Subject: Re: [RFC PATCH] network: return errors if we know tcp_connect failed

From: Patrick McHardy <[email-masked]>
Date: Fri, 12 Nov 2010 17:54:29 +0100

Am 12.11.2010 17:35, schrieb David Lamparter:
> 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.

Indeed. We could even make the NF_DROP return value configurable
by encoding it in the verdict.

> 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.

There is no NF_REJECT.

> 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.

Returning NF_DROP results in -EPERM getting reported back. As Eric
noticed, this is ignored for SYN packets.

> 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.

NF_DROP returns -EPERM, the REJECT targets send packets to reject
a connection. Whether this is reported immediately depends on the
error and the protocol in question. Using a TCP reset immediately
resets the connection.