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

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Date:   Mon, 13 Nov 2017 23:29:49 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Neil Horman <nhorman@...driver.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 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;

>
> Neil
>