netdev - Re: [PATCH] fix race in AF

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Date:	Sat, 2 Jun 2007 19:11:48 -0300
From:	"Arnaldo Carvalho de Melo" <acme@...stprotocols.net>
To:	"Miklos Szeredi" <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix race in AF_UNIX

On 6/2/07, Miklos Szeredi <miklos@...redi.hu> wrote:
> From: Miklos Szeredi <mszeredi@...e.cz>
>
> A recv() on an AF_UNIX, SOCK_STREAM socket can race with a
> send()+close() on the peer, causing recv() to return zero, even though
> the sent data should be received.
>
> This happens if the send() and the close() is performed between
> skb_dequeue() and checking sk->sk_shutdown in unix_stream_recvmsg():
>
> process A  skb_dequeue() returns NULL, there's no data in the socket queue
> process B  new data is inserted onto the queue by unix_stream_sendmsg()
> process B  sk->sk_shutdown is set to SHUTDOWN_MASK by unix_release_sock()
> process A  sk->sk_shutdown is checked, unix_release_sock() returns zero
>
> I'm surprised nobody noticed this, it's not hard to trigger.  Maybe
> it's just (un)luck with the timing.
>
> It's possible to work around this bug in userspace, by retrying the
> recv() once in case of a zero return value.
>
> Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
> ---
>
> Index: linux-2.6.22-rc2/net/unix/af_unix.c
> ===================================================================
> --- linux-2.6.22-rc2.orig/net/unix/af_unix.c    2007-06-02 23:45:47.000000000 +0200
> +++ linux-2.6.22-rc2/net/unix/af_unix.c 2007-06-02 23:45:49.000000000 +0200
> @@ -1711,20 +1711,23 @@ static int unix_stream_recvmsg(struct ki
>                 int chunk;
>                 struct sk_buff *skb;
>
> +               unix_state_rlock(sk);

this function doesn't exist anymore, see:

http://git.kernel.org/?p=linux/kernel/git/davem/net-2.6.git;a=commitdiff;h=e97b6936b03dd0a62991f361c048cca38ac00198

There is also another AF_UNIX patch, also related to a race.

- Arnaldo
-
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