netdev - Re: [PATCH net] sctp: do not free asoc when it is already dead in sctp

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Message-ID: <20171114124328.GA21954@hmswarspite.think-freely.org>
Date:   Tue, 14 Nov 2017 07:43:28 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem <davem@...emloft.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] sctp: do not free asoc when it is already dead in
 sctp_sendmsg

On Mon, Nov 13, 2017 at 11:29:49PM +0800, Xin Long wrote:
> On Mon, Nov 13, 2017 at 10:46 PM, Neil Horman <nhorman@...driver.com> wrote:
> > On Mon, Nov 13, 2017 at 12:43:50PM +0800, Xin Long wrote:
> >> Now in sctp_sendmsg sctp_wait_for_sndbuf could schedule out without
> >> holding sock sk. It means the current asoc can be freed elsewhere,
> >> like when receiving an abort packet.
> >>
> >> If the asoc is just created in sctp_sendmsg and sctp_wait_for_sndbuf
> >> returns err, the asoc will be freed again due to new_asoc is not nil.
> >> An use-after-free issue would be triggered by this.
> >>
> >> This patch is to fix it by setting new_asoc with nil if the asoc is
> >> already dead when cpu schedules back, so that it will not be freed
> >> again in sctp_sendmsg.
> >>
> >> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> >> Signed-off-by: Xin Long <lucien.xin@...il.com>
> >> ---
> >>  net/sctp/socket.c | 15 +++++++++------
> >>  1 file changed, 9 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> >> index 6f45d17..f575976 100644
> >> --- a/net/sctp/socket.c
> >> +++ b/net/sctp/socket.c
> >> @@ -83,8 +83,8 @@
> >>  /* Forward declarations for internal helper functions. */
> >>  static int sctp_writeable(struct sock *sk);
> >>  static void sctp_wfree(struct sk_buff *skb);
> >> -static int sctp_wait_for_sndbuf(struct sctp_association *, long *timeo_p,
> >> -                             size_t msg_len);
> >> +static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >> +                             size_t msg_len, struct sctp_association **new);
> >>  static int sctp_wait_for_packet(struct sock *sk, int *err, long *timeo_p);
> >>  static int sctp_wait_for_connect(struct sctp_association *, long *timeo_p);
> >>  static int sctp_wait_for_accept(struct sock *sk, long timeo);
> >> @@ -1962,7 +1962,7 @@ static int sctp_sendmsg(struct sock *sk, struct msghdr *msg, size_t msg_len)
> >>
> >>       timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
> >>       if (!sctp_wspace(asoc)) {
> >> -             err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
> >> +             err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len, &new_asoc);
> >>               if (err)
> >>                       goto out_free;
> >>       }
> >> @@ -7822,7 +7822,7 @@ void sctp_sock_rfree(struct sk_buff *skb)
> >>
> >>  /* Helper function to wait for space in the sndbuf.  */
> >>  static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >> -                             size_t msg_len)
> >> +                             size_t msg_len, struct sctp_association **new)
> >>  {
> >>       struct sock *sk = asoc->base.sk;
> >>       int err = 0;
> >> @@ -7839,10 +7839,13 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
> >>       for (;;) {
> >>               prepare_to_wait_exclusive(&asoc->wait, &wait,
> >>                                         TASK_INTERRUPTIBLE);
> >> +             if (asoc->base.dead) {
> >> +                     *new = NULL;
> >> +                     goto do_error;
> >> +             }
> >>               if (!*timeo_p)
> >>                       goto do_nonblock;
> >> -             if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING ||
> >> -                 asoc->base.dead)
> >> +             if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)
> >>                       goto do_error;
> >>               if (signal_pending(current))
> >>                       goto do_interrupted;
> >> --
> >> 2.1.0
> >>
> >>
> > Why pass a pointer to a pointer into the wait function?  It seems like you could
> > just check the return code for err == -EPIPE and set the association to null in
> > sctp_sendmsg.  That would avoid passing another parameter, and cut down on some
> > complexity here.
> "if (sk->sk_err || asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)"
> also goes to err == -EPIPE path, with it, the new_asoc are supposed to
> be freed in old codes.
> 
> do you think it's good to not free it now when sk->sk_err or SHUTDOWN_PENDING ?
> or I can add another err path like:
> +do_dead:
> +       err = -ESRCH;
> +       goto out;
> 
That would work, but it also seems just as easy to check in sctp_sendmsg. that
is to say, if sctp_wait_for_sndbuf return -EPIPE sctp_sndmsg jumps to out_free,
where we can check to see if (new_assoc && new_assoc->base.dead) as a gating
factor on free.

Neil
> >
> > Neil
> >
>