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SubjectRe: [syzbot] KASAN: use-after-free Write in sco_sock_timeout
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On 29/8/21 4:29 pm, Hillf Danton wrote:
> On Fri, 27 Aug 2021 15:58:34 +0800 Desmond Cheong Zhi Xi wrote:
>> On 27/8/21 9:19 am, Hillf Danton wrote:
>>> On Thu, 26 Aug 2021 09:29:24 -0700
>>>> syzbot found the following issue on:
>>>>
>>>> HEAD commit: e3f30ab28ac8 Merge branch 'pktgen-samples-next'
>>>> git tree: net-next
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=13249c96300000
>>>> kernel config: https://syzkaller.appspot.com/x/.config?x=ef482942966bf763
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=2bef95d3ab4daa10155b
>>>> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
>>>> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16a29ea9300000
>>>>
>>>> The issue was bisected to:
>>>>
>>>> commit e1dee2c1de2b4dd00eb44004a4bda6326ed07b59
>>>> Author: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>>>> Date: Tue Aug 10 04:14:10 2021 +0000
>>>>
>>>> Bluetooth: fix repeated calls to sco_sock_kill
>
> To fix the uaf, grab another hold to sock to make the timeout work safe.
>
> #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git e3f30ab28ac8
>
> --- a/net/bluetooth/sco.c
> +++ b/net/bluetooth/sco.c
> @@ -190,15 +190,14 @@ static void sco_conn_del(struct hci_conn
> sco_conn_unlock(conn);
>
> if (sk) {
> - sock_hold(sk);
> lock_sock(sk);
> sco_sock_clear_timer(sk);
> sco_chan_del(sk, err);
> release_sock(sk);
> - sock_put(sk);
>
> /* Ensure no more work items will run before freeing conn. */
> cancel_delayed_work_sync(&conn->timeout_work);
> + sock_put(sk);

Hi Hillf,

Saw that this passed the reproducer. But on closer inspection, I think
what's happening is that sco_conn_del is never run.

So the extra sock_hold prevents a UAF, but that's because now the
reference count never goes to 0. In my opinion, something closer to your
previous proposal (+ also addressing other calls to __sco_sock_close)
where we call cancel_delayed_work_sync after the channel is deleted
would address the root cause better.

Just my two cents.

> }
>
> hcon->sco_data = NULL;
> @@ -212,6 +211,8 @@ static void __sco_chan_add(struct sco_co
>
> sco_pi(sk)->conn = conn;
> conn->sk = sk;
> + /* make timeout_work safe; will be put in sco_conn_del() */
> + sock_hold(sk);
>
> INIT_DELAYED_WORK(&conn->timeout_work, sco_sock_timeout);
>
> --
>

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