ipx: fix locking regression in ipx_sendmsg and ipx_recvmsg

[ Upstream commit 01462405f0c093b2f8dfddafcadcda6c9e4c5cdf ]

This fixes an old regression introduced by commit
b0d0d915 (ipx: remove the BKL).

When a recvmsg syscall blocks waiting for new data, no data can be sent on the
same socket with sendmsg because ipx_recvmsg() sleeps with the socket locked.

This breaks mars-nwe (NetWare emulator):
- the ncpserv process reads the request using recvmsg
- ncpserv forks and spawns nwconn
- ncpserv calls a (blocking) recvmsg and waits for new requests
- nwconn deadlocks in sendmsg on the same socket

Commit b0d0d915 has simply replaced BKL locking with
lock_sock/release_sock. Unlike now, BKL got unlocked while
sleeping, so a blocking recvmsg did not block a concurrent
sendmsg.

Only keep the socket locked while actually working with the socket data and
release it prior to calling skb_recv_datagram().

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/ipx/af_ipx.c b/net/ipx/af_ipx.c
index e0897377b3b44bd0d7d3ebedaea9f669c6fcc2cb..2665bf4b8d052e08ccd618a0d1a249c44b13ba25 100644 (file)
--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -1778,6 +1778,7 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
        struct ipxhdr *ipx = NULL;
        struct sk_buff *skb;
        int copied, rc;
+       bool locked = true;
 
        lock_sock(sk);
        /* put the autobinding in */
@@ -1804,6 +1805,8 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
        if (sock_flag(sk, SOCK_ZAPPED))
                goto out;
 
+       release_sock(sk);
+       locked = false;
        skb = skb_recv_datagram(sk, flags & ~MSG_DONTWAIT,
                                flags & MSG_DONTWAIT, &rc);
        if (!skb)
@@ -1837,7 +1840,8 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
 out_free:
        skb_free_datagram(sk, skb);
 out:
-       release_sock(sk);
+       if (locked)
+               release_sock(sk);
        return rc;
 }