[PATCH] knfsd: Fix race that can disable NFS server.

[PATCH] knfsd: Fix race that can disable NFS server.

[NeilBrown]

This patch is suitable for just about any 2.6 kernel.
It should go in 2.6.19 and 2.6.18.2 and possible even the .17 and .16
stable series.

### Comments for Changeset

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@suse.de>

### Diffstat output
 ./net/sunrpc/svcsock.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff .prev/net/sunrpc/svcsock.c ./net/sunrpc/svcsock.c
--- .prev/net/sunrpc/svcsock.c	2006-10-20 11:49:18.000000000 +1000
+++ ./net/sunrpc/svcsock.c	2006-10-20 11:49:47.000000000 +1000
@@ -1002,7 +1002,7 @@ 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;

Re: [PATCH] knfsd: Fix race that can disable NFS server.

[Adrian Bunk]

On Fri, Oct 20, 2006 at 11:52:44AM +1000, NeilBrown wrote:

> This patch is suitable for just about any 2.6 kernel.
> It should go in 2.6.19 and 2.6.18.2 and possible even the .17 and .16
> stable series.


Thanks, applied to 2.6.16.


> ### Comments for Changeset
> 
> 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@suse.de>
> 
> ### Diffstat output
>  ./net/sunrpc/svcsock.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff .prev/net/sunrpc/svcsock.c ./net/sunrpc/svcsock.c
> --- .prev/net/sunrpc/svcsock.c	2006-10-20 11:49:18.000000000 +1000
> +++ ./net/sunrpc/svcsock.c	2006-10-20 11:49:47.000000000 +1000
> @@ -1002,7 +1002,7 @@ 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;

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed


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