[syzbot] general protection fault in l2cap_chan_timeout (2)

[syzbot] general protection fault in l2cap_chan_timeout (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ad9f25d3 Merge tag 'netfs-lib-fixes-20200525' of git://git..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=173d383dd00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=266cda122a0b56c
dashboard link: https://syzkaller.appspot.com/bug?extid=008cdbf7a9044c2c2f99

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+008cdbf7a9044c2c2f99@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc000000005a: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000002d0-0x00000000000002d7]
CPU: 0 PID: 8 Comm: kworker/0:2 Not tainted 5.13.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events l2cap_chan_timeout
RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:941 [inline]
RIP: 0010:__mutex_lock+0xf6/0x10c0 kernel/locking/mutex.c:1104
Code: d0 7c 08 84 d2 0f 85 cc 0c 00 00 8b 15 e3 55 5f 07 85 d2 75 29 48 8d 7d 60 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 db 0e 00 00 48 3b 6d 60 0f 85 5a 0a 00 00 bf 01
RSP: 0018:ffffc90000cd7b78 EFLAGS: 00010216
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000001
RDX: 000000000000005a RSI: 0000000000000000 RDI: 00000000000002d0
RBP: 0000000000000270 R08: ffffffff880a40d9 R09: 0000000000000000
R10: ffffffff814b4be0 R11: 0000000000000000 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff888072e47020 R15: ffff8880b9c34a40
FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcac15f1d58 CR3: 00000000628fa000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 l2cap_chan_timeout+0x69/0x2f0 net/bluetooth/l2cap_core.c:422
 process_one_work+0x98d/0x1600 kernel/workqueue.c:2276
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2422
 kthread+0x3b1/0x4a0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
Modules linked in:
---[ end trace d3dc393d48928266 ]---
RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:941 [inline]
RIP: 0010:__mutex_lock+0xf6/0x10c0 kernel/locking/mutex.c:1104
Code: d0 7c 08 84 d2 0f 85 cc 0c 00 00 8b 15 e3 55 5f 07 85 d2 75 29 48 8d 7d 60 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 db 0e 00 00 48 3b 6d 60 0f 85 5a 0a 00 00 bf 01
RSP: 0018:ffffc90000cd7b78 EFLAGS: 00010216
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000001
RDX: 000000000000005a RSI: 0000000000000000 RDI: 00000000000002d0
RBP: 0000000000000270 R08: ffffffff880a40d9 R09: 0000000000000000
R10: ffffffff814b4be0 R11: 0000000000000000 R12: 0000000000000000
R13: dffffc0000000000 R14: ffff888072e47020 R15: ffff8880b9c34a40
FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b33621000 CR3: 00000000628fa000 CR4: 0000000000350ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600


---
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Re: [syzbot] general protection fault in l2cap_chan_timeout (2)

[Luiz Augusto von Dentz]

Hi,

On Mon, May 31, 2021 at 2:04 AM Hillf Danton <hdanton@sina.com> wrote:
>
> On Mon, 31 May 2021 00:19:17 -0700
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    ad9f25d3 Merge tag 'netfs-lib-fixes-20200525' of git://git..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=173d383dd00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=266cda122a0b56c
> > dashboard link: https://syzkaller.appspot.com/bug?extid=008cdbf7a9044c2c2f99
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+008cdbf7a9044c2c2f99@syzkaller.appspotmail.com
> >
> > general protection fault, probably for non-canonical address 0xdffffc000000005a: 0000 [#1] PREEMPT SMP KASAN
> > KASAN: null-ptr-deref in range [0x00000000000002d0-0x00000000000002d7]
> > CPU: 0 PID: 8 Comm: kworker/0:2 Not tainted 5.13.0-rc3-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Workqueue: events l2cap_chan_timeout
> > RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:941 [inline]
> > RIP: 0010:__mutex_lock+0xf6/0x10c0 kernel/locking/mutex.c:1104
> > Code: d0 7c 08 84 d2 0f 85 cc 0c 00 00 8b 15 e3 55 5f 07 85 d2 75 29 48 8d 7d 60 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 db 0e 00 00 48 3b 6d 60 0f 85 5a 0a 00 00 bf 01
> > RSP: 0018:ffffc90000cd7b78 EFLAGS: 00010216
> > RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000001
> > RDX: 000000000000005a RSI: 0000000000000000 RDI: 00000000000002d0
> > RBP: 0000000000000270 R08: ffffffff880a40d9 R09: 0000000000000000
> > R10: ffffffff814b4be0 R11: 0000000000000000 R12: 0000000000000000
> > R13: dffffc0000000000 R14: ffff888072e47020 R15: ffff8880b9c34a40
> > FS:  0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fcac15f1d58 CR3: 00000000628fa000 CR4: 0000000000350ef0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
> > Call Trace:
> >  l2cap_chan_timeout+0x69/0x2f0 net/bluetooth/l2cap_core.c:422
> >  process_one_work+0x98d/0x1600 kernel/workqueue.c:2276
> >  worker_thread+0x64c/0x1120 kernel/workqueue.c:2422
> >  kthread+0x3b1/0x4a0 kernel/kthread.c:313
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
>
> To fix the uaf reported, 1) releases connection through rcu 2) detects race
> under rcu lock in the delayed work callback.
>
> Note it is only for idea and thoughts are welcome if it makes sense to you.
>
> +++ x/net/bluetooth/l2cap_core.c
> @@ -414,12 +414,25 @@ static void l2cap_chan_timeout(struct wo
>  {
>         struct l2cap_chan *chan = container_of(work, struct l2cap_chan,
>                                                chan_timer.work);
> -       struct l2cap_conn *conn = chan->conn;
> +       struct l2cap_conn *conn;
>         int reason;
>
> +       rcu_read_lock();
> +       conn = chan->conn;
> +       if (conn && !kref_get_unless_zero(&conn->ref))
> +               conn = NULL;
> +       rcu_read_unlock();
> +
> +       if (!conn)
> +               goto put;
> +
>         BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
>
>         mutex_lock(&conn->chan_lock);
> +
> +       if (!chan->conn)
> +               goto put;
> +
>         /* __set_chan_timer() calls l2cap_chan_hold(chan) while scheduling
>          * this work. No need to call l2cap_chan_hold(chan) here again.
>          */
> @@ -438,9 +451,13 @@ static void l2cap_chan_timeout(struct wo
>         chan->ops->close(chan);
>
>         l2cap_chan_unlock(chan);
> +put:
>         l2cap_chan_put(chan);
>
> +       if (!conn)
> +               return;
>         mutex_unlock(&conn->chan_lock);
> +       l2cap_conn_put(conn);
>  }
>
>  struct l2cap_chan *l2cap_chan_create(void)
> @@ -1915,12 +1932,19 @@ static void l2cap_conn_del(struct hci_co
>         l2cap_conn_put(conn);
>  }
>
> +static void l2cap_conn_rcu_fn(struct rcu_head *r)
> +{
> +       struct l2cap_conn *conn = container_of(r, struct l2cap_conn, rcu);
> +
> +       kfree(conn);
> +}
> +
>  static void l2cap_conn_free(struct kref *ref)
>  {
>         struct l2cap_conn *conn = container_of(ref, struct l2cap_conn, ref);
>
>         hci_conn_put(conn->hcon);
> -       kfree(conn);

Shouldn't we actually cancel the timeout work if the connection is
freed here? At least I don't see a valid reason to have a l2cap_chan
without l2cap_conn.

> +       call_rcu(&conn->rcu, l2cap_conn_rcu_fn);
>  }
>
>  struct l2cap_conn *l2cap_conn_get(struct l2cap_conn *conn)



-- 
Luiz Augusto von Dentz

Re: [syzbot] general protection fault in l2cap_chan_timeout (2)

[Luiz Augusto von Dentz]

Hi,

On Tue, Jun 1, 2021 at 12:54 AM Hillf Danton <hdanton@sina.com> wrote:
>
> On Mon, 31 May 2021 19:11:08 -0700 Luiz Augusto von Dentz wrote:
> >
> >Shouldn't we actually cancel the timeout work if the connection is
> >freed here? At least I don't see a valid reason to have a l2cap_chan
> >without l2cap_conn.
>
> A far neater approach at the cost of making l2cap_conn_put() blocking and
> nobody currently seems to care about it.

I wonder what is going on here, there doesn't seem to be any code path
where the chan_timer is not cleared since the code path should be:

l2cap_conn_del -> l2cap_chan_del -> __clear_chan_timer -> cancel_delayed_work
                                                             chan->conn = NULL

Perhaps the problem is that cancel_delayed_work does not actually
prevent l2cap_chan_timeout to run if that is already pending, so maybe
something like this would work:

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 9ebb85df4db4..f6e423111dfc 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -414,17 +414,23 @@ static void l2cap_chan_timeout(struct work_struct *work)
 {
        struct l2cap_chan *chan = container_of(work, struct l2cap_chan,
                                               chan_timer.work);
-       struct l2cap_conn *conn = chan->conn;
+       struct l2cap_conn *conn;
        int reason;

        BT_DBG("chan %p state %s", chan, state_to_string(chan->state));

-       mutex_lock(&conn->chan_lock);
        /* __set_chan_timer() calls l2cap_chan_hold(chan) while scheduling
         * this work. No need to call l2cap_chan_hold(chan) here again.
         */
        l2cap_chan_lock(chan);

+       conn = chan->conn;
+       if (!conn)
+               /* l2cap_conn_del might have run */
+               goto unlock;
+
+       mutex_lock(&conn->chan_lock);
+
        if (chan->state == BT_CONNECTED || chan->state == BT_CONFIG)
                reason = ECONNREFUSED;
        else if (chan->state == BT_CONNECT &&
@@ -437,10 +443,11 @@ static void l2cap_chan_timeout(struct work_struct *work)

        chan->ops->close(chan);

+       mutex_unlock(&conn->chan_lock);
+
+unlock:
        l2cap_chan_unlock(chan);
        l2cap_chan_put(chan);
-
-       mutex_unlock(&conn->chan_lock);
 }


-- 
Luiz Augusto von Dentz