netdev - Re: [PATCH v3 2/3] genetlink: disallow subscribing to unknown mcast groups

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Message-Id: <20150116.172038.1258592815037521446.davem@davemloft.net>
Date:	Fri, 16 Jan 2015 17:20:38 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	johannes@...solutions.net
Cc:	netdev@...r.kernel.org, jeff.layton@...marydata.com,
	sedat.dilek@...il.com, johannes.berg@...el.com
Subject: Re: [PATCH v3 2/3] genetlink: disallow subscribing to unknown
 mcast groups

From: Johannes Berg <johannes@...solutions.net>
Date: Fri, 16 Jan 2015 11:37:13 +0100

> From: Johannes Berg <johannes.berg@...el.com>
> 
> Jeff Layton reported that he could trigger the multicast unbind warning
> in generic netlink using trinity. I originally thought it was a race
> condition between unregistering the generic netlink family and closing
> the socket, but there's a far simpler explanation: genetlink currently
> allows subscribing to groups that don't (yet) exist, and the warning is
> triggered when unsubscribing again while the group still doesn't exist.
> 
> Originally, I had a warning in the subscribe case and accepted it out of
> userspace API concerns, but the warning was of course wrong and removed
> later.
> 
> However, I now think that allowing userspace to subscribe to groups that
> don't exist is wrong and could possibly become a security problem:
> Consider a (new) genetlink family implementing a permission check in
> the mcast_bind() function similar to the like the audit code does today;
> it would be possible to bypass the permission check by guessing the ID
> and subscribing to the group it exists. This is only possible in case a
> family like that would be dynamically loaded, but it doesn't seem like a
> huge stretch, for example wireless may be loaded when you plug in a USB
> device.
> 
> To avoid this reject such subscription attempts.
> 
> If this ends up causing userspace issues we may need to add a workaround
> in af_netlink to deny such requests but not return an error.
> 
> Reported-by: Jeff Layton <jeff.layton@...marydata.com>
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Applied.
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