netdev - Re: [PATCH net] tcp: Fix potential use-after-free due to double kfree().

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Date:   Wed, 20 Jan 2021 14:07:35 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        Benjamin Herrenschmidt <benh@...zon.com>,
        Ricardo Dias <rdias@...glestore.com>
Subject: Re: [PATCH net] tcp: Fix potential use-after-free due to double kfree().

On Wed, Jan 20, 2021 at 2:17 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 18 Jan 2021 14:59:20 +0900 Kuniyuki Iwashima wrote:
> > Receiving ACK with a valid SYN cookie, cookie_v4_check() allocates struct
> > request_sock and then can allocate inet_rsk(req)->ireq_opt. After that,
> > tcp_v4_syn_recv_sock() allocates struct sock and copies ireq_opt to
> > inet_sk(sk)->inet_opt. Normally, tcp_v4_syn_recv_sock() inserts the full
> > socket into ehash and sets NULL to ireq_opt. Otherwise,
> > tcp_v4_syn_recv_sock() has to reset inet_opt by NULL and free the full
> > socket.
> >
> > The commit 01770a1661657 ("tcp: fix race condition when creating child
> > sockets from syncookies") added a new path, in which more than one cores
> > create full sockets for the same SYN cookie. Currently, the core which
> > loses the race frees the full socket without resetting inet_opt, resulting
> > in that both sock_put() and reqsk_put() call kfree() for the same memory:
> >
> >   sock_put
> >     sk_free
> >       __sk_free
> >         sk_destruct
> >           __sk_destruct
> >             sk->sk_destruct/inet_sock_destruct
> >               kfree(rcu_dereference_protected(inet->inet_opt, 1));
> >
> >   reqsk_put
> >     reqsk_free
> >       __reqsk_free
> >         req->rsk_ops->destructor/tcp_v4_reqsk_destructor
> >           kfree(rcu_dereference_protected(inet_rsk(req)->ireq_opt, 1));
> >
> > Calling kmalloc() between the double kfree() can lead to use-after-free, so
> > this patch fixes it by setting NULL to inet_opt before sock_put().
> >
> > As a side note, this kind of issue does not happen for IPv6. This is
> > because tcp_v6_syn_recv_sock() clones both ipv6_opt and pktopts which
> > correspond to ireq_opt in IPv4.
> >
> > Fixes: 01770a166165 ("tcp: fix race condition when creating child sockets from syncookies")
> > CC: Ricardo Dias <rdias@...glestore.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> > Reviewed-by: Benjamin Herrenschmidt <benh@...zon.com>
>
> Ricardo, Eric, any reason this was written this way?

Well, I guess that was a plain bug.

IPv4 options are not used often I think.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>