Re: [PATCH] tcp: assign the sock correctly to an outgoing SYNACK packet

Eric Dumazet
2013-04-08T16:19:27Z

On Mon, 2013-04-08 at 11:45 -0400, Paul Moore wrote:
> Commit 90ba9b1986b5ac4b2d184575847147ea7c4280a2 converted
> tcp_make_synack() to use alloc_skb() directly instead of calling
> sock_wmalloc(), the goal being the elimination of two atomic
> operations. Unfortunately, in doing so the change broke certain
> SELinux/NetLabel configurations by no longer correctly assigning
> the sock to the outgoing packet.
>
> This patch fixes this regression by doing the skb->sk assignment
> directly inside tcp_make_synack().
>
> Reported-by: Miroslav Vadkerti <mvadkert@redhat.com>
> Signed-off-by: Paul Moore <pmoore@redhat.com>
> ---
> net/ipv4/tcp_output.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 5d0b438..23cc295 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2705,6 +2705,8 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
> dst_release(dst);
> return NULL;
> }
> + skb->sk = sk;
> +

Hmm...

Keeping a pointer on a socket without taking a refcount is not going to
work.

We are trying to make the stack scale, so you need to add a selinux call
to take a ref count only if needed.

That is : If selinux is not used, we don't need to slow down the stack.