Re: GPF in net sybsystem

On Wed, May 5, 2021 at 10:36 AM Pavel Skripkin [off-list ref] wrote:

Hi, netdev developers!

I've spent some time debugging this bug
https://syzkaller.appspot.com/bug?id=c670fb9da2ce08f7b5101baa9426083b39ee9f90
and, I believe, I found the root case:

static int nr_accept(struct socket *sock, struct socket *newsock, int flags,
                     bool kern)
{
....
        for (;;) {
                prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
                ...
                if (!signal_pending(current)) {
                        release_sock(sk);
                        schedule();
                        lock_sock(sk);
                        continue;
                }
                ...
        }
...
}

When calling process will be scheduled, another proccess can release
this socket and set sk->sk_wq to NULL. (In this case nr_release()
will call sock_orphan(sk)). In this case GPF will happen in
prepare_to_wait().

Are you sure?

How could another process release this socket when its fd is still
refcnt'ed? That is, accept() still does not return yet at the point of
schedule().

Also, the above pattern is pretty common in networking subsystem,
see sk_wait_event(), so how come it is only problematic for netrom?

Thanks.