netdev - Re: [PATCH net] net: unix: inherit SOCK_PASS{CRED,SEC} flags from socket to fix race

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Message-Id: <20131019.185057.1209480553711126452.davem@davemloft.net>
Date:	Sat, 19 Oct 2013 18:50:57 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	dborkman@...hat.com
Cc:	netdev@...r.kernel.org, edumazet@...gle.com, ebiederm@...ssion.com
Subject: Re: [PATCH net] net: unix: inherit SOCK_PASS{CRED,SEC} flags from
 socket to fix race

From: Daniel Borkmann <dborkman@...hat.com>
Date: Thu, 17 Oct 2013 22:51:31 +0200

> In the case of credentials passing in unix stream sockets (dgram
> sockets seem not affected), we get a rather sparse race after
> commit 16e5726 ("af_unix: dont send SCM_CREDENTIALS by default").
> 
> We have a stream server on receiver side that requests credential
> passing from senders (e.g. nc -U). Since we need to set SO_PASSCRED
> on each spawned/accepted socket on server side to 1 first (as it's
> not inherited), it can happen that in the time between accept() and
> setsockopt() we get interrupted, the sender is being scheduled and
> continues with passing data to our receiver. At that time SO_PASSCRED
> is neither set on sender nor receiver side, hence in cmsg's
> SCM_CREDENTIALS we get eventually pid:0, uid:65534, gid:65534
> (== overflow{u,g}id) instead of what we actually would like to see.
> 
> On the sender side, here nc -U, the tests in maybe_add_creds()
> invoked through unix_stream_sendmsg() would fail, as at that exact
> time, as mentioned, the sender has neither SO_PASSCRED on his side
> nor sees it on the server side, and we have a valid 'other' socket
> in place. Thus, sender believes it would just look like a normal
> connection, not needing/requesting SO_PASSCRED at that time.
> 
> As reverting 16e5726 would not be an option due to the significant
> performance regression reported when having creds always passed,
> one way/trade-off to prevent that would be to set SO_PASSCRED on
> the listener socket and allow inheriting these flags to the spawned
> socket on server side in accept(). It seems also logical to do so
> if we'd tell the listener socket to pass those flags onwards, and
> would fix the race.
> 
> Before, strace:
> 
> recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"blub\n", 4096}],
>         msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET,
>         cmsg_type=SCM_CREDENTIALS{pid=0, uid=65534, gid=65534}},
>         msg_flags=0}, 0) = 5
> 
> After, strace:
> 
> recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"blub\n", 4096}],
>         msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET,
>         cmsg_type=SCM_CREDENTIALS{pid=11580, uid=1000, gid=1000}},
>         msg_flags=0}, 0) = 5
> 
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>

Applied and queued up for -stable, thanks Daniel.
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