Subject: [PATCH 08/12] socket: close race condition between sock_close() and sockfs_setattr()

From: Cong Wang <[email-masked]>
Date: Thu, 7 Jun 2018 13:39:49 -0700

[ Upstream commit 6d8c50dcb029872b298eea68cc6209c866fd3e14 ]

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 <[email-masked]>
Cc: Tetsuo Handa <[email-masked]>
Cc: Lorenzo Colitti <[email-masked]>
Cc: Al Viro <[email-masked]>
Signed-off-by: Cong Wang <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/socket.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index 08847c3b8c39..26b1fab01daf 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -538,7 +538,10 @@ static int sockfs_setattr(struct dentry *dentry, struct iattr *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;
@@ -587,12 +590,16 @@ EXPORT_SYMBOL(sock_alloc);
  *	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);
 	}
@@ -606,6 +613,11 @@ 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(__u16 tsflags, __u8 *tx_flags)
@@ -1146,7 +1158,7 @@ static int sock_mmap(struct file *file, struct vm_area_struct *vma)
 
 static int sock_close(struct inode *inode, struct file *filp)
 {
-	sock_release(SOCKET_I(inode));
+	__sock_release(SOCKET_I(inode), inode);
 	return 0;
 }
 
-- 
2.17.1