linux-kernel - Re: net/sctp: use-after-free in __sctp

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Date:   Thu, 3 Nov 2016 19:45:04 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-sctp@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: net/sctp: use-after-free in __sctp_connect

On Thu, Nov 3, 2016 at 7:35 PM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
> On Thu, Nov 03, 2016 at 07:02:47PM +0100, Andrey Konovalov wrote:
>> On Thu, Nov 3, 2016 at 6:52 PM, Marcelo Ricardo Leitner
>> <marcelo.leitner@...il.com> wrote:
>> > On Thu, Nov 03, 2016 at 06:11:01PM +0100, Andrey Konovalov wrote:
>> >> On Wed, Nov 2, 2016 at 11:42 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>> >> > On Wed, Oct 19, 2016 at 6:57 PM, Marcelo Ricardo Leitner
>> >> > <marcelo.leitner@...il.com> wrote:
>> >> >> On Wed, Oct 19, 2016 at 02:25:24PM +0200, Andrey Konovalov wrote:
>> >> >>> Hi,
>> >> >>>
>> >> >>> I've got the following error report while running the syzkaller fuzzer:
>> >> >>>
>> >> >>> ==================================================================
>> >> >>> BUG: KASAN: use-after-free in __sctp_connect+0xabe/0xbf0 at addr
>> >> >>> ffff88006b1dc610
>> >> >>
>> >> >> Seems this is the same that Dmitry Vyukov had reported back in Jan 13th.
>> >> >> So far I couldn't identify the reason.
>> >> >> "Good" to know it's still there, thanks for reporting it.
>> >>
>> >> Hi Marcelo,
>> >>
>> >
>> > Hi
>> >
>> >> So I've looked at the code.
>> >> As far as I understand, the problem is a race condition between
>> >> setsockopt(SCTP_SOCKOPT_CONNECTX) and shutdown on an sctp socket.
>> >> setsockopt() calls sctp_wait_for_connect(), which exits the for loop
>> >> on the sk->sk_shutdown & RCV_SHUTDOWN if clause, and then frees asoc
>> >> with sctp_association_put() and returns err = 0.
>> >> Then __sctp_connect() checks that err == 0 and reads asoc->assoc_id
>> >> from the freed asoc.
>> >
>> > Suddenly this seems familiar. Your description makes sense, thanks for
>> > looking deeper into this, Andrey.
>> >
>> > This fix should do it, can you please try it? I'll post it properly
>> > if it works.
>>
>> Yes, it fixes the issue.
>>
>> Tested-by: Andrey Konovalov <andreyknvl@...gle.com>
>>
>> Thanks for the fix!
>
> Ahm this other fix looks better: do the read before calling
> sctp_wait_for_connect() as that id won't change in this call and the
> application shouldn't trust this number if an error is returned, so
> there should be no issues by returning it in such situation.
>
> Can you please confirm this one also works? Thanks!

Sure!

This one also works.

Tested-by: Andrey Konovalov <andreyknvl@...gle.com>

>
> ---8<---
>
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 6cdc61c21438..be1d9bb98230 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -1214,9 +1214,12 @@ static int __sctp_connect(struct sock *sk,
>
>         timeo = sock_sndtimeo(sk, f_flags & O_NONBLOCK);
>
> -       err = sctp_wait_for_connect(asoc, &timeo);
> -       if ((err == 0 || err == -EINPROGRESS) && assoc_id)
> +       if (assoc_id)
>                 *assoc_id = asoc->assoc_id;
> +       err = sctp_wait_for_connect(asoc, &timeo);
> +       /* Note: the asoc may be freed after the return of
> +        * sctp_wait_for_connect.
> +        */
>
>         /* Don't free association on exit. */
>         asoc = NULL;