netdev - Re: [PATCH bpf] xsk: fix xsk_diag use-after-free error during socket cleanup

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Date: Thu, 31 Aug 2023 07:15:06 +0200
From: Magnus Karlsson <magnus.karlsson@...il.com>
To: magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org, 
	daniel@...earbox.net, netdev@...r.kernel.org, maciej.fijalkowski@...el.com
Cc: jonathan.lemon@...il.com, bpf@...r.kernel.org, 
	syzbot+822d1359297e2694f873@...kaller.appspotmail.com
Subject: Re: [PATCH bpf] xsk: fix xsk_diag use-after-free error during socket cleanup

On Wed, 30 Aug 2023 at 17:17, Magnus Karlsson <magnus.karlsson@...il.com> wrote:
>
> From: Magnus Karlsson <magnus.karlsson@...el.com>
>
> Fix a use-after-free error that is possible if the xsk_diag interface
> is used at the same time as the socket is being closed. In the early
> days of AF_XDP, the way we tested that a socket was not bound or being
> closed was to simply check if the netdevice pointer in the xsk socket
> structure was NULL. Later, a better system was introduced by having an
> explicit state variable in the xsk socket struct. For example, the
> state of a socket that is going down is XSK_UNBOUND.
>
> The commit in the Fixes tag below deleted the old way of signalling
> that a socket is going down, setting dev to NULL. This in the belief
> that all code using the old way had been exterminated. That was
> unfortunately not true as the xsk diagnostics code was still using the
> old way and thus does not work as intended when a socket is going
> down. Fix this by introducing a test against the state variable. If
> the socket is going down, simply abort the diagnostic's netlink
> operation.
>
> Fixes: 18b1ab7aa76b ("xsk: Fix race at socket teardown")
> Reported-by: syzbot+822d1359297e2694f873@...kaller.appspotmail.com
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> ---
>  net/xdp/xsk_diag.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/xdp/xsk_diag.c b/net/xdp/xsk_diag.c
> index c014217f5fa7..da3100bfa1c5 100644
> --- a/net/xdp/xsk_diag.c
> +++ b/net/xdp/xsk_diag.c
> @@ -111,6 +111,9 @@ static int xsk_diag_fill(struct sock *sk, struct sk_buff *nlskb,
>         sock_diag_save_cookie(sk, msg->xdiag_cookie);
>
>         mutex_lock(&xs->mutex);
> +       if (xs->state == XSK_UNBOUND)

Sorry, but I have to spin a v2. There should be a READ_ONCE() here of the state.

> +               goto out_nlmsg_trim;
> +
>         if ((req->xdiag_show & XDP_SHOW_INFO) && xsk_diag_put_info(xs, nlskb))
>                 goto out_nlmsg_trim;
>
>
> base-commit: 35d2b7ffffc1d9b3dc6c761010aa3338da49165b
> --
> 2.42.0
>