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

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Date: Wed, 30 Aug 2023 20:58:09 +0200
From: Magnus Karlsson <magnus.karlsson@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org, 
	daniel@...earbox.net, netdev@...r.kernel.org, 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 19:03, Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Wed, Aug 30, 2023 at 05:17:03PM +0200, Magnus Karlsson 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
>
> I thought our understanding is: socket is alive, we use diag interface
> against it but netdev that we bound socket to is being torn down.

If the socket was not going down at the same time, we would still have
a reference to the netdevice and it would not disappear. So the socket
needs to be going down for this to happen.

> since xs->dev was freed but not NULLed, xsk_diag_put_info() uses this ptr
> to retrieve ifindex.
>
> > 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
>
> Again, I believe it was not the socket going down but rather the netdev?
>
> > 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
>
> Nit: I see syzbot wanted you to include:
> Reported-and-tested-by: syzbot+822d13...@...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)
> > +             goto out_nlmsg_trim;
>
> With the above I feel like we can get rid of xs->dev test in
> xsk_diag_put_info(), no?

It has to stay since the socket does not get a reference to the device
until it is bound. It is fine to use the xsk_diag interface on an
unbound socket to query its state.

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