netdev - Re: [RFC PATCH] network: return errors if we know tcp

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Message-ID: <20101112163543.GB122902@jupiter.n2.diac24.net>
Date:	Fri, 12 Nov 2010 17:35:43 +0100
From:	David Lamparter <equinox@...c24.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Eric Paris <eparis@...hat.com>, Hua Zhong <hzhong@...il.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kuznet@....inr.ac.ru, pekkas@...core.fi,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
	paul.moore@...com
Subject: Re: [RFC PATCH] network: return errors if we know tcp_connect failed

On Fri, Nov 12, 2010 at 05:15:32PM +0100, Eric Dumazet wrote:
> Le vendredi 12 novembre 2010 à 11:08 -0500, Eric Paris a écrit :
> 
> > 2) What should the generic TCP code (tcp_connect()) do if the skb failed
> > to send.  Should it return error codes back up the stack somehow or
> > should they continue to be ignored?  Obviously continuing to just ignore
> > information we have doesn't make me happy (otherwise I wouldn't have
> > started scratching this itch).  But the point about ENOBUFS is well
> > taken.  Maybe I should make tcp_connect(), or the caller to
> > tcp_connect() more intelligent about specific error codes?
> > 
> > I'm looking for a path forward.  If SELinux is rejecting the SYN packets
> > on connect() I want to pass that info to userspace rather than just
> > hanging.  What's the best way to accomplish that?
> > 
> 
> Eric, if you can differentiate a permanent reject, instead of a
> temporary one (congestion, or rate limiting, or ENOBUF, or ...), then
> yes, you could make tcp_connect() report to user the permanent error,
> and ignore the temporary one.

If the netfilter targets DROP/REJECT match the NF_DROP/NF_REJECT
counterparts, which i guess they do but i didn't read the source ;),
then SELinux should use NF_REJECT in my opinion.

NF_DROP does exactly what the name says, it drops the packet aka
basically puts it in /dev/null. As with writing to /dev/null, you don't
get an error for that. Even more, if in the meantime the DROP rule does
not match anymore, the 2nd or 3rd SYN from the connect() can come
through and establish a connection (think of "-m statistic" & co.)

This is very different from REJECT.

If REJECT doesn't immediately get reported to the application, that *is*
a bug, but last time i checked i got EPERM immediately. I would fix
SELinux to use the same mechanism.


-David

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