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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: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Rainer Weikusat 2016-01-03 18:56:38 +00:00 committed by David S. Miller
parent b5bdacf3bb
commit c845acb324
1 changed files with 40 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

66
net/unix/af_unix.c
View File

@ -953,32 +953,20 @@ fail:
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 @@ out_unlock:
spin_unlock(&unix_table_lock);
out_up:
mutex_unlock(&u->readlock);
out_path:
if (dentry)
done_path_create(&path, dentry);
out:
return err;
}