CVE-2018-12232: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition') (CWE-362)
value: In net/socket.c in the Linux kernel through 4.17.1, there is a race condition between fchownat and close in cases where they target the same socket file descriptor, related to the sock_close and sockfs_setattr functions. fchownat does not increment the file descriptor reference count, which allows close to set the socket to NULL during fchownat's execution, leading to a NULL pointer dereference and system crash.
CWE description: The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
CVSS v3 base score: 5.9
Published: 2018-06-12T12:29Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/socket.c
Commit date: 2018-06-10 12:25:53 -0700
Commit message: socket: close race condition between sock_close() and sockfs_setattr()

fchownat() doesn't even hold refcnt of fd until it figures out
fd is really needed (otherwise is ignored) and releases it after
it resolves the path. This means sock_close() could race with
sockfs_setattr(), which leads to a NULL pointer dereference
since typically we set sock->sk to NULL in ->release().

As pointed out by Al, this is unique to sockfs. So we can fix this
in socket layer by acquiring inode_lock in sock_close() and
checking against NULL in sockfs_setattr().

sock_release() is called in many places, only the sock_close()
path matters here. And fortunately, this should not affect normal
sock_close() as it is only called when the last fd refcnt is gone.
It only affects sock_close() with a parallel sockfs_setattr() in
progress, which is not common.

Fixes: 86741ec25462 ("net: core: Add a UID field to struct sock.")
Reported-by: shankarapailoor <shankarapailoor@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Lorenzo Colitti <lorenzo@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/socket.c b/net/socket.c
index af57d85bcb48..8a109012608a 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -539,11 +539,14 @@ static int sockfs_setattr(struct dentry *dentry, struct iattr *iattr)
 	int err = simple_setattr(dentry, iattr);
 
 	if (!err && (iattr->ia_valid & ATTR_UID)) {
 		struct socket *sock = SOCKET_I(d_inode(dentry));
 
-		sock->sk->sk_uid = iattr->ia_uid;
+		if (sock->sk)
+			sock->sk->sk_uid = iattr->ia_uid;
+		else
+			err = -ENOENT;
 	}
 
 	return err;
 }
 
@@ -588,16 +591,20 @@ EXPORT_SYMBOL(sock_alloc);
  *	The socket is released from the protocol stack if it has a release
  *	callback, and the inode is then released if the socket is bound to
  *	an inode not a file.
  */
 
-void sock_release(struct socket *sock)
+static void __sock_release(struct socket *sock, struct inode *inode)
 {
 	if (sock->ops) {
 		struct module *owner = sock->ops->owner;
 
+		if (inode)
+			inode_lock(inode);
 		sock->ops->release(sock);
+		if (inode)
+			inode_unlock(inode);
 		sock->ops = NULL;
 		module_put(owner);
 	}
 
 	if (rcu_dereference_protected(sock->wq, 1)->fasync_list)
@@ -607,10 +614,15 @@ void sock_release(struct socket *sock)
 		iput(SOCK_INODE(sock));
 		return;
 	}
 	sock->file = NULL;
 }
+
+void sock_release(struct socket *sock)
+{
+	__sock_release(sock, NULL);
+}
 EXPORT_SYMBOL(sock_release);
 
 void __sock_tx_timestamp(__u16 tsflags, __u8 *tx_flags)
 {
 	u8 flags = *tx_flags;
@@ -1169,11 +1181,11 @@ static int sock_mmap(struct file *file, struct vm_area_struct *vma)
 	return sock->ops->mmap(file, sock, vma);
 }
 
 static int sock_close(struct inode *inode, struct file *filp)
 {
-	sock_release(SOCKET_I(inode));
+	__sock_release(SOCKET_I(inode), inode);
 	return 0;
 }
 
 /*
  *	Update the socket async list

Vulnerable code: 		sock->sk->sk_uid = iattr->ia_uid;
void sock_release(struct socket *sock)
	sock_release(SOCKET_I(inode));
Fixed code: 		if (sock->sk)
			sock->sk->sk_uid = iattr->ia_uid;
		else
			err = -ENOENT;
static void __sock_release(struct socket *sock, struct inode *inode)
		if (inode)
			inode_lock(inode);
		if (inode)
			inode_unlock(inode);

void sock_release(struct socket *sock)
{
	__sock_release(sock, NULL);
}
	__sock_release(SOCKET_I(inode), inode);