netdev - RE: [PATCH v2 1/2] sctp: add new getsockopt option SCTP_SOCKOPT_PEELOFF

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Date:	Wed, 22 Jul 2015 13:13:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	Neil Horman <nhorman@...driver.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	"linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] sctp: add new getsockopt option
 SCTP_SOCKOPT_PEELOFF_KERNEL

From: Marcelo Ricardo Leitner
> Sent: 14 July 2015 18:13
> 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@...il.com>
...
> +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;

You can't need copy_from_user() here, the buffer would surely be kernel.

	David

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