BACKPORT: 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>

(cherry picked from commit 6d8c50dcb029872b298eea68cc6209c866fd3e14)
(use i_mutex directly instead of inode_lock())
Bug: 125367761
Test: used reproducer from Ied4bbca5c7eb80c201fec6e0aabc95c24acc1b59
Change-Id: Icf0824632734bd9d50dda65f87fc130c21b137ac
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 15 insertions, 3 deletions

diff --git a/net/socket.c b/net/socket.c
index b7f357804..2e6a155e2 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -524,7 +524,10 @@ static int sockfs_setattr(struct dentry *dentry, struct iattr *iattr)
 	if (!err && (iattr->ia_valid & ATTR_UID)) {
 		struct socket *sock = SOCKET_I(dentry->d_inode);
 
-		sock->sk->sk_uid = iattr->ia_uid;
+		if (sock->sk)
+			sock->sk->sk_uid = iattr->ia_uid;
+		else
+			err = -ENOENT;
 	}
 
 	return err;
@@ -592,12 +595,16 @@ const struct file_operations bad_sock_fops = {
  *	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)
+			mutex_lock(&inode->i_mutex);
 		sock->ops->release(sock);
+		if (inode)
+			mutex_unlock(&inode->i_mutex);
 		sock->ops = NULL;
 		module_put(owner);
 	}
@@ -615,6 +622,11 @@ void sock_release(struct socket *sock)
 	}
 	sock->file = NULL;
 }
+
+void sock_release(struct socket *sock)
+{
+	__sock_release(sock, NULL);
+}
 EXPORT_SYMBOL(sock_release);
 
 void sock_tx_timestamp(struct sock *sk, __u8 *tx_flags)
@@ -1199,7 +1211,7 @@ static int sock_close(struct inode *inode, struct file *filp)
 		   pr_debug(KERN_INFO "[mtk_net][socekt]socket_close[%lu] \n",inode->i_ino); 
 		}
 	#endif
-	sock_release(SOCKET_I(inode));
+	__sock_release(SOCKET_I(inode), inode);
 	
 	return 0;
 }