linux-kernel - Re: [PATCH v2] net: ipv6: fix use-after-free Read in __xfrm6_tunnel_spi

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Date:   Sat, 25 Jul 2020 22:35:12 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     B K Karthik <bkkarthik@...u.pes.edu>
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        skhan@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v2] net: ipv6: fix use-after-free Read in __xfrm6_tunnel_spi_lookup

On Sat, Jul 25, 2020 at 8:09 PM B K Karthik <bkkarthik@...u.pes.edu> wrote:
> @@ -103,10 +103,10 @@ static int __xfrm6_tunnel_spi_check(struct net *net, u32 spi)
>  {
>         struct xfrm6_tunnel_net *xfrm6_tn = xfrm6_tunnel_pernet(net);
>         struct xfrm6_tunnel_spi *x6spi;
> -       int index = xfrm6_tunnel_spi_hash_byspi(spi);
> +       int index = xfrm6_tunnel_spi_hash_byaddr((const xfrm_address_t *)spi);
>
>         hlist_for_each_entry(x6spi,
> -                            &xfrm6_tn->spi_byspi[index],
> +                            &xfrm6_tn->spi_byaddr[index],
>                              list_byspi) {
>                 if (x6spi->spi == spi)

How did you convince yourself this is correct? This lookup is still
using spi. :)

More importantly, can you explain how UAF happens? Apparently
the syzbot stack traces you quote make no sense at all. I also
looked at other similar reports, none of them makes sense to me.

Thanks.