[Patch net] socket: close race condition between sock_close() and sockfs_setattr()

[Cong Wang]

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 <shankarapail...@gmail.com>
Cc: Tetsuo Handa <penguin-ker...@i-love.sakura.ne.jp>
Cc: Lorenzo Colitti <lore...@google.com>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/socket.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index af57d85bcb48..8a109012608a 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -541,7 +541,10 @@ 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;
@@ -590,12 +593,16 @@ 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);
        }
@@ -609,6 +616,11 @@ 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)
@@ -1171,7 +1183,7 @@ 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;
 }
 
-- 
2.13.0