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 09:46:53 -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@...emloft.net,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, 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 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.

Neil