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 a new mutex.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Martin MOKREJŠ <mmokrejs@ribosome.natur.cuni.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

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