Re: [Patch bpf-next] sock_map: fix a potential use-after-free in sock_map_close()

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> On Thu, Apr 8, 2021 at 5:26 PM John Fastabend <john.fastabend@gmail.com> wrote:
> >
> > Cong Wang wrote:
> > > From: Cong Wang <cong.wang@bytedance.com>
> > >
> > > The last refcnt of the psock can be gone right after
> > > sock_map_remove_links(), so sk_psock_stop() could trigger a UAF.
> > > The reason why I placed sk_psock_stop() there is to avoid RCU read
> > > critical section, and more importantly, some callee of
> > > sock_map_remove_links() is supposed to be called with RCU read lock,
> > > we can not simply get rid of RCU read lock here. Therefore, the only
> > > choice we have is to grab an additional refcnt with sk_psock_get()
> > > and put it back after sk_psock_stop().
> > >
> > > Reported-by: syzbot+7b6548ae483d6f4c64ae@syzkaller.appspotmail.com
> > > Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
> > > Cc: John Fastabend <john.fastabend@gmail.com>
> > > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > > Cc: Jakub Sitnicki <jakub@cloudflare.com>
> > > Cc: Lorenz Bauer <lmb@cloudflare.com>
> > > Signed-off-by: Cong Wang <cong.wang@bytedance.com>
> > > ---
> > >  net/core/sock_map.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > > index f473c51cbc4b..6f1b82b8ad49 100644
> > > --- a/net/core/sock_map.c
> > > +++ b/net/core/sock_map.c
> > > @@ -1521,7 +1521,7 @@ void sock_map_close(struct sock *sk, long timeout)
> > >
> > >       lock_sock(sk);
> > >       rcu_read_lock();
> >
> > It looks like we can drop the rcu_read_lock()/unlock() section then if we
> > take a reference on the psock? Before it was there to ensure we didn't
> > lose the psock from some other context, but with a reference held this
> > can not happen.
> 
> Some callees under sock_map_remove_links() still assert RCU read
> lock, so we can not simply drop the RCU read lock here. Some
> additional efforts are needed to take care of those assertions, which
> can be a separate patch.
> 
> Thanks.

OK at least this case exists,

 sock_map_close
  sock_map_remove_links
   sock_map_unlink
    sock_hash_delete_from_link
      WARN_ON_ONCE(!rcu_read_lock_held()); 

also calls into sock_map_unref through similar path use sk_psock(sk)
depending on rcu critical section.

Its certainly non-trivial to remove. I don't really like taking a ref
here but seems necessary for now.

Acked-by: John Fastabend <john.fastabend@gmail.com>