Re: [PATCH net] tcp/dccp: remove reqsk_put() from inet_child_forget()

From: Eric Dumazet <redacted>
Date: Mon, 11 Sep 2017 15:58:38 -0700

From: Eric Dumazet <edumazet@google.com>

Back in linux-4.4, I inadvertently put a call to reqsk_put() in
inet_child_forget(), forgetting it could be called from two different
points.

In the case it is called from inet_csk_reqsk_queue_add(), we want to
keep the reference on the request socket, since it is released later by
the caller (tcp_v{4|6}_rcv())

This bug never showed up because atomic_dec_and_test() was not signaling
the underflow, and SLAB_DESTROY_BY RCU semantic for request sockets
prevented the request to be put in quarantine.

Recent conversion of socket refcount from atomic_t to refcount_t finally
exposed the bug.

So move the reqsk_put() to inet_csk_listen_stop() to fix this.

Thanks to Shankara Pailoor for using syzkaller and providing
a nice set of .config and C repro.

 ...

Fixes: ebb516af60e1 ("tcp/dccp: fix race at listener dismantle phase")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Shankara Pailoor <redacted>
Tested-by: Shankara Pailoor <redacted>

Applied and queued up for -stable.

Thanks.