Re: [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@xxxxxxxxx>
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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