep
> > and hold it under rcu_read_lock to make sure that sk in sctp_sock_dump
> > is still alive when calling lock_sock().
>
> Could you clarify a little more where the RCU lock is held, it's not
> obvious.
sure, will do.

>
> > Note that delaying endpint free won't delay the port release, as the port
> > release happens in sctp_endpoint_destroy() before calling call_rcu().
> > Also, freeing endpoint by call_rcu() makes it safe to access the sk by
> > asoc->base.sk in sctp_assocs_seq_show() and sctp_rcv().
> >
> > Thanks Jones to bring this issue up.
> >
> > Reported-by: syzbot+9276d76e83e3bcde6c99@...kaller.appspotmail.com
> > Reported-by: Lee Jones <lee.jones@...aro.org>
> > Fixes: d25adbeb0cdb ("sctp: fix an use-after-free issue in sctp_sock_dump")
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
>
> > diff --git a/net/sctp/endpointola.c b/net/sctp/endpointola.c
> > index 48c9c2c7602f..81fb97d382d7 100644
> > --- a/net/sctp/endpointola.c
> > +++ b/net/sctp/endpointola.c
> > @@ -184,6 +184,17 @@ void sctp_endpoint_free(struct sctp_endpoint *ep)
> >  }
> >
> >  /* Final destructor for endpoint.  */
> > +static void sctp_endpoint_destroy_rcu(struct rcu_head *head)
> > +{
> > +     struct sctp_endpoint *ep = container_of(head, struct sctp_endpoint, rcu);
> > +     struct sock *sk = ep->base.sk;
> > +
> > +     sctp_sk(sk)->ep = NULL;
> > +     sock_put(sk);
> > +
> > +     SCTP_DBG_OBJCNT_DEC(ep);
> > +}
> > +
> >  static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
> >  {
> >       struct sock *sk;
> > @@ -213,18 +224,13 @@ static void sctp_endpoint_destroy(struct sctp_endpoint *ep)
> >       if (sctp_sk(sk)->bind_hash)
> >               sctp_put_port(sk);
> >
> > -     sctp_sk(sk)->ep = NULL;
> > -     /* Give up our hold on the sock */
> > -     sock_put(sk);
> > -
> > -     kfree(ep);
>
> where does this kfree() go after the change?
Good catch, somehow I didn't move it into sctp_endpoint_destroy_rcu().
will post v2.

Thanks!

>
> > -     SCTP_DBG_OBJCNT_DEC(ep);
> > +     call_rcu(&ep->rcu, sctp_endpoint_destroy_rcu);
> >  }