Re: [PATCH bpf v2] xsk: fix xsk_diag use-after-free error during socket cleanup

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Aug 31, 2023 at 12:01:17PM +0200, Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@intel.com>
> 
> Fix a use-after-free error that is possible if the xsk_diag interface
> is used after the socket has been unbound from the device. This can
> happen either due to the socket being closed or the device
> disappearing. In the early days of AF_XDP, the way we tested that a
> socket was not bound to a device 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 on the way to being
> closed and has been unbound from the device is XSK_UNBOUND.
> 
> The commit in the Fixes tag below deleted the old way of signalling
> that a socket is unbound, 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 in the state XSK_UNBOUND, simply abort the diagnostic's
> netlink operation.
> 
> Fixes: 18b1ab7aa76b ("xsk: Fix race at socket teardown")
> Reported-and-tested-by: syzbot+822d1359297e2694f873@syzkaller.appspotmail.com
> Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

> ---
> v1 -> v2:
>   * Added READ_ONCE for the state variable [Magnus]
>   * Improved commit message [Maciej]
> 
>  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..22b36c8143cf 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 (READ_ONCE(xs->state) == XSK_UNBOUND)
> +		goto out_nlmsg_trim;
> +
>  	if ((req->xdiag_show & XDP_SHOW_INFO) && xsk_diag_put_info(xs, nlskb))
>  		goto out_nlmsg_trim;
> 
> 
> base-commit: 7d35eb1a184a3f0759ad9e9cde4669b5c55b2063
> --
> 2.42.0