Re: Possible netlink autobind regression

[Tejun Heo]

Hello, Herbert.

On Thu, Sep 17, 2015 at 01:15:03PM +0800, Herbert Xu wrote:
> netlink: Fix autobind race condition that leads to zero port ID
> 
> The commit c0bb07df7d981e4091432754e30c9c720e2c0c78 ("netlink:
> Reset portid after netlink_insert failure") introduced a race
> condition where if two threads tried to autobind the same socket
> one of them may end up with a zero port ID.
>
> This patch reverts that commit and instead fixes it by introducing
> a separte "bound" variable to indicate whether a user-space socket
> has been bound.
> 
> Fixes: c0bb07df7d98 ("netlink: Reset portid after netlink_insert failure")
> Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Cong Wang <cwang@xxxxxxxxxxxxxxxx>

Maybe add that this led to a deadlock and add a Link tag to this
thread?

> @@ -1083,10 +1083,12 @@ static int netlink_insert(struct sock *sk, u32 portid)
>  	if (err) {
>  		if (err == -EEXIST)
>  			err = -EADDRINUSE;
> -		nlk_sk(sk)->portid = 0;
>  		sock_put(sk);
> +		goto err;
>  	}
>  
> +	nlk_sk(sk)->bound = !!portid;

!! isn't necessasry and this creates ordering between two stores.
->bound must be visible only after ->portid is visible, so this should
be smp_store_release().

> @@ -2371,7 +2373,7 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
>  		dst_group = nlk->dst_group;
>  	}
>  
> -	if (!nlk->portid) {
> +	if (!nlk->bound) {

And all unlocked reads should be smp_load_acquire().

>  		err = netlink_autobind(sock);
>  		if (err)
>  			goto out;

Thanks.

-- 
tejun
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