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 <dvyu...@google.com>
> Signed-off-by: Xin Long <lucien....@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.
Neil
