linux-kernel - Re: [PATCH 30/61] knfsd: Fix race that can disable NFS server.

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Message-ID: <20061101071111.GB543@1wt.eu>
Date:	Wed, 1 Nov 2006 08:11:11 +0100
From:	Willy Tarreau <w@....eu>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	linux-kernel@...r.kernel.org, NeilBrown <neilb@...e.de>
Subject: Re: [PATCH 30/61] knfsd: Fix race that can disable NFS server.

On Tue, Oct 31, 2006 at 09:34:10PM -0800, Chris Wright wrote:
> -stable review patch.  If anyone has any objections, please let us know.
> ------------------
> 
> From: NeilBrown <neilb@...e.de>
> 
> This is a long standing bug that seems to have only recently become
> apparent, presumably due to increasing use of NFS over TCP - many
> distros seem to be making it the default.
> 
> The SK_CONN bit gets set when a listening socket may be ready
> for an accept, just as SK_DATA is set when data may be available.
> 
> It is entirely possible for svc_tcp_accept to be called with neither
> of these set.  It doesn't happen often but there is a small race in
> svc_sock_enqueue as SK_CONN and SK_DATA are tested outside the
> spin_lock.  They could be cleared immediately after the test and
> before the lock is gained.
> 
> This normally shouldn't be a problem.  The sockets are non-blocking so
> trying to read() or accept() when ther is nothing to do is not a problem.
> 
> However: svc_tcp_recvfrom makes the decision "Should I accept() or
> should I read()" based on whether SK_CONN is set or not.  This usually
> works but is not safe.  The decision should be based on whether it is
> a TCP_LISTEN socket or a TCP_CONNECTED socket.
> 
> 
> Signed-off-by: Neil Brown <neilb@...e.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
> Signed-off-by: Chris Wright <chrisw@...s-sol.org>
> 
> ---
>  net/sunrpc/svcsock.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- linux-2.6.18.1.orig/net/sunrpc/svcsock.c
> +++ linux-2.6.18.1/net/sunrpc/svcsock.c
> @@ -902,7 +902,7 @@ svc_tcp_recvfrom(struct svc_rqst *rqstp)
>  		return 0;
>  	}
>  
> -	if (test_bit(SK_CONN, &svsk->sk_flags)) {
> +	if (svsk->sk_sk->sk_state == TCP_LISTEN) {
>  		svc_tcp_accept(svsk);
>  		svc_sock_received(svsk);
>  		return 0;

This one seems valid for 2.4 too. Neil, do you confirm ?

Willy

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