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Message-ID: <bad67d05-366b-bebe-cbdb-6555386497de@gmail.com>
Date: Thu, 2 Sep 2021 15:32:27 -0400
From: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To: Eric Dumazet <eric.dumazet@...il.com>, marcel@...tmann.org,
johan.hedberg@...il.com, luiz.dentz@...il.com, davem@...emloft.net,
kuba@...nel.org, sudipm.mukherjee@...il.com
Cc: linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
gregkh@...uxfoundation.org,
linux-kernel-mentees@...ts.linuxfoundation.org,
syzbot+2f6d7c28bb4bf7e82060@...kaller.appspotmail.com
Subject: Re: [PATCH v6 1/6] Bluetooth: schedule SCO timeouts with delayed_work
On 2/9/21 3:17 pm, Eric Dumazet wrote:
>
>
> On 8/9/21 9:14 PM, Desmond Cheong Zhi Xi wrote:
>> struct sock.sk_timer should be used as a sock cleanup timer. However,
>> SCO uses it to implement sock timeouts.
>>
>> This causes issues because struct sock.sk_timer's callback is run in
>> an IRQ context, and the timer callback function sco_sock_timeout takes
>> a spin lock on the socket. However, other functions such as
>> sco_conn_del and sco_conn_ready take the spin lock with interrupts
>> enabled.
>>
>> This inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} lock usage could
>> lead to deadlocks as reported by Syzbot [1]:
>> CPU0
>> ----
>> lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
>> <Interrupt>
>> lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
>>
>> To fix this, we use delayed work to implement SCO sock timouts
>> instead. This allows us to avoid taking the spin lock on the socket in
>> an IRQ context, and corrects the misuse of struct sock.sk_timer.
>>
>> As a note, cancel_delayed_work is used instead of
>> cancel_delayed_work_sync in sco_sock_set_timer and
>> sco_sock_clear_timer to avoid a deadlock. In the future, the call to
>> bh_lock_sock inside sco_sock_timeout should be changed to lock_sock to
>> synchronize with other functions using lock_sock. However, since
>> sco_sock_set_timer and sco_sock_clear_timer are sometimes called under
>> the locked socket (in sco_connect and __sco_sock_close),
>> cancel_delayed_work_sync might cause them to sleep until an
>> sco_sock_timeout that has started finishes running. But
>> sco_sock_timeout would also sleep until it can grab the lock_sock.
>>
>> Using cancel_delayed_work is fine because sco_sock_timeout does not
>> change from run to run, hence there is no functional difference
>> between:
>> 1. waiting for a timeout to finish running before scheduling another
>> timeout
>> 2. scheduling another timeout while a timeout is running.
>>
>> Link: https://syzkaller.appspot.com/bug?id=9089d89de0502e120f234ca0fc8a703f7368b31e [1]
>> Reported-by: syzbot+2f6d7c28bb4bf7e82060@...kaller.appspotmail.com
>> Tested-by: syzbot+2f6d7c28bb4bf7e82060@...kaller.appspotmail.com
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
>> ---
>> net/bluetooth/sco.c | 35 +++++++++++++++++++++++++++++------
>> 1 file changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
>> index ffa2a77a3e4c..62e638f971a9 100644
>> --- a/net/bluetooth/sco.c
>> +++ b/net/bluetooth/sco.c
>> @@ -48,6 +48,8 @@ struct sco_conn {
>> spinlock_t lock;
>> struct sock *sk;
>>
>> + struct delayed_work timeout_work;
>> +
>> unsigned int mtu;
>> };
>>
>> @@ -74,9 +76,20 @@ struct sco_pinfo {
>> #define SCO_CONN_TIMEOUT (HZ * 40)
>> #define SCO_DISCONN_TIMEOUT (HZ * 2)
>>
>> -static void sco_sock_timeout(struct timer_list *t)
>> +static void sco_sock_timeout(struct work_struct *work)
>> {
>> - struct sock *sk = from_timer(sk, t, sk_timer);
>> + struct sco_conn *conn = container_of(work, struct sco_conn,
>> + timeout_work.work);
>> + struct sock *sk;
>> +
>> + sco_conn_lock(conn);
>> + sk = conn->sk;
>> + if (sk)
>> + sock_hold(sk);
>
> syzbot complains here that sk refcount can be zero at this time.
>
> refcount_t: addition on 0; use-after-free.
> WARNING: CPU: 0 PID: 10451 at lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
> Modules linked in:
> CPU: 0 PID: 10451 Comm: kworker/0:8 Not tainted 5.14.0-rc7-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Workqueue: events sco_sock_timeout
> RIP: 0010:refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
> Code: 09 31 ff 89 de e8 d7 c9 9e fd 84 db 0f 85 36 ff ff ff e8 8a c3 9e fd 48 c7 c7 20 8f e3 89 c6 05 e8 7f 81 09 01 e8 f0 98 16 05 <0f> 0b e9 17 ff ff ff e8 6b c3 9e fd 0f b6 1d cd 7f 81 09 31 ff 89
> RSP: 0018:ffffc9001766fce8 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff88802cea3880 RSI: ffffffff815d87a5 RDI: fffff52002ecdf8f
> RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
> R10: ffffffff815d25de R11: 0000000000000000 R12: ffff88806d23ce08
> R13: ffff8880712c8080 R14: ffff88802edf4500 R15: ffff8880b9c51240
> FS: 0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f3748c20000 CR3: 0000000017644000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> __refcount_add include/linux/refcount.h:199 [inline]
> __refcount_inc include/linux/refcount.h:250 [inline]
> refcount_inc include/linux/refcount.h:267 [inline]
> sock_hold include/net/sock.h:702 [inline]
> sco_sock_timeout+0x216/0x290 net/bluetooth/sco.c:88
> process_one_work+0x98d/0x1630 kernel/workqueue.c:2276
> worker_thread+0x658/0x11f0 kernel/workqueue.c:2422
> kthread+0x3e5/0x4d0 kernel/kthread.c:319
>
>
>> + sco_conn_unlock(conn);
>> +
>> + if (!sk)
>> + return;
>>
>> BT_DBG("sock %p state %d", sk, sk->sk_state);
>>
>> @@ -91,14 +104,21 @@ static void sco_sock_timeout(struct timer_list *t)
>>
>> static void sco_sock_set_timer(struct sock *sk, long timeout)
>> {
>> + if (!sco_pi(sk)->conn)
>> + return;
>> +
>> BT_DBG("sock %p state %d timeout %ld", sk, sk->sk_state, timeout);
>> - sk_reset_timer(sk, &sk->sk_timer, jiffies + timeout);
>> + cancel_delayed_work(&sco_pi(sk)->conn->timeout_work);
>> + schedule_delayed_work(&sco_pi(sk)->conn->timeout_work, timeout);
>
>> }
>>
>> static void sco_sock_clear_timer(struct sock *sk)
>> {
>> + if (!sco_pi(sk)->conn)
>> + return;
>> +
>> BT_DBG("sock %p state %d", sk, sk->sk_state);
>> - sk_stop_timer(sk, &sk->sk_timer);
>> + cancel_delayed_work(&sco_pi(sk)->conn->timeout_work);
>
>
>> }
>>
>> /* ---- SCO connections ---- */
>> @@ -179,6 +199,9 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
>> bh_unlock_sock(sk);
>> sco_sock_kill(sk);
>> sock_put(sk);
>> +
>> + /* Ensure no more work items will run before freeing conn. */
>
> Maybe you should have done this cancel_delayed_work_sync() before the prior sock_put(sk) ?
>
>> + cancel_delayed_work_sync(&conn->timeout_work);
>> }
>>
>> hcon->sco_data = NULL;
>> @@ -193,6 +216,8 @@ static void __sco_chan_add(struct sco_conn *conn, struct sock *sk,
>> sco_pi(sk)->conn = conn;
>> conn->sk = sk;
>>
>> + INIT_DELAYED_WORK(&conn->timeout_work, sco_sock_timeout);
>> +
>> if (parent)
>> bt_accept_enqueue(parent, sk, true);
>> }
>> @@ -500,8 +525,6 @@ static struct sock *sco_sock_alloc(struct net *net, struct socket *sock,
>>
>> sco_pi(sk)->setting = BT_VOICE_CVSD_16BIT;
>>
>> - timer_setup(&sk->sk_timer, sco_sock_timeout, 0);
>> -
>> bt_sock_link(&sco_sk_list, sk);
>> return sk;
>> }
>>
Hi Eric,
This actually seems to be a pre-existing error in sco_sock_connect that
we now hit in sco_sock_timeout.
Any thoughts on the following patch to address the problem?
Link:
https://lore.kernel.org/lkml/20210831065601.101185-1-desmondcheongzx@gmail.com/
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