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

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Date: Thu, 31 Aug 2023 07:21:24 +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 23:57, Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Wed, Aug 30, 2023 at 08:58:09PM +0200, Magnus Karlsson wrote:
> > 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.
>
> No, I am able to trigger this now on my local system with KASAN turned on
> via:
>
> window 0:
> sudo ./xdpsock -i enp24s0f0np0 -r -z -q 17
>
> window 1:
> watch -n 0.1 "ss --xdp -e"
>
> window 2:
> sudo rmmod ice
>
> we hold the device via dev_get_by_index() in xsk_bind() but dev_put() is
> called from xsk_unbind_dev() which can happen either from xsk_release() or
> xsk_notifier(), our case refers to the latter.

Nice reproducer! My definition of "going down" is probably not clear.
In both the cases above, the state is set to XSK_UNBOUND and the
reference to the device is dropped, i.e. the socket is on its path to
oblivion. In any case, I will send a v2 to fix the missing READ_ONCE()
and I will try to make this "going down" clearer in the commit
message.

> I don't see currently how ss gets the ifname but after rmmoding ice I am
> getting something bogus over there:
>
> Recv-Q Send-Q Local Address:Port Peer Address:PortProcess
> 0      0               if18:q17              *     ino:18691 sk:2001
>         rx(entries:2048)
>         umem(id:0,size:16777216,num_pages:4096,chunk_size:4096,headroom:0,ifindex:0,qid:17,zc:1,refs:1)
>         fr(entries:4096)
>         cr(entries:2048)
>         stats(rx dropped:0,rx invalid:0,rx queue full:0,rx fill ring empty:0,tx invalid:0,tx ring empty:0)
>
> 'if18' instead 'enp24s0f0np0'. With your patch we bail out early so we
> wouldn't have that problem AFAICT.

"if18"? Interesting. Good thing we get rid of this with the patch.

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