[PATCH 3.12 095/101] ipx: fix locking regression in ipx_sendmsg and ipx_recvmsg

From: Jiri Slaby
Date: Wed Dec 03 2014 - 06:32:16 EST

From: Jiri Bohac <jbohac@xxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


[ 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

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@xxxxxxx>
Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
net/ipx/af_ipx.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipx/af_ipx.c b/net/ipx/af_ipx.c
index e096025b477f..6857ae49dc8c 100644
--- 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;

/* 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,
skb_free_datagram(sk, skb);
- release_sock(sk);
+ if (locked)
+ release_sock(sk);
return rc;


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