Subject: Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram_poll()

From: Rainer Weikusat <[email-masked]>
Date: Mon, 05 Oct 2015 18:20:45 +0100

Eric Dumazet <[email-masked]> writes:
> On Mon, 2015-10-05 at 17:31 +0100, Rainer Weikusat wrote:
>
>>  	atomic_long_set(&u->inflight, 0);
>>  	INIT_LIST_HEAD(&u->link);
>> @@ -2135,8 +2139,16 @@ static unsigned int unix_poll(struct fil
>>  static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
>>  				    poll_table *wait)
>>  {
>> -	struct sock *sk = sock->sk, *other;
>> -	unsigned int mask, writable;
>> +	struct sock *sk = sock->sk, *other, *pp;
>> +	struct unix_sock *u;
>> +	unsigned int mask, writable, dead;
>> +
>> +	u = unix_sk(sk);
>> +	pp = u->poll_peer;
>> +	if (pp) {
>> +		u->poll_peer = NULL;
>> +		sock_put(pp);
>> +	}
>
>
> This looks racy.
> Multiple threads could use poll() at the same time,
> and you would have too many sock_put()

That's one of the reasons why I wrote "might work": The use of a single
structure member without any locking for the sock_poll_wait suggests
that this is taken care of in some other way, as does the absence of any
comment about that in the 'public' LDDs ("Linux Device Drivers"),
however, I don't really know if this is true. If not, this simple idea
can't work.