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Commit c845acb3 authored by Rainer Weikusat's avatar Rainer Weikusat Committed by David S. Miller
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af_unix: Fix splice-bind deadlock 

On 2015/11/06, Dmitry Vyukov reported a deadlock involving the splice
system call and AF_UNIX sockets,

http://lists.openwall.net/netdev/2015/11/06/24

The situation was analyzed as

(a while ago) A: socketpair()
B: splice() from a pipe to /mnt/regular_file
	does sb_start_write() on /mnt
C: try to freeze /mnt
	wait for B to finish with /mnt
A: bind() try to bind our socket to /mnt/new_socket_name
	lock our socket, see it not bound yet
	decide that it needs to create something in /mnt
	try to do sb_start_write() on /mnt, block (it's
	waiting for C).
D: splice() from the same pipe to our socket
	lock the pipe, see that socket is connected
	try to lock the socket, block waiting for A
B:	get around to actually feeding a chunk from
	pipe to file, try to lock the pipe.  Deadlock.

on 2015/11/10 by Al Viro,

http://lists.openwall.net/netdev/2015/11/10/4



The patch fixes this by removing the kern_path_create related code from
unix_mknod and executing it as part of unix_bind prior acquiring the
readlock of the socket in question. This means that A (as used above)
will sb_start_write on /mnt before it acquires the readlock, hence, it
won't indirectly block B which first did a sb_start_write and then
waited for a thread trying to acquire the readlock. Consequently, A
being blocked by C waiting for B won't cause a deadlock anymore
(effectively, both A and B acquire two locks in opposite order in the
situation described above).

Dmitry Vyukov(<dvyukov@google.com>) tested the original patch.

Signed-off-by: default avatarRainer Weikusat <rweikusat@mobileactivedefense.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent b5bdacf3

net/unix/af_unix.c

+40 −26
Original line number Diff line number Diff line
@@ -953,32 +953,20 @@ static struct sock *unix_find_other(struct net *net, 
	return NULL;

}



static int unix_mknod(const char *sun_path, umode_t mode, struct path *res)

static int unix_mknod(struct dentry *dentry, struct path *path, umode_t mode,

		      struct path *res)

{

	struct dentry *dentry;

	struct path path;

	int err = 0;

	/*

	 * Get the parent directory, calculate the hash for last

	 * component.

	 */

	dentry = kern_path_create(AT_FDCWD, sun_path, &path, 0);

	err = PTR_ERR(dentry);

	if (IS_ERR(dentry))

		return err;

	int err;



	/*

	 * All right, let's create it.

	 */

	err = security_path_mknod(&path, dentry, mode, 0);

	err = security_path_mknod(path, dentry, mode, 0);

	if (!err) {

		err = vfs_mknod(d_inode(path.dentry), dentry, mode, 0);

		err = vfs_mknod(d_inode(path->dentry), dentry, mode, 0);

		if (!err) {

			res->mnt = mntget(path.mnt);

			res->mnt = mntget(path->mnt);

			res->dentry = dget(dentry);

		}

	}

	done_path_create(&path, dentry);



	return err;

}
@@ -989,10 +977,12 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) 
	struct unix_sock *u = unix_sk(sk);

	struct sockaddr_un *sunaddr = (struct sockaddr_un *)uaddr;

	char *sun_path = sunaddr->sun_path;

	int err;

	int err, name_err;

	unsigned int hash;

	struct unix_address *addr;

	struct hlist_head *list;

	struct path path;

	struct dentry *dentry;



	err = -EINVAL;

	if (sunaddr->sun_family != AF_UNIX)
@@ -1008,14 +998,34 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) 
		goto out;

	addr_len = err;



	name_err = 0;

	dentry = NULL;

	if (sun_path[0]) {

		/* Get the parent directory, calculate the hash for last

		 * component.

		 */

		dentry = kern_path_create(AT_FDCWD, sun_path, &path, 0);



		if (IS_ERR(dentry)) {

			/* delay report until after 'already bound' check */

			name_err = PTR_ERR(dentry);

			dentry = NULL;

		}

	}



	err = mutex_lock_interruptible(&u->readlock);

	if (err)

		goto out;

		goto out_path;



	err = -EINVAL;

	if (u->addr)

		goto out_up;



	if (name_err) {

		err = name_err == -EEXIST ? -EADDRINUSE : name_err;

		goto out_up;

	}



	err = -ENOMEM;

	addr = kmalloc(sizeof(*addr)+addr_len, GFP_KERNEL);

	if (!addr)
@@ -1026,11 +1036,11 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) 
	addr->hash = hash ^ sk->sk_type;

	atomic_set(&addr->refcnt, 1);



	if (sun_path[0]) {

		struct path path;

	if (dentry) {

		struct path u_path;

		umode_t mode = S_IFSOCK |

		       (SOCK_INODE(sock)->i_mode & ~current_umask());

		err = unix_mknod(sun_path, mode, &path);

		err = unix_mknod(dentry, &path, mode, &u_path);

		if (err) {

			if (err == -EEXIST)

				err = -EADDRINUSE;
@@ -1038,9 +1048,9 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) 
			goto out_up;

		}

		addr->hash = UNIX_HASH_SIZE;

		hash = d_backing_inode(path.dentry)->i_ino & (UNIX_HASH_SIZE-1);

		hash = d_backing_inode(dentry)->i_ino & (UNIX_HASH_SIZE - 1);

		spin_lock(&unix_table_lock);

		u->path = path;

		u->path = u_path;

		list = &unix_socket_table[hash];

	} else {

		spin_lock(&unix_table_lock);
@@ -1063,6 +1073,10 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len) 
	spin_unlock(&unix_table_lock);

out_up:

	mutex_unlock(&u->readlock);

out_path:

	if (dentry)

		done_path_create(&path, dentry);



out:

	return err;

}