On Tue, Jul 14, 2015 at 02:13:24PM -0300, Marcelo Ricardo Leitner wrote:
> SCTP has this operation to peel off associations from a given socket and
> create a new socket using this association. We currently have two ways
> to use this operation:
> - via getsockopt(), on which it will also create and return a file
> descriptor for this new socket
> - via sctp_do_peeloff(), which is for kernel only
>
> The caveat with using sctp_do_peeloff() directly is that it creates a
> dependency to SCTP module, while all other operations are handled via
> kernel_{socket,sendmsg,getsockopt...}() interface. This causes the
> kernel to load SCTP module even when it's not really used.
>
> This patch then creates a new sockopt that is to be used only by kernel
> users of this protocol. This new sockopt will not allocate a file
> descriptor but instead just return the socket pointer directly.
>
> Kernel users are actually identified by if the parent socket has or not
> a fd attached to it. If not, it's a kernel a user.
>
> If called by an user application, it will just return -EPERM.
>
> Even though it's not intended for user applications, it's listed under
> uapi header. That's because hidding this wouldn't add any extra security
> and to keep the sockopt list in one place, so it's easy to check
> available numbers to use.
>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
> ---
> include/uapi/linux/sctp.h | 12 ++++++++++++
> net/sctp/socket.c | 37 +++++++++++++++++++++++++++++++++++++
> 2 files changed, 49 insertions(+)
>
> diff --git a/include/uapi/linux/sctp.h b/include/uapi/linux/sctp.h
> index ce70fe6b45df3e841c35accbdb6379c16563893c..b3aad3ce456ab3c1ebf4d81fdb7269ba40b3d92a 100644
> --- a/include/uapi/linux/sctp.h
> +++ b/include/uapi/linux/sctp.h
> @@ -105,6 +105,10 @@ typedef __s32 sctp_assoc_t;
> #define SCTP_SOCKOPT_BINDX_ADD 100 /* BINDX requests for adding addrs */
> #define SCTP_SOCKOPT_BINDX_REM 101 /* BINDX requests for removing addrs. */
> #define SCTP_SOCKOPT_PEELOFF 102 /* peel off association. */
> +#define SCTP_SOCKOPT_PEELOFF_KERNEL 103 /* peel off association.
> + * only valid for kernel
> + * users
> + */
> /* Options 104-106 are deprecated and removed. Do not use this space */
> #define SCTP_SOCKOPT_CONNECTX_OLD 107 /* CONNECTX old requests. */
> #define SCTP_GET_PEER_ADDRS 108 /* Get all peer address. */
> @@ -892,6 +896,14 @@ typedef struct {
> int sd;
> } sctp_peeloff_arg_t;
>
> +/* This is the union that is passed as an argument(optval) to
> + * getsockopt(SCTP_SOCKOPT_PEELOFF_KERNEL).
> + */
> +typedef union {
> + sctp_assoc_t associd;
> + struct socket *socket;
> +} sctp_peeloff_kernel_arg_t;
> +
> /*
> * Peer Address Thresholds socket option
> */
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index f1a65398f3118ab5d3a884e9c875620560e6b5ef..7968de7a1aeabd5cd0a0398461dbf2081bd4c5b7 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -4504,6 +4504,39 @@ out:
> return retval;
> }
>
> +static int sctp_getsockopt_peeloff_kernel(struct sock *sk, int len,
> + char __user *optval, int __user *optlen)
> +{
> + sctp_peeloff_kernel_arg_t peeloff;
> + struct socket *newsock;
> + int retval = 0;
> +
> + /* We only allow this operation if parent socket also hadn't a
> + * file descriptor allocated to it, mainly as a way to make sure
> + * that this is really a kernel socket.
> + */
> + if (sk->sk_socket->file)
> + return -EPERM;
> +
> + if (len < sizeof(sctp_peeloff_kernel_arg_t))
> + return -EINVAL;
> + len = sizeof(sctp_peeloff_kernel_arg_t);
> + if (copy_from_user(&peeloff, optval, len))
> + return -EFAULT;
> +
> + retval = sctp_do_peeloff(sk, peeloff.associd, &newsock);
> + if (retval < 0)
> + goto out;
> +
> + peeloff.socket = newsock;
> + if (copy_to_user(optval, &peeloff, len)) {
> + sock_release(newsock);
> + return -EFAULT;
> + }
> +out:
> + return retval;
> +}
> +
> /* 7.1.13 Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)
> *
> * Applications can enable or disable heartbeats for any peer address of
> @@ -5991,6 +6024,10 @@ static int sctp_getsockopt(struct sock *sk, int level, int optname,
> case SCTP_SOCKOPT_PEELOFF:
> retval = sctp_getsockopt_peeloff(sk, len, optval, optlen);
> break;
> + case SCTP_SOCKOPT_PEELOFF_KERNEL:
> + retval = sctp_getsockopt_peeloff_kernel(sk, len, optval,
> + optlen);
> + break;
Do we need to do something here to prevent user space calls from inadvertently
accessing this option?
Neil
> case SCTP_PEER_ADDR_PARAMS:
> retval = sctp_getsockopt_peer_addr_params(sk, len, optval,
> optlen);
> --
> 2.4.1
>
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