* [PATCH] ALSA: dummy: Revalidate CD Volume before updating its access
@ 2026-10-16 2:17 Cen Zhang
2026-10-16 12:51 ` Cen Zhang
0 siblings, 1 reply; 2+ messages in thread
From: Cen Zhang @ 2026-10-16 2:17 UTC (permalink / raw)
To: perex, tiwai, thomas.weissschuh, sammiee5311, tglx, phucduc.bui,
kees, clemens
Cc: linux-sound, linux-kernel, baijiaju1990, jjzuming, zzzccc427
The CD Volume control must remain live while snd_dummy_iobox_put()
updates its access flags and sends an INFO notification. The mixer
constructor caches a raw pointer to that control, but removing it does
not invalidate the cached pointer.
This can happen when a kernel client removes CD Volume through
snd_ctl_remove_id() while the dummy card remains connected and External
I/O Box is still present. Userspace ELEM_REMOVE rejects this driver
control, so a kernel control-removal caller is required. With I/O Box
initially set to 1, the following ordering is possible:
1. The kernel client takes controls_rwsem for write in
snd_ctl_remove_id(). __snd_ctl_remove() unlinks CD Volume, sends
REMOVE notifications and frees it through snd_ctl_free_one(). The
semaphore is released and removal returns.
2. After removal returns, a native ELEM_WRITE changes External I/O Box
from 1 to 0. snd_ctl_elem_write() takes controls_rwsem for write
and calls snd_dummy_iobox_put(), which follows the cached CD Volume
pointer to update vd[0].access.
The access update starts with a use-after-free read. The subsequent
write and notification ID access also use the freed allocation. The
semaphore serializes removal and the callback, but does not preserve a
control removed before callback entry.
Save the control ID after successful addition and look it up again
inside snd_dummy_iobox_put(). The existing control-write semaphore keeps
a found control live through the access update and INFO notification.
Skip those operations when the lookup fails, preserving the I/O Box
value change and the surviving control's update.
KASAN report as below:
BUG: KASAN: slab-use-after-free in snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
Read of size 4 at addr ffff88810d8b4c98 by task alsa-trigger/415
CPU: 0 UID: 0 PID: 415 Comm: alsa-trigger Tainted: G O 7.2.0-rc6-pmb-bt-functional-v1+ #1 PREEMPT(lazy)
Tainted: [O]=OOT_MODULE
Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x93/0xd0
print_report+0xce/0x630
? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
? srso_alias_return_thunk+0x5/0xfbef5
? __virt_addr_valid+0x20d/0x410
? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
kasan_report+0xe0/0x110
? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
snd_ctl_elem_write+0x376/0x5a0
snd_ctl_ioctl+0x997/0xdd0
? __pfx_snd_ctl_ioctl+0x10/0x10
? __pfx_do_sys_openat2+0x10/0x10
? srso_alias_return_thunk+0x5/0xfbef5
? selinux_file_ioctl+0xaa/0x280
? __pfx_snd_ctl_ioctl+0x10/0x10
__x64_sys_ioctl+0x163/0x1d0
do_syscall_64+0x115/0x6a0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc9c21a991b
Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00
RSP: 002b:00007ffc08d51f80 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc9c21a991b
RDX: 00007ffc08d51fe0 RSI: 00000000c4c85513 RDI: 0000000000000003
RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffc08d52600 R14: 00007fc9c22d8000 R15: 00005578bae81d58
</TASK>
Allocated by task 415:
kasan_save_stack+0x33/0x60
kasan_save_track+0x14/0x30
__kasan_kmalloc+0xaa/0xb0
__kmalloc_noprof+0x2d7/0x770
snd_ctl_new+0x51/0x190
snd_ctl_new1+0x13c/0x650
snd_dummy_probe+0xba5/0x1240 [snd_dummy]
platform_probe+0xbe/0x1d0
really_probe+0x21f/0xa90
__driver_probe_device+0x1ec/0x490
driver_probe_device+0x4f/0x130
__device_attach_driver+0x1d2/0x2e0
bus_for_each_drv+0x155/0x1d0
__device_attach+0x1c3/0x490
device_initial_probe+0x96/0xb0
bus_probe_device+0x69/0x160
device_add+0xfff/0x1700
platform_device_add+0x318/0x7b0
platform_device_register_full+0x596/0x7b0
pmbd_remove_cd_exit+0x12a/0xf90 [remove_cd]
do_one_initcall+0xf3/0x4e0
do_init_module+0x281/0x8e0
load_module+0x658c/0x7cc0
init_module_from_file+0x15b/0x180
idempotent_init_module+0x22d/0x770
__x64_sys_finit_module+0xd2/0x160
do_syscall_64+0x115/0x6a0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 415:
kasan_save_stack+0x33/0x60
kasan_save_track+0x14/0x30
kasan_save_free_info+0x3b/0x60
__kasan_slab_free+0x5f/0x80
kfree+0x307/0x580
snd_ctl_free_one+0x6b/0x110
__snd_ctl_remove+0x26f/0x360
snd_ctl_remove_id+0x4f/0x70
0xffffffffc04060c9
do_one_initcall+0xf3/0x4e0
do_init_module+0x281/0x8e0
load_module+0x658c/0x7cc0
init_module_from_file+0x15b/0x180
idempotent_init_module+0x22d/0x770
__x64_sys_finit_module+0xd2/0x160
do_syscall_64+0x115/0x6a0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
The buggy address belongs to the object at ffff88810d8b4c00
which belongs to the cache kmalloc-192 of size 192
The buggy address is located 152 bytes inside of
freed 192-byte region [ffff88810d8b4c00, ffff88810d8b4cc0)
The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10d8b4
flags: 0x200000000000000(node=0|zone=2)
page_type: f5(slab)
raw: 0200000000000000 ffff8881000423c0 dead000000000122 0000000000000000
raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff88810d8b4b80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
ffff88810d8b4c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88810d8b4c80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
^
ffff88810d8b4d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff88810d8b4d80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
Fixes: 16e434670e95 ("ALSA: dummy: allow disabling mixer controls")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
index 6b0a06e5c87638264e5d22deda1fe4bc028febe7..d422fb3fa25d1279df2e971e0f5546a329db4049 100644
--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -134,7 +134,7 @@ struct snd_dummy {
int mixer_volume[MIXER_ADDR_LAST+1][2];
int capture_source[MIXER_ADDR_LAST+1][2];
int iobox;
- struct snd_kcontrol *cd_volume_ctl;
+ struct snd_ctl_elem_id cd_volume_id;
struct snd_kcontrol *cd_switch_ctl;
};
@@ -836,6 +836,7 @@ static int snd_dummy_iobox_put(struct snd_kcontrol *kcontrol,
struct snd_ctl_elem_value *value)
{
struct snd_dummy *dummy = snd_kcontrol_chip(kcontrol);
+ struct snd_kcontrol *cd_volume_ctl;
int changed;
if (value->value.enumerated.item[0] > 1)
@@ -845,20 +846,27 @@ static int snd_dummy_iobox_put(struct snd_kcontrol *kcontrol,
if (changed) {
dummy->iobox = value->value.enumerated.item[0];
+ /* The control-write path holds controls_rwsem. */
+ cd_volume_ctl = snd_ctl_find_id(dummy->card,
+ &dummy->cd_volume_id);
if (dummy->iobox) {
- dummy->cd_volume_ctl->vd[0].access &=
- ~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
+ if (cd_volume_ctl)
+ cd_volume_ctl->vd[0].access &=
+ ~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
dummy->cd_switch_ctl->vd[0].access &=
~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
} else {
- dummy->cd_volume_ctl->vd[0].access |=
- SNDRV_CTL_ELEM_ACCESS_INACTIVE;
+ if (cd_volume_ctl)
+ cd_volume_ctl->vd[0].access |=
+ SNDRV_CTL_ELEM_ACCESS_INACTIVE;
dummy->cd_switch_ctl->vd[0].access |=
SNDRV_CTL_ELEM_ACCESS_INACTIVE;
}
- snd_ctl_notify(dummy->card, SNDRV_CTL_EVENT_MASK_INFO,
- &dummy->cd_volume_ctl->id);
+ if (cd_volume_ctl)
+ snd_ctl_notify(dummy->card,
+ SNDRV_CTL_EVENT_MASK_INFO,
+ &cd_volume_ctl->id);
snd_ctl_notify(dummy->card, SNDRV_CTL_EVENT_MASK_INFO,
&dummy->cd_switch_ctl->id);
}
@@ -903,7 +911,7 @@ static int snd_card_dummy_new_mixer(struct snd_dummy *dummy)
if (err < 0)
return err;
if (!strcmp(kcontrol->id.name, "CD Volume"))
- dummy->cd_volume_ctl = kcontrol;
+ dummy->cd_volume_id = kcontrol->id;
else if (!strcmp(kcontrol->id.name, "CD Capture Switch"))
dummy->cd_switch_ctl = kcontrol;
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] ALSA: dummy: Revalidate CD Volume before updating its access
2026-10-16 2:17 [PATCH] ALSA: dummy: Revalidate CD Volume before updating its access Cen Zhang
@ 2026-10-16 12:51 ` Cen Zhang
0 siblings, 0 replies; 2+ messages in thread
From: Cen Zhang @ 2026-10-16 12:51 UTC (permalink / raw)
To: perex, tiwai, thomas.weissschuh, sammiee5311, tglx, phucduc.bui,
kees, clemens
Cc: linux-sound, linux-kernel, baijiaju1990, jjzuming
Hi all,
Cen Zhang <zzzccc427@gmail.com> 于2026年10月16日周五 10:17写道：
>
> The CD Volume control must remain live while snd_dummy_iobox_put()
> updates its access flags and sends an INFO notification. The mixer
> constructor caches a raw pointer to that control, but removing it does
> not invalidate the cached pointer.
>
> This can happen when a kernel client removes CD Volume through
> snd_ctl_remove_id() while the dummy card remains connected and External
> I/O Box is still present. Userspace ELEM_REMOVE rejects this driver
> control, so a kernel control-removal caller is required. With I/O Box
> initially set to 1, the following ordering is possible:
>
> 1. The kernel client takes controls_rwsem for write in
> snd_ctl_remove_id(). __snd_ctl_remove() unlinks CD Volume, sends
> REMOVE notifications and frees it through snd_ctl_free_one(). The
> semaphore is released and removal returns.
> 2. After removal returns, a native ELEM_WRITE changes External I/O Box
> from 1 to 0. snd_ctl_elem_write() takes controls_rwsem for write
> and calls snd_dummy_iobox_put(), which follows the cached CD Volume
> pointer to update vd[0].access.
>
> The access update starts with a use-after-free read. The subsequent
> write and notification ID access also use the freed allocation. The
> semaphore serializes removal and the callback, but does not preserve a
> control removed before callback entry.
>
> Save the control ID after successful addition and look it up again
> inside snd_dummy_iobox_put(). The existing control-write semaphore keeps
> a found control live through the access update and INFO notification.
> Skip those operations when the lookup fails, preserving the I/O Box
> value change and the surviving control's update.
>
> KASAN report as below:
>
> BUG: KASAN: slab-use-after-free in snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
> Read of size 4 at addr ffff88810d8b4c98 by task alsa-trigger/415
>
> CPU: 0 UID: 0 PID: 415 Comm: alsa-trigger Tainted: G O 7.2.0-rc6-pmb-bt-functional-v1+ #1 PREEMPT(lazy)
> Tainted: [O]=OOT_MODULE
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x93/0xd0
> print_report+0xce/0x630
> ? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? __virt_addr_valid+0x20d/0x410
> ? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
> kasan_report+0xe0/0x110
> ? snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
> snd_dummy_iobox_put+0x391/0x440 [snd_dummy]
> snd_ctl_elem_write+0x376/0x5a0
> snd_ctl_ioctl+0x997/0xdd0
> ? __pfx_snd_ctl_ioctl+0x10/0x10
> ? __pfx_do_sys_openat2+0x10/0x10
> ? srso_alias_return_thunk+0x5/0xfbef5
> ? selinux_file_ioctl+0xaa/0x280
> ? __pfx_snd_ctl_ioctl+0x10/0x10
> __x64_sys_ioctl+0x163/0x1d0
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fc9c21a991b
> Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00
> RSP: 002b:00007ffc08d51f80 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007fc9c21a991b
> RDX: 00007ffc08d51fe0 RSI: 00000000c4c85513 RDI: 0000000000000003
> RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ffc08d52600 R14: 00007fc9c22d8000 R15: 00005578bae81d58
> </TASK>
>
> Allocated by task 415:
> kasan_save_stack+0x33/0x60
> kasan_save_track+0x14/0x30
> __kasan_kmalloc+0xaa/0xb0
> __kmalloc_noprof+0x2d7/0x770
> snd_ctl_new+0x51/0x190
> snd_ctl_new1+0x13c/0x650
> snd_dummy_probe+0xba5/0x1240 [snd_dummy]
> platform_probe+0xbe/0x1d0
> really_probe+0x21f/0xa90
> __driver_probe_device+0x1ec/0x490
> driver_probe_device+0x4f/0x130
> __device_attach_driver+0x1d2/0x2e0
> bus_for_each_drv+0x155/0x1d0
> __device_attach+0x1c3/0x490
> device_initial_probe+0x96/0xb0
> bus_probe_device+0x69/0x160
> device_add+0xfff/0x1700
> platform_device_add+0x318/0x7b0
> platform_device_register_full+0x596/0x7b0
> pmbd_remove_cd_exit+0x12a/0xf90 [remove_cd]
> do_one_initcall+0xf3/0x4e0
> do_init_module+0x281/0x8e0
> load_module+0x658c/0x7cc0
> init_module_from_file+0x15b/0x180
> idempotent_init_module+0x22d/0x770
> __x64_sys_finit_module+0xd2/0x160
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Freed by task 415:
> kasan_save_stack+0x33/0x60
> kasan_save_track+0x14/0x30
> kasan_save_free_info+0x3b/0x60
> __kasan_slab_free+0x5f/0x80
> kfree+0x307/0x580
> snd_ctl_free_one+0x6b/0x110
> __snd_ctl_remove+0x26f/0x360
> snd_ctl_remove_id+0x4f/0x70
> 0xffffffffc04060c9
> do_one_initcall+0xf3/0x4e0
> do_init_module+0x281/0x8e0
> load_module+0x658c/0x7cc0
> init_module_from_file+0x15b/0x180
> idempotent_init_module+0x22d/0x770
> __x64_sys_finit_module+0xd2/0x160
> do_syscall_64+0x115/0x6a0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> The buggy address belongs to the object at ffff88810d8b4c00
> which belongs to the cache kmalloc-192 of size 192
> The buggy address is located 152 bytes inside of
> freed 192-byte region [ffff88810d8b4c00, ffff88810d8b4cc0)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10d8b4
> flags: 0x200000000000000(node=0|zone=2)
> page_type: f5(slab)
> raw: 0200000000000000 ffff8881000423c0 dead000000000122 0000000000000000
> raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000
> page dumped because: kasan: bad access detected
>
> Memory state around the buggy address:
> ffff88810d8b4b80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
> ffff88810d8b4c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88810d8b4c80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> ^
> ffff88810d8b4d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff88810d8b4d80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
> Fixes: 16e434670e95 ("ALSA: dummy: allow disabling mixer controls")
> Assisted-by: LLM
> Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
> ---
>
> diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
> index 6b0a06e5c87638264e5d22deda1fe4bc028febe7..d422fb3fa25d1279df2e971e0f5546a329db4049 100644
> --- a/sound/drivers/dummy.c
> +++ b/sound/drivers/dummy.c
> @@ -134,7 +134,7 @@ struct snd_dummy {
> int mixer_volume[MIXER_ADDR_LAST+1][2];
> int capture_source[MIXER_ADDR_LAST+1][2];
> int iobox;
> - struct snd_kcontrol *cd_volume_ctl;
> + struct snd_ctl_elem_id cd_volume_id;
> struct snd_kcontrol *cd_switch_ctl;
> };
>
> @@ -836,6 +836,7 @@ static int snd_dummy_iobox_put(struct snd_kcontrol *kcontrol,
> struct snd_ctl_elem_value *value)
> {
> struct snd_dummy *dummy = snd_kcontrol_chip(kcontrol);
> + struct snd_kcontrol *cd_volume_ctl;
> int changed;
>
> if (value->value.enumerated.item[0] > 1)
> @@ -845,20 +846,27 @@ static int snd_dummy_iobox_put(struct snd_kcontrol *kcontrol,
> if (changed) {
> dummy->iobox = value->value.enumerated.item[0];
>
> + /* The control-write path holds controls_rwsem. */
> + cd_volume_ctl = snd_ctl_find_id(dummy->card,
> + &dummy->cd_volume_id);
> if (dummy->iobox) {
> - dummy->cd_volume_ctl->vd[0].access &=
> - ~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> + if (cd_volume_ctl)
> + cd_volume_ctl->vd[0].access &=
> + ~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> dummy->cd_switch_ctl->vd[0].access &=
> ~SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> } else {
> - dummy->cd_volume_ctl->vd[0].access |=
> - SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> + if (cd_volume_ctl)
> + cd_volume_ctl->vd[0].access |=
> + SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> dummy->cd_switch_ctl->vd[0].access |=
> SNDRV_CTL_ELEM_ACCESS_INACTIVE;
> }
>
> - snd_ctl_notify(dummy->card, SNDRV_CTL_EVENT_MASK_INFO,
> - &dummy->cd_volume_ctl->id);
> + if (cd_volume_ctl)
> + snd_ctl_notify(dummy->card,
> + SNDRV_CTL_EVENT_MASK_INFO,
> + &cd_volume_ctl->id);
> snd_ctl_notify(dummy->card, SNDRV_CTL_EVENT_MASK_INFO,
> &dummy->cd_switch_ctl->id);
> }
> @@ -903,7 +911,7 @@ static int snd_card_dummy_new_mixer(struct snd_dummy *dummy)
> if (err < 0)
> return err;
> if (!strcmp(kcontrol->id.name, "CD Volume"))
> - dummy->cd_volume_ctl = kcontrol;
> + dummy->cd_volume_id = kcontrol->id;
> else if (!strcmp(kcontrol->id.name, "CD Capture Switch"))
> dummy->cd_switch_ctl = kcontrol;
>
This is AI obfuscation. I'm very, very sorry for this noise. It is all my fault.
Sorry for this again :(
Best regards,
Cen Zhang
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2026-10-16 12:51 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
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2026-10-16 2:17 [PATCH] ALSA: dummy: Revalidate CD Volume before updating its access Cen Zhang
2026-10-16 12:51 ` Cen Zhang
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