[PATCH 3.14 17/73] ipx: fix locking regression in ipx_sendmsg and ipx_recvmsg

Greg Kroah-Hartman
2014-12-05T23:30:19Z

From: Jiri Bohac <jbohac@suse.cz>

[ 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>
---
net/ipx/af_ipx.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

--- a/net/ipx/af_ipx.c
+++ b/net/ipx/af_ipx.c
@@ -1763,6 +1763,7 @@ static int ipx_recvmsg(struct kiocb *ioc
struct ipxhdr *ipx = NULL;
struct sk_buff *skb;
int copied, rc;
+ bool locked = true;

lock_sock(sk);
/* put the autobinding in */
@@ -1789,6 +1790,8 @@ static int ipx_recvmsg(struct kiocb *ioc
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)
@@ -1822,7 +1825,8 @@ static int ipx_recvmsg(struct kiocb *ioc
out_free:
skb_free_datagram(sk, skb);
out:
- release_sock(sk);
+ if (locked)
+ release_sock(sk);
return rc;
}