[alsa-devel] [PATCH] ALSA: fix oops in snd_pcm_info() caused by ASoC DPCM

[Russell King - ARM Linux]

Liam, Mark,

As discussed privately, here's the fix for the ASoC oops I've been seeing
with DPCM in ALSA.  This avoids unnecessarily exposing the ASoC internal
PCM devices to userspace, which is complimentary to your patch fixing the
deref of the PCM ops.

8<========
From: Russell King <rmk+kernel at arm.linux.org.uk>
Subject: [PATCH] ALSA: fix oops in snd_pcm_info() caused by ASoC DPCM

Unable to handle kernel NULL pointer dereference at virtual address 00000008
pgd = d5300000
[00000008] *pgd=0d265831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] PREEMPT ARM
CPU: 0 PID: 2295 Comm: vlc Not tainted 3.11.0+ #755
task: dee74800 ti: e213c000 task.ti: e213c000
PC is at snd_pcm_info+0xc8/0xd8
LR is at 0x30232065
pc : [<c031b52c>]    lr : [<30232065>]    psr: a0070013
sp : e213dea8  ip : d81cb0d0  fp : c05f7678
r10: c05f7770  r9 : fffffdfd  r8 : 00000000
r7 : d8a968a8  r6 : d8a96800  r5 : d8a96200  r4 : d81cb000
r3 : 00000000  r2 : d81cb000  r1 : 00000001  r0 : d8a96200
Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 10c5387d  Table: 15300019  DAC: 00000015
Process vlc (pid: 2295, stack limit = 0xe213c248)
[<c031b52c>] (snd_pcm_info) from [<c031b570>] (snd_pcm_info_user+0x34/0x9c)
[<c031b570>] (snd_pcm_info_user) from [<c03164a4>] (snd_pcm_control_ioctl+0x274/0x280)
[<c03164a4>] (snd_pcm_control_ioctl) from [<c0311458>] (snd_ctl_ioctl+0xc0/0x55c)
[<c0311458>] (snd_ctl_ioctl) from [<c00eca84>] (do_vfs_ioctl+0x80/0x31c)
[<c00eca84>] (do_vfs_ioctl) from [<c00ecd5c>] (SyS_ioctl+0x3c/0x60)
[<c00ecd5c>] (SyS_ioctl) from [<c000e500>] (ret_fast_syscall+0x0/0x48)
Code: e1a00005 e59530dc e3a01001 e1a02004 (e5933008)
---[ end trace cb3d9bdb8dfefb3c ]---

This is provoked when the ASoC front end is open along with its backend,
(which causes the backend to have a runtime assigned to it) and then the
SNDRV_CTL_IOCTL_PCM_INFO is requested for the (visible) backend device.

Resolve this by ensuring that ASoC internal backend devices are not
visible to userspace, just as the commentry for snd_pcm_new_internal()
says it should be.

Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
---
 sound/core/pcm.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index 17f45e8aa89c..e1e9e0c999fe 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -49,6 +49,8 @@ static struct snd_pcm *snd_pcm_get(struct snd_card *card, int device)
 	struct snd_pcm *pcm;
 
 	list_for_each_entry(pcm, &snd_pcm_devices, list) {
+		if (pcm->internal)
+			continue;
 		if (pcm->card == card && pcm->device == device)
 			return pcm;
 	}
@@ -60,6 +62,8 @@ static int snd_pcm_next(struct snd_card *card, int device)
 	struct snd_pcm *pcm;
 
 	list_for_each_entry(pcm, &snd_pcm_devices, list) {
+		if (pcm->internal)
+			continue;
 		if (pcm->card == card && pcm->device > device)
 			return pcm->device;
 		else if (pcm->card->number > card->number)
-- 
1.7.4.4