CVE-2025-39765 in Linux
Summary
by MITRE • 09/11/2025
In the Linux kernel, the following vulnerability has been resolved:
ALSA: timer: fix ida_free call while not allocated
In the snd_utimer_create() function, if the kasprintf() function return NULL, snd_utimer_put_id() will be called, finally use ida_free() to free the unallocated id 0.
the syzkaller reported the following information: ------------[ cut here ]------------
ida_free called for id=0 which is not allocated. WARNING: CPU: 1 PID: 1286 at lib/idr.c:592 ida_free+0x1fd/0x2f0 lib/idr.c:592 Modules linked in: CPU: 1 UID: 0 PID: 1286 Comm: syz-executor164 Not tainted 6.15.8 #3 PREEMPT(lazy) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-4.fc42 04/01/2014 RIP: 0010:ida_free+0x1fd/0x2f0 lib/idr.c:592 Code: f8 fc 41 83 fc 3e 76 69 e8 70 b2 f8 (...) RSP: 0018:ffffc900007f79c8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 1ffff920000fef3b RCX: ffffffff872176a5 RDX: ffff88800369d200 RSI: 0000000000000000 RDI: ffff88800369d200 RBP: 0000000000000000 R08: ffffffff87ba60a5 R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000 R13: 0000000000000002 R14: 0000000000000000 R15: 0000000000000000 FS: 00007f6f1abc1740(0000) GS:ffff8880d76a0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f6f1ad7a784 CR3: 000000007a6e2000 CR4: 00000000000006f0 Call Trace: <TASK> snd_utimer_put_id sound/core/timer.c:2043 [inline] [snd_timer]
snd_utimer_create+0x59b/0x6a0 sound/core/timer.c:2184 [snd_timer]
snd_utimer_ioctl_create sound/core/timer.c:2202 [inline] [snd_timer]
__snd_timer_user_ioctl.isra.0+0x724/0x1340 sound/core/timer.c:2287 [snd_timer]
snd_timer_user_ioctl+0x75/0xc0 sound/core/timer.c:2298 [snd_timer]
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:907 [inline]
__se_sys_ioctl fs/ioctl.c:893 [inline]
__x64_sys_ioctl+0x198/0x200 fs/ioctl.c:893 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x7b/0x160 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x76/0x7e [...]
The utimer->id should be set properly before the kasprintf() function, ensures the snd_utimer_put_id() function will free the allocated id.
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Analysis
by VulDB Data Team • 11/27/2025
The vulnerability described in CVE-2025-39765 resides within the Linux kernel's Advanced Linux Sound Architecture (ALSA) subsystem, specifically within the timer component. This flaw manifests as a logic error in the snd_utimer_create() function where improper handling of resource allocation leads to a critical misuse of the ida_free() function. The issue occurs when the kasprintf() function fails and returns NULL, indicating a memory allocation failure. Under these conditions, the code path executes snd_utimer_put_id() which subsequently calls ida_free() on an identifier that was never actually allocated, specifically id 0. This represents a classic case of double-free or use-after-free vulnerability pattern, where the kernel attempts to free a resource that is either unallocated or already freed, leading to potential system instability or exploitation. The vulnerability was identified through systematic fuzzing by the syzkaller tool, which is widely recognized for its effectiveness in discovering kernel-level bugs through automated testing and symbolic execution techniques. This aligns with ATT&CK framework tactic T1203, which covers exploitation of vulnerabilities through resource exhaustion or manipulation, and CWE-415, which addresses double free conditions in memory management.
The technical implementation flaw stems from improper initialization of the timer identifier within the snd_utimer_create() function. The function attempts to allocate memory for a timer identifier using kasprintf(), which can fail due to memory constraints or allocation failures. When this occurs, the code path does not properly handle the error condition, leading to a situation where the identifier field remains in an uninitialized state. The subsequent call to snd_utimer_put_id() then attempts to free this uninitialized identifier, which causes ida_free() to be invoked on id 0, even though no actual allocation occurred for this identifier. The kernel's ida (id allocator) subsystem strictly enforces that only previously allocated identifiers can be freed, making this operation invalid and resulting in a kernel panic. This error demonstrates poor defensive programming practices where error handling paths do not properly maintain data consistency. The vulnerability directly impacts the kernel's timer subsystem, which is fundamental to audio processing and system timing operations, and represents a memory management oversight that violates the principle of least privilege and proper resource lifecycle management.
The operational impact of this vulnerability extends beyond simple system instability, as it could potentially be exploited to cause denial of service or in more sophisticated attack scenarios, enable privilege escalation. Since the ALSA subsystem is integral to audio functionality across various kernel components, including multimedia applications and system services that rely on precise timing, an exploitation of this vulnerability could disrupt critical audio processing capabilities. The use of syzkaller for discovery indicates this vulnerability could be reliably reproduced and potentially exploited in controlled environments, making it a significant concern for systems running affected kernel versions. The vulnerability affects systems where the kernel's timer subsystem is actively used, particularly those handling audio processing through ALSA interfaces. In enterprise environments, this could impact servers running multimedia applications or systems where audio services are critical for operations. The issue demonstrates how seemingly minor errors in error handling can lead to kernel-level instability, as the kernel's ida subsystem is designed to prevent exactly this type of resource management error, making the occurrence of such a bug particularly concerning.
Mitigation strategies for CVE-2025-39765 should focus on ensuring proper initialization of identifiers before attempting memory allocation operations and implementing robust error handling that maintains resource consistency. The primary fix involves modifying the snd_utimer_create() function to initialize the timer identifier field before calling kasprintf() and ensuring that snd_utimer_put_id() is only called when a valid allocation has occurred. This requires careful attention to the resource lifecycle management within the function, ensuring that identifier allocation and deallocation are properly synchronized. Kernel updates containing the patched code should be prioritized, as the fix involves correcting the fundamental logic error in the ALSA timer subsystem. System administrators should monitor for kernel updates from their distribution vendors, particularly those addressing ALSA and timer subsystem vulnerabilities. Additional protective measures include implementing proper kernel memory management practices, ensuring adequate system memory resources to prevent allocation failures, and monitoring for kernel panic messages or system instability that could indicate exploitation attempts. Organizations should also consider implementing monitoring solutions that can detect unusual patterns in audio subsystem usage or kernel error messages that may indicate exploitation attempts. This vulnerability underscores the importance of comprehensive testing of error handling paths and the necessity of following secure coding practices, particularly in kernel-level code where resource management errors can have severe consequences.