CVE-2026-74640 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
ALSA: FCP: fix OOB write in fcp_meter_ctl_get()
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 <[email protected]>
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel contains a critical out-of-bounds write vulnerability within the ALSA FCP driver's fcp_meter_ctl_get function, classified under CWE-787 as an Out-of-Bounds Write. This flaw arises from a discrepancy between the validation limits applied during control setup and the actual capacity of the destination data structure used during runtime operations. Specifically, when configuring the Level Meter map via fcp_ioctl_set_meter_map, the driver validates that both the map size and meter slots do not exceed 255 elements. However, this validated count is subsequently passed to fcp_add_new_ctl as the channel count for a control element. The core ALSA subsystem stores these values in a struct snd_ctl_elem_value object, which contains an integer array declared with a fixed maximum capacity of only 128 entries. This fundamental mismatch creates a scenario where user-supplied input can exceed the allocated buffer size by up to double its intended limit.
The operational impact is severe because the vulnerability allows for arbitrary memory corruption within kernel space. During execution, fcp_meter_ctl_get iterates through the channel count and writes 64-bit values directly into the value array without performing any additional bounds checking at that stage. Since each entry occupies eight bytes on LP64 architectures, writing more than 128 entries results in a slab-out-of-bounds write. KASAN analysis confirms that with a map size of 255, writes extend up to 888 bytes past the allocated region and into adjacent kernel memory structures. The attacker can control both the extent of the overflow and the specific contents written by manipulating the meter_level_map array, which selects which device response words are stored in each slot. This capability enables potential privilege escalation or denial of service through heap corruption, as the overwritten data may include critical kernel pointers or state variables used by other subsystems.
From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1059, specifically command and script interpretation via system utilities if leveraged for further exploitation, but more accurately represents memory corruption primitives often associated with local privilege escalation paths such as those found in CVE-2023-xxxx series kernel flaws. Although installing an oversized map requires CAP_SYS_RAWIO privileges initially, the resulting control object persists beyond the lifecycle of the hardware descriptor that created it. Consequently, any unprivileged process capable of reading controls on the corresponding device node can trigger the out-of-bounds write during a standard ioctl call to snd_ctl_elem_read. This effectively lowers the attack complexity from requiring high-level system administration rights to merely needing access to specific sound devices, significantly expanding the potential attack surface for local attackers within multi-user environments or virtualized containers where such device nodes might be exposed.
The root cause lies in insufficient validation logic at multiple layers of the ALSA control framework. The core function snd_ctl_check_elem_info is only invoked under CONFIG_SND_CTL_DEBUG configurations; without this debug flag, the system relies on snd_ctl_skip_validation which acts as a compile-time true, bypassing checks entirely. Furthermore, __snd_ctl_add_replace validates kcontrol->count but fails to inspect elem->channels for consistency with the underlying buffer capacity. To mitigate this issue, developers must enforce strict bounds checking against the ABI limit of 128 rather than the driver-specific limit of 255 during both configuration and retrieval phases. The fix involves bounding the map size by the actual array limits and ensuring the store loop in fcp_meter_ctl_get terminates before exceeding the value array boundaries regardless of what elem->channels holds. This ensures that even if malicious input attempts to exploit the discrepancy, the kernel memory remains protected from unauthorized writes, preserving system integrity and preventing potential exploitation chains stemming from heap corruption.