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

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 31 Aug 2023 12:55:59 +0200

On Thu, Aug 31, 2023 at 12:39:21PM +0200, Maciej Fijalkowski wrote:
> On Thu, Aug 31, 2023 at 12:01:17PM +0200, Magnus Karlsson wrote:
> > From: Magnus Karlsson <[email-masked]>
> > 
> > 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: [email-masked]
> > Signed-off-by: Magnus Karlsson <[email-masked]>
> 
> Reviewed-by: Maciej Fijalkowski <[email-masked]>

FWIW also tested that issue is no longer triggered on my local system:
Tested-by: Maciej Fijalkowski <[email-masked]>

> 
> > ---
> > 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