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Hi,

Commit 16200948d835 ("ALSA: usb-audio: Fix race at stopping the stream") fixes a race-codition but it also introduces another really nasty data race regression which makes my usb sound card [1] completely useless, throwing the kernel into a panic if anything from userspace tries to start playback.

The problem is this: ep->data_subs is now set to NULL every time inside wait_clear_urbs(). ep->data_subs is initalized only in one place in start_endpoints(), then it is immediately wiped by the pre-existing call to wait_clear_urbs() inside snd_usb_endpoint_start().

To ilustrate, this is what happens in the non-irq part of the code:

Step 1 (inside start_endpoints): ep->data_subs = subs; Step 2 (inside start_endpoints): snd_usb_endpoint_start(ep, can_sleep); Step 3 (inside snd_usb_endpoint_start): wait_clear_urbs(ep); Step 4 (inside wait_clear_urbs): ep->data_subs = NULL;

Here's a simplified call stack for the IRQ part (full one at the end):

(NULL dereference, param "subs" is passed NULL value of ep->data_subs) retire_playback_urb retire_outbound_urb snd_complete_urb (...) xhci_irq

Looking at the git log there seems to be quite a lot of history in this part of the codebase, dating back to 2012 or earlier. My fix is based on 015618b90 ("ALSA: snd-usb: Fix URB cancellation at stream start") and e9ba389c5 ("ALSA: usb-audio: Fix scheduling-while-atomic bug in PCM capture stream") with a few modifications of my own. My idea is to do the ep->data_subs wiping before endpoint initialization in a manner similar to these older commits by using stop_endpoints() in snd_usb_pcm_prepare() and at the same time keep the ep->data_subs = NULL in wait_clear_urbs() to not trigger the recently fixed stream stopping race again.

Full call stack:

[ 131.093240] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010 [ 131.094313] IP: retire_playback_urb+0x1b/0x160 [snd_usb_audio] [ 131.095046] PGD 0 [ 131.095047] [ 131.096509] Oops: 0000 [#1] PREEMPT SMP [ 131.097255] Modules linked in: fuse snd_usb_audio snd_usbmidi_lib snd_rawmidi snd_seq_device ctr ccm arc4 ath9k intel_rapl ath9k_common x86_pkg_temp_thermal ath9k_hw intel_powerclamp coretemp joydev mousedev ath kvm_intel mac80211 kvm input_leds hid_generic psmouse irqbypass usbhid hid crct10dif_pclmul crc32_pclmul serio_raw crc32c_intel atkbd ghash_clmulni_intel snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic snd_hda_intel pcbc aesni_intel libps2 cfg8 0211 aes_x86_64 crypto_simd dell_laptop glue_helper r8169 dell_smbios snd_hda_codec sr_mod cryptd led_class mii rfkill snd_hwdep cdrom snd_hda_core ac dcdbas i8042 xhci_pci serio xhci_hcd snd_pcm tpm_tis battery tpm_tis_core lpc_ich tpm snd_timer evdev shpchp i2c_i801 sch_fq_codel [ 131.105551] CPU: 2 PID: 165 Comm: irq/29-xhci_hcd Not tainted 4.9.0-gd824cdc58ba0 #10 [ 131.107516] Hardware name: Dell Inc. Inspiron 3521/018DYG, BIOS A14 07/31/2015 [ 131.109592] task: ffff880154a70000 task.stack: ffffc90000f48000 [ 131.111746] RIP: 0010:retire_playback_urb+0x1b/0x160 [snd_usb_audio] [ 131.113899] RSP: 0018:ffffc90000f4bc10 EFLAGS: 00010082 [ 131.116080] RAX: ffffffffa04cabe0 RBX: 0000000000000000 RCX: 0000000000000000 [ 131.118284] RDX: 0000000000000000 RSI: ffff8801435a4c00 RDI: 0000000000000000 [ 131.120505] RBP: ffffc90000f4bc40 R08: 0000000000000001 R09: 0000000000000001 [ 131.122807] R10: 0000000000000001 R11: ffffffff82f60d6d R12: ffff8801432a0238 [ 131.125265] R13: ffff8801435a4c00 R14: 0000000000000000 R15: ffff8801535c83b8 [ 131.127723] FS: 0000000000000000(0000) GS:ffff88015a000000(0000) knlGS:0000000000000000 [ 131.130217] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 131.132568] CR2: 0000000000000010 CR3: 0000000001e0b000 CR4: 00000000001406e0 [ 131.135007] Call Trace: [ 131.137329] snd_complete_urb+0x190/0x2b0 [snd_usb_audio] [ 131.139526] __usb_hcd_giveback_urb+0x8e/0x160 [ 131.141948] usb_hcd_giveback_urb+0x43/0x110 [ 131.144104] xhci_giveback_urb_in_irq.isra.22+0x7d/0xb0 [xhci_hcd] [ 131.146521] finish_td.constprop.38+0x1de/0x2f0 [xhci_hcd] [ 131.148736] xhci_irq+0x13a2/0x1ca0 [xhci_hcd] [ 131.150972] ? trace_hardirqs_on+0xd/0x10 [ 131.153209] ? _raw_spin_unlock_irq+0x2c/0x50 [ 131.155390] ? irq_thread+0xb5/0x1d0 [ 131.157771] xhci_msi_irq+0x11/0x20 [xhci_hcd] [ 131.159938] irq_forced_thread_fn+0x2f/0x70 [ 131.162072] ? irq_thread+0xb5/0x1d0 [ 131.164141] irq_thread+0x149/0x1d0 [ 131.166132] ? irq_finalize_oneshot.part.2+0xe0/0xe0 [ 131.168142] ? wake_threads_waitq+0x30/0x30 [ 131.170149] kthread+0x10f/0x150 [ 131.172144] ? irq_thread_dtor+0xc0/0xc0 [ 131.174139] ? kthread_create_on_node+0x40/0x40 [ 131.176116] ret_from_fork+0x2a/0x40 [ 131.178074] Code: 8b 77 64 4c 89 e7 e8 e5 fe ff ff eb c3 0f 1f 00 0f 1f 44 00 00 55 31 d2 48 89 e5 41 57 41 56 41 55 41 54 53 48 89 fb 48 83 ec 08 <48> 8b 47 10 48 8b 4f 70 48 c7 c7 88 7b 4d a0 4c 8b a0 78 01 00 [ 131.182382] RIP: retire_playback_urb+0x1b/0x160 [snd_usb_audio] RSP: ffffc90000f4bc10 [ 131.184562] CR2: 0000000000000010

[1] 041e:3232 Creative Technology SoundBlaster X-FI HD [2] http://mailman.alsa-project.org/pipermail/alsa-devel/2016-December/115425.ht...

I can regenerate this bug with my EMU 0404 USB. My understanding of this bug is quite similar to your perspective. This bug is quite critical. In my understanding, we encounter this bug in all of cases in which snd-usb-audio is applied for PCM playback substream.

Below workaround can also suppress this bug.

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c index a2cdf33..9b34f76 100644 --- a/sound/usb/endpoint.c +++ b/sound/usb/endpoint.c @@ -537,11 +537,14 @@ 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;

+ msleep(200); + + ep->data_subs = NULL; + return 0; }

If initialization of 'struct snd_usb_endpoint.data_subs' can be done after all of queued URBs are processed and corresponding complete interrupts are cought, we can solve this critical bug.

Signed-off-by: Ioan-Adrian Ratiu adi@adirat.com

sound/usb/endpoint.c | 11 ++--------- sound/usb/endpoint.h | 2 +- sound/usb/pcm.c | 13 ++++++------- 3 files changed, 9 insertions(+), 17 deletions(-)

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c index a2cdf3370afe..4465f324c2c2 100644 --- a/sound/usb/endpoint.c +++ b/sound/usb/endpoint.c @@ -920,9 +920,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep, /**

• snd_usb_endpoint_start: start an snd_usb_endpoint

• • @ep: the endpoint to start
• • @can_sleep: flag indicating whether the operation is executed in

• • non-atomic context
    

• • @ep: the endpoint to start
  • A call to this function will increment the use count of the endpoint.
  • In case it is not already running, the URBs for this endpoint will be

@@ -932,7 +930,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,

• Returns an error if the URB submission failed, 0 in all other cases.

*/ -int snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep) +int snd_usb_endpoint_start(struct snd_usb_endpoint *ep) { int err; unsigned int i; @@ -944,11 +942,6 @@ int snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep) if (++ep->use_count != 1) return 0;

• /* just to be sure */
• deactivate_urbs(ep, false);
• if (can_sleep)

• wait_clear_urbs(ep);
  

• ep->active_mask = 0; ep->unlink_mask = 0; ep->phase = 0;

diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h index 6428392d8f62..584f295d7c77 100644 --- a/sound/usb/endpoint.h +++ b/sound/usb/endpoint.h @@ -18,7 +18,7 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep, struct audioformat *fmt, struct snd_usb_endpoint *sync_ep);

-int snd_usb_endpoint_start(struct snd_usb_endpoint *ep, bool can_sleep); +int snd_usb_endpoint_start(struct snd_usb_endpoint *ep); void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep); void snd_usb_endpoint_sync_pending_stop(struct snd_usb_endpoint *ep); int snd_usb_endpoint_activate(struct snd_usb_endpoint *ep); diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index 34c6d4f2c0b6..db26f767f851 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -218,7 +218,7 @@ int snd_usb_init_pitch(struct snd_usb_audio *chip, int iface, } }

-static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep) +static int start_endpoints(struct snd_usb_substream *subs) { int err;

@@ -231,7 +231,7 @@ static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep) dev_dbg(&subs->dev->dev, "Starting data EP @%p\n", ep);

ep->data_subs = subs;

• err = snd_usb_endpoint_start(ep, can_sleep);
  

• err = snd_usb_endpoint_start(ep);
  

  if (err < 0) { clear_bit(SUBSTREAM_FLAG_DATA_EP_STARTED, &subs->flags); return err;

@@ -260,7 +260,7 @@ static int start_endpoints(struct snd_usb_substream *subs, bool can_sleep) dev_dbg(&subs->dev->dev, "Starting sync EP @%p\n", ep);

ep->sync_slave = subs->data_endpoint;

• err = snd_usb_endpoint_start(ep, can_sleep);
  

• err = snd_usb_endpoint_start(ep);
  

  if (err < 0) { clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags); return err;

@@ -809,8 +809,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) goto unlock; }

• snd_usb_endpoint_sync_pending_stop(subs->sync_endpoint);
• snd_usb_endpoint_sync_pending_stop(subs->data_endpoint);

• stop_endpoints(subs, true);

  ret = set_format(subs, subs->cur_audiofmt); if (ret < 0)

@@ -850,7 +849,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) /* for playback, submit the URBs now; otherwise, the first hwptr_done * updates for all URBs would happen at the same time when starting */ if (subs->direction == SNDRV_PCM_STREAM_PLAYBACK)

• ret = start_endpoints(subs, true);
  

• return start_endpoints(subs);
  
  

  unlock: snd_usb_unlock_shutdown(subs->stream->chip);

@@ -1666,7 +1665,7 @@ static int snd_usb_substream_capture_trigger(struct snd_pcm_substream *substream

switch (cmd) { case SNDRV_PCM_TRIGGER_START:

• err = start_endpoints(subs, false);
  

• err = start_endpoints(subs);
  

  if (err < 0) return err;

This patch works better, but not the best. It's a bit intrusive.

What we need to synchronize process contexts to interrupt context till all of queued URBs are handled. This seems to be a main purpose of wait_clear_bits, while current implementation of snd_complete_urb()/wait_clear_bits() is not enough.

Thanks for your reporting.

Takashi Sakamoto