Re: [PATCH] ALSA: FCP: fix OOB write in fcp_meter_ctl_get()

[Takashi Iwai <tiwai@suse.de>]

On Tue, 04 Aug 2026 14:36:11 +0200,
Baul Lee wrote:
> 
> fcp_ioctl_set_meter_map() bounds the user-supplied Level Meter map size
> by the driver's own limit of 255
> 
> 	if (map.map_size < 1 || map.map_size > 255 ||
> 	    map.meter_slots < 1 || map.meter_slots > 255)
> 		return -EINVAL;
> 
> and passes it to fcp_add_new_ctl() as the control's channel count, where
> it is stored as elem->channels.
> 
> Every control read writes into struct snd_ctl_elem_value, whose integer
> array is declared long value[128], so the limit is 128, not 255.
> fcp_meter_ctl_get() stores one 64-bit word per channel into that array
> with no bound of its own:
> 
> 	for (i = 0; i < elem->channels; i++) {
> 		int idx = private->meter_level_map[i];
> 		int value = idx < 0 ? 0 : le32_to_cpu(resp[idx]);
> 
> 		ucontrol->value.integer.value[i] = value;
> 	}
> 
> snd_ctl_elem_read_user() serves that object from
> memdup_user(_control, sizeof(*control)), 1224 bytes on LP64 out of
> kmalloc-2048.  offsetof(struct snd_ctl_elem_value, value) is 72, so
> element i is written at byte 72 + 8 * i and element 144 already lands
> past the allocation.  At map_size 255 the last store ends at byte 2112,
> 888 bytes past the object and 64 bytes into the adjacent slab object.
> The stored words come from the device and meter_level_map[] selects
> which word lands in which slot, so extent and contents are both
> controlled.
> 
> The core does not catch this.  snd_ctl_check_elem_info() is reached only
> from __snd_ctl_elem_info(), which snd_ctl_elem_read() calls under
> CONFIG_SND_CTL_DEBUG; without that option snd_ctl_skip_validation() is a
> compile-time true.  __snd_ctl_add_replace() validates kcontrol->count and
> never inspects elem->channels.
> 
> Installing an oversized map needs CAP_SYS_RAWIO, but the control outlives
> the hwdep descriptor that created it, so the out-of-bounds stores are
> issued by any process able to read controls on /dev/snd/controlC0.
> 
> KASAN on 7.2.0-rc5 (arm64), triggered by an unprivileged control read:
> 
>   BUG: KASAN: slab-out-of-bounds in fcp_meter_ctl_get
>   Write of size 8 at addr ffff000017af04c8 by task fcp_trigger/185
>    __asan_store8
>    fcp_meter_ctl_get
>    snd_ctl_elem_read
>    snd_ctl_ioctl
>   Allocated by task 185:
>    memdup_user
>    snd_ctl_ioctl
>   The buggy address is located 0 bytes to the right of
>    allocated 1224-byte region [ffff000017af0000, ffff000017af04c8)
> 
> Bound the map size by the ABI limit rather than by 255, and bound the
> store loop at the sink so it cannot run past the value array whatever
> elem->channels holds.
> 
> Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>
> 
> Fixes: 46757a3e7d50 ("ALSA: FCP: Add Focusrite Control Protocol driver")
> Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
> Reported-by: Baul Lee <baul.lee@xbow.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Baul Lee <baul.lee@xbow.com>

Applied now.  Thanks.


Takashi