[PATCH 4.16 08/64] socket: close race condition between sock_close() and sockfs_setattr()

Greg Kroah-Hartman
2018-06-24T15:34:36Z

From: Cong Wang <xiyou.wangcong@gmail.com>

[ 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 <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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/socket.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

--- a/net/socket.c
+++ b/net/socket.c
@@ -538,7 +538,10 @@ static int sockfs_setattr(struct dentry
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,

static int sock_close(struct inode *inode, struct file *filp)
{
- sock_release(SOCKET_I(inode));
+ __sock_release(SOCKET_I(inode), inode);
return 0;
}