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Message-ID: <87d1ti1ogp.fsf@doppelsaurus.mobileactivedefense.com>
Date: Sun, 03 Jan 2016 18:56:38 +0000
From: Rainer Weikusat <rweikusat@...ileactivedefense.com>
To: David Miller <davem@...emloft.net>
Cc: <dvyukov@...gle.com>, <netdev@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] 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@...gle.com>) tested the original patch.
Signed-off-by: Rainer Weikusat <rweikusat@...ileactivedefense.com>
---
This fixes two 'wrong' error returns, namely, return -EADDRINUSE if
kern_path_create returned -EEXIST but delay returning an error from
kern_path_create until after the u->addr check as the -EINVAL should IMO
take precedence here.
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index b1314c0..e6d3556 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -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;
}
--
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