linux-kernel - [PATCH 3.12 095/101] ipx: fix locking regression in ipx_sendmsg and ipx

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Message-Id: <f39a6f3d592372cd369b6a04a8562f3e6f41ee47.1417605922.git.jslaby@suse.cz>
Date:	Wed,  3 Dec 2014 12:26:57 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Jiri Bohac <jbohac@...e.cz>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 095/101] ipx: fix locking regression in ipx_sendmsg and ipx_recvmsg

From: Jiri Bohac <jbohac@...e.cz>

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
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@...e.cz>
Reviewed-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 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;
 
 	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;
 }
 
-- 
2.1.3

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