netdev - Re: [Bugme-new] [Bug 12515] New: possible circular locking #0: (sk_lock-AF_PACKET){--..}, at: [<c1279838>] sock

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Date:	Fri, 30 Jan 2009 23:49:47 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>
Cc:	akpm@...ux-foundation.org, netdev@...r.kernel.org,
	bugme-daemon@...zilla.kernel.org, vegard.nossum@...il.com,
	a.p.zijlstra@...llo.nl, jarkao2@...il.com, davem@...emloft.net
Subject: Re: [Bugme-new] [Bug 12515] New: possible circular locking #0:
	(sk_lock-AF_PACKET){--..}, at: [<c1279838>]
	sock_setsockopt+0x12b/0x4a4

On Fri, Jan 30, 2009 at 05:12:50PM +1100, Herbert Xu wrote:
> 
> Well, doing the copy under sk_lock is pretty common through all
> protocols.  So I think it'd be safer to change the other path,
> which is doing the odd thing here, i.e., ->mmap() grabbing the
> socket lock while holding mmap_sem.
> 
> In fact, it would appear that we don't really need the socket lock
> in ->mmap() since it only needs to ensure that pg_vec* doesn't
> get yanked or changed.  So this patch should work:
> 
> packet: Avoid lock_sock in mmap handler

Dave pointed out that a spin lock is illegal for this purpose
as vm_insert_page can do a GFP_KERNEL allocation.  So I've added
a mutex for this.

I've also widened the critical section in packet_set_ring since
we need the mapped check to be within it.

packet: Avoid lock_sock in mmap handler

As the mmap handler gets called under mmap_sem, and we may grab
mmap_sem elsewhere under the socket lock to access user data, we
should avoid grabbing the socket lock in the mmap handler.

Since the only thing we care about in the mmap handler is for
pg_vec* to be invariant, i.e., to exclude packet_set_ring, we
can achieve this by simply using sk_receive_queue.lock.

I resisted the temptation to create a new spin lock because the
mmap path isn't exactly common.

Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 5f94db2..9454d4a 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -77,6 +77,7 @@
 #include <linux/poll.h>
 #include <linux/module.h>
 #include <linux/init.h>
+#include <linux/mutex.h>
 
 #ifdef CONFIG_INET
 #include <net/inet_common.h>
@@ -175,6 +176,7 @@ struct packet_sock {
 #endif
 	struct packet_type	prot_hook;
 	spinlock_t		bind_lock;
+	struct mutex		pg_vec_lock;
 	unsigned int		running:1,	/* prot_hook is attached*/
 				auxdata:1,
 				origdev:1;
@@ -1069,6 +1071,7 @@ static int packet_create(struct net *net, struct socket *sock, int protocol)
 	 */
 
 	spin_lock_init(&po->bind_lock);
+	mutex_init(&po->pg_vec_lock);
 	po->prot_hook.func = packet_rcv;
 
 	if (sock->type == SOCK_PACKET)
@@ -1865,6 +1868,7 @@ static int packet_set_ring(struct sock *sk, struct tpacket_req *req, int closing
 	synchronize_net();
 
 	err = -EBUSY;
+	mutex_lock(&po->pg_vec_lock);
 	if (closing || atomic_read(&po->mapped) == 0) {
 		err = 0;
 #define XC(a, b) ({ __typeof__ ((a)) __t; __t = (a); (a) = (b); __t; })
@@ -1886,6 +1890,7 @@ static int packet_set_ring(struct sock *sk, struct tpacket_req *req, int closing
 		if (atomic_read(&po->mapped))
 			printk(KERN_DEBUG "packet_mmap: vma is busy: %d\n", atomic_read(&po->mapped));
 	}
+	mutex_unlock(&po->pg_vec_lock);
 
 	spin_lock(&po->bind_lock);
 	if (was_running && !po->running) {
@@ -1918,7 +1923,7 @@ static int packet_mmap(struct file *file, struct socket *sock, struct vm_area_st
 
 	size = vma->vm_end - vma->vm_start;
 
-	lock_sock(sk);
+	mutex_lock(&po->pg_vec_lock);
 	if (po->pg_vec == NULL)
 		goto out;
 	if (size != po->pg_vec_len*po->pg_vec_pages*PAGE_SIZE)
@@ -1941,7 +1946,7 @@ static int packet_mmap(struct file *file, struct socket *sock, struct vm_area_st
 	err = 0;
 
 out:
-	release_sock(sk);
+	mutex_unlock(&po->pg_vec_lock);
 	return err;
 }
 #endif

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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