netdev - Re: [Question] Should `CAP_NET_ADMIN` be needed when opening `/dev/ppp`?

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Date:	Tue, 3 May 2016 12:12:40 +0200
From:	Guillaume Nault <g.nault@...halink.fr>
To:	Wang Shanker <shankerwangmiao@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Question] Should `CAP_NET_ADMIN` be needed when opening
 `/dev/ppp`?

On Sun, May 01, 2016 at 09:38:57PM +0800, Wang Shanker wrote:
> static int ppp_open(struct inode *inode, struct file *file)
> {
> 	/*
> 	 * This could (should?) be enforced by the permissions on /dev/ppp.
> 	 */
> 	if (!capable(CAP_NET_ADMIN))
> 		return -EPERM;
> 	return 0;
> }
> ```
> 
> I wonder why CAP_NET_ADMIN is needed here, rather than leaving it to the
> permission of the device node. If there is no need, I suggest that the
> CAP_NET_ADMIN check be removed.
> 
If this test was removed here, then it'd have to be added again in the
PPPIOCNEWUNIT ioctl, at the very least, because creating a netdevice
should require CAP_NET_ADMIN. Therefore that wouldn't help for your
case.
I don't know why the test was placed in ppp_open() in the first place,
but changing it now would have side effects on user space. So I'd
rather leave the code as is.