Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Mon, 2022-05-02 at 10:40 +0900, Tetsuo Handa wrote:
> syzbot is reporting use-after-free read in tcp_retransmit_timer() [1],
> for TCP socket used by RDS is accessing sock_net() without acquiring a
> refcount on net namespace. Since TCP's retransmission can happen after
> a process which created net namespace terminated, we need to explicitly
> acquire a refcount.
> 
> Link: https://syzkaller.appspot.com/bug?extid=694120e1002c117747ed [1]
> Reported-by: syzbot <syzbot+694120e1002c117747ed@syzkaller.appspotmail.com>
> Fixes: 26abe14379f8e2fa ("net: Modify sk_alloc to not reference count the netns of kernel sockets.")
> Fixes: 8a68173691f03661 ("net: sk_clone_lock() should only do get_net() if the parent is not a kernel socket")
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Tested-by: syzbot <syzbot+694120e1002c117747ed@syzkaller.appspotmail.com>
> ---
> Changes in v2:
>   Add Fixes: tag.
>   Move to inside lock_sock() section.
> 
> I chose 26abe14379f8e2fa and 8a68173691f03661 which went to 4.2 for Fixes: tag,
> for refcount was implicitly taken when 70041088e3b97662 ("RDS: Add TCP transport
> to RDS") was added to 2.6.32.
> 
>  net/rds/tcp.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/rds/tcp.c b/net/rds/tcp.c
> index 5327d130c4b5..2f638f8b7b1e 100644
> --- a/net/rds/tcp.c
> +++ b/net/rds/tcp.c
> @@ -495,6 +495,14 @@ void rds_tcp_tune(struct socket *sock)
>  
>  	tcp_sock_set_nodelay(sock->sk);
>  	lock_sock(sk);
> +	/* TCP timer functions might access net namespace even after
> +	 * a process which created this net namespace terminated.
> +	 */
> +	if (!sk->sk_net_refcnt) {
> +		sk->sk_net_refcnt = 1;
> +		get_net_track(net, &sk->ns_tracker, GFP_KERNEL);
> +		sock_inuse_add(net, 1);
> +	}
>  	if (rtn->sndbuf_size > 0) {
>  		sk->sk_sndbuf = rtn->sndbuf_size;
>  		sk->sk_userlocks |= SOCK_SNDBUF_LOCK;

This looks equivalent to the fix presented here:

https://lore.kernel.org/all/CANn89i+484ffqb93aQm1N-tjxxvb3WDKX0EbD7318RwRgsatjw@mail.gmail.com/

but the latter looks a more generic solution. @Tetsuo could you please
test the above in your setup?

Thanks!

Paolo