mirror of https://lore.kernel.org/lkml/
 help / color / mirror / Atom feed
* [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
end of thread, other threads:[~2026-10-16 12:51 UTC | newest]

Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2026-10-16  2:17 [PATCH] ALSA: dummy: Revalidate CD Volume before updating its access Cen Zhang
2026-10-16 12:51 ` Cen Zhang

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox
all inboxes | Powered by JetHome®