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

[Xin Long <lucien.xin@gmail.com>]

On Tue, Nov 14, 2017 at 8:43 PM, Neil Horman <nhorman@tuxdriver.com> wrote:
> 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@tuxdriver.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@google.com>
>> >> Signed-off-by: Xin Long <lucien.xin@gmail.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.
we can't do that, as nowhere holds new_assoc, it may be already destroyed.
to dereference it (new_assoc->base.dead) will cause use-after-free panic.

I may do like this:
                err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
-               if (err)
+               if (err) {
+                       if (err == -ESRCH) {
+                               /* asoc is already dead; */
+                               new_asoc = NULL;
+                               err = -EPIPE;
+                       }
                        goto out_free;
+               }

agree ?

>
> Neil
>> >
>> > Neil
>> >
>>