Re: [PATCH 3/3 v3] ALSA: usb-audio: fix race in snd_usb_endpoint_stop
From: Jiada Wang
Date: Mon Dec 05 2016 - 05:12:09 EST
Hi, Takashi
On 11/30/2016 01:00 AM, Takashi Iwai wrote:
On Wed, 30 Nov 2016 08:59:23 +0100,
Jiada Wang wrote:
From: Mark Craske<Mark_Craske@xxxxxxxxxx>
Kernel crash seen once:
Unable to handle kernel NULL pointer dereference at virtual address 00000008
pgd = a1d7c000
[00000008] *pgd=31c93831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] PREEMPT SMP ARM
CPU: 0 PID: 250 Comm: dbus-daemon Not tainted 3.14.51-03479-gf50bdf4 #1
task: a3ae61c0 ti: a08c8000 task.ti: a08c8000
PC is at retire_capture_urb+0x10/0x1f4 [snd_usb_audio]
LR is at snd_complete_urb+0x140/0x1f0 [snd_usb_audio]
pc : [<7f0eb22c>] lr : [<7f0e57fc>] psr: 200e0193
sp : a08c9c98 ip : a08c9ce8 fp : a08c9ce4
r10: 0000000a r9 : 00000102 r8 : 94cb3000
r7 : 94cb3000 r6 : 94d0f000 r5 : 94d0e8e8 r4 : 94d0e000
r3 : 7f0eb21c r2 : 00000000 r1 : 94cb3000 r0 : 00000000
Flags: nzCv IRQs off FIQs on Mode SVC_32 ISA ARM Segment user
Control: 10c5387d Table: 31d7c04a DAC: 00000015
Process dbus-daemon (pid: 250, stack limit = 0xa08c8238)
Stack: (0xa08c9c98 to 0xa08ca000)
...
Backtrace:
[<7f0eb21c>] (retire_capture_urb [snd_usb_audio]) from [<7f0e57fc>] (snd_complete_urb+0x140/0x1f0 [snd_usb_audio])
[<7f0e56bc>] (snd_complete_urb [snd_usb_audio]) from [<80371118>] (__usb_hcd_giveback_urb+0x78/0xf4)
[<803710a0>] (__usb_hcd_giveback_urb) from [<80371514>] (usb_giveback_urb_bh+0x8c/0xc0)
[<80371488>] (usb_giveback_urb_bh) from [<80028e3c>] (tasklet_hi_action+0xc4/0x148)
[<80028d78>] (tasklet_hi_action) from [<80028358>] (__do_softirq+0x190/0x380)
[<800281c8>] (__do_softirq) from [<80028858>] (irq_exit+0x8c/0xfc)
[<800287cc>] (irq_exit) from [<8000ea88>] (handle_IRQ+0x8c/0xc8)
[<8000e9fc>] (handle_IRQ) from [<800085e8>] (gic_handle_irq+0xbc/0xf8)
[<8000852c>] (gic_handle_irq) from [<80509044>] (__irq_svc+0x44/0x78)
[<80508820>] (_raw_spin_unlock_irq) from [<8004b880>] (finish_task_switch+0x5c/0x100)
[<8004b824>] (finish_task_switch) from [<805052f0>] (__schedule+0x48c/0x6d8)
[<80504e64>] (__schedule) from [<805055d4>] (schedule+0x98/0x9c)
[<8050553c>] (schedule) from [<800116c8>] (do_work_pending+0x30/0xd0)
[<80011698>] (do_work_pending) from [<8000e160>] (work_pending+0xc/0x20)
Code: e1a0c00d e92ddff0 e24cb004 e24dd024 (e5902008)
Kernel panic - not syncing: Fatal exception in interrupt
There is a race between retire_capture_urb()& stop_endpoints() which is
setting ep->data_subs to NULL. The underlying cause is in
snd_usb_endpoint_stop(), which sets
ep->data_subs = NULL;
ep->sync_slave = NULL;
ep->retire_data_urb = NULL;
ep->prepare_data_urb = NULL;
An improvement, suggested by Andreas Pape (ADIT) is to read parameters
into local variables. This should solve race between stop and retire
where it is legal to store the pointers to local variable as they are
not freed in stop path but just set to NULL.
Well, it's tricky. A saner way would be to clear the stuff really
after all users are gone.
An untested patch is below. Let me know if it really works.
Thanks for your proposal, I am afraid, we only found the race issue once
during
our automation test, so I can't test for your patch,
but from what I can see, your patch makes sense to me.
The only difference when apply with your patch is, now in case
stop_endpoints() is called
from TRIGGER_STOP, these stuff won't get cleared, but I think this isn't
an issue, is it?
Thanks,
Jiada
However, additional races may still happen in close+hw_free against
retire, as there pointers may be freed in addition. For example, while
in retire_capture_urb() runtime->dma_area might be freed/nulled.
This shouldn't be a problem, as stop_endpoints() waits until all
active URBS get killed.
thanks,
Takashi
---
diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
index c470251cea4b..f3fce9abece9 100644
--- a/sound/usb/endpoint.c
+++ b/sound/usb/endpoint.c
@@ -384,6 +384,9 @@ static void snd_complete_urb(struct urb *urb)
if (unlikely(atomic_read(&ep->chip->shutdown)))
goto exit_clear;
+ if (unlikely(!test_bit(EP_FLAG_RUNNING,&ep->flags)))
+ goto exit_clear;
+
if (usb_pipeout(ep->pipe)) {
retire_outbound_urb(ep, ctx);
/* can be stopped during retire callback */
@@ -534,6 +537,11 @@ static int wait_clear_urbs(struct snd_usb_endpoint *ep)
alive, ep->ep_num);
clear_bit(EP_FLAG_STOPPING,&ep->flags);
+ ep->data_subs = NULL;
+ ep->sync_slave = NULL;
+ ep->retire_data_urb = NULL;
+ ep->prepare_data_urb = NULL;
+
return 0;
}
@@ -1006,10 +1014,6 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
if (--ep->use_count == 0) {
deactivate_urbs(ep, false);
- ep->data_subs = NULL;
- ep->sync_slave = NULL;
- ep->retire_data_urb = NULL;
- ep->prepare_data_urb = NULL;
set_bit(EP_FLAG_STOPPING,&ep->flags);
}
}