CVE-2026-98776 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
binderfs: fix UAF write in binder_add_device
binderfs_binder_device_create() publishes the new dentry with d_make_persistent() and then calls simple_done_creating(), which drops the parent directory lock and the creator's dentry reference. It then calls binder_add_device() to register the device in the global binder_devices list.
After simple_done_creating() releases the parent directory lock, a concurrent unlinkat() can remove the new device entry. Dropping the creator's dentry reference can then trigger binderfs_evict_inode(), freeing the device. binder_add_device() later accesses the freed object, causing UAF write.
Found by a modified Syzkaller:
BUG: KASAN: slab-use-after-free in hlist_add_head include/linux/list.h:1073 [inline]
BUG: KASAN: slab-use-after-free in binder_add_device+0xa9/0xc0 drivers/android/binder.c:7068 Write of size 8 at addr ffff88805b340c00 by task syz.1.532/11389
CPU: 0 UID: 0 PID: 11389 Comm: syz.1.532 Not tainted 7.2.0 #4 PREEMPT(full) 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 lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x10e/0x1f0 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline]
print_report+0xf7/0x600 mm/kasan/report.c:482 kasan_report+0xe4/0x120 mm/kasan/report.c:595 hlist_add_head include/linux/list.h:1073 [inline]
binder_add_device+0xa9/0xc0 drivers/android/binder.c:7068 binderfs_binder_device_create.isra.0+0x724/0x990 drivers/android/binderfs.c:196 binder_ctl_ioctl+0x186/0x1b0 drivers/android/binderfs.c:241 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7ff9027a833d Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ff903674018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 RAX: ffffffffffffffda RBX: 00007ff902a35fa0 RCX: 00007ff9027a833d RDX: 0000200000000500 RSI: 00000000c1086201 RDI: 0000000000000004 RBP: 00007ff902850733 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 00007ff902a36038 R14: 00007ff902a35fa0 R15: 00007ffd7166bfa0 </TASK>
Allocated by task 11389: kasan_save_stack+0x33/0x60 mm/kasan/common.c:57 kasan_save_track+0x14/0x30 mm/kasan/common.c:78 poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415 kasan_kmalloc include/linux/kasan.h:263 [inline]
__kmalloc_cache_noprof+0x2e4/0x6f0 mm/slub.c:5489 _kmalloc_noprof include/linux/slab.h:988 [inline]
_kzalloc_noprof include/linux/slab.h:1309 [inline]
binderfs_binder_device_create.isra.0+0x17a/0x990 drivers/android/binderfs.c:148 binder_ctl_ioctl+0x186/0x1b0 drivers/android/binderfs.c:241 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 11389: kasan_save_stack+0x33/0x60 mm/kasan/common.c:57 kasan_save_track+0x14/0x30 mm/kasan/common.c:78 kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:584 poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285 kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2677 [inline]
slab_free mm/slub.c:6377 [inline]
kfree+0x2fc/0x6e0 mm/slub.c:6692 binderf ---truncated---
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Analysis
by VulDB Data Team • 10/19/2026
The Linux kernel's binderfs subsystem contains a critical use-after-free vulnerability within the device creation logic, specifically in the binder_add_device function. This flaw arises from an improper synchronization sequence during the initialization of new binder devices. The root cause lies in the interaction between filesystem operations and internal state management. When creating a new binder device via ioctl calls, the kernel executes binderfs_binder_device_create to allocate memory for the device structure and register it with the virtual file system. This process involves publishing the directory entry using d_make_persistent followed by calling simple_done_creating. The latter function is responsible for finalizing the creation of the inode and releasing associated locks, including the parent directory lock and the creator's reference to the dentry.
The vulnerability manifests because the release of these synchronization primitives occurs before the device is fully registered in the global binder_devices list via binder_add_device. Once simple_done_creating releases the parent directory lock, a concurrent thread can execute an unlinkat system call targeting the newly created but not yet safely registered device entry. This removal triggers the eviction process for the inode associated with that dentry. Because the creator's reference to the dentry was already dropped by simple_done_creating, the binderfs_evict_inode function is invoked immediately upon unlinking. This eviction routine frees the underlying memory allocated for the binder_device structure using kfree.
Subsequently, when control returns from simple_done_creating and proceeds to call binder_add_device, the code attempts to access the now-freed device object to add it to the global hlist. Specifically, the function calls hlist_add_head on a pointer that points to memory already returned to the slab allocator. This constitutes a classic use-after-free condition where the kernel writes data into freed heap memory. The KASAN report confirms this by showing a write operation at an address previously allocated and then poisoned as free space. Such corruption can lead to unpredictable system behavior, including crashes, privilege escalation if the attacker controls the contents of the reclaimed slab cache, or denial of service through kernel oopses.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, which describes errors resulting from using memory after it has been freed without proper re-initialization or reference counting checks. In terms of attack vectors and techniques, the scenario involves race conditions triggered by concurrent system calls, mapping to MITRE ATT&CK technique T1059, Command and Scripting Interpreter, specifically through ioctl abuse in a local context. The ability to trigger this via standard file operations like unlinkat combined with binder control ioctls means that any unprivileged user with access to the binderfs mount point can potentially exploit this race condition if they possess sufficient concurrency capabilities or timing precision.
The operational impact of this vulnerability is severe due to its location in kernel space. A successful exploitation could allow a local attacker to execute arbitrary code within the kernel context, effectively gaining root privileges on the affected system. Even without full code execution, the memory corruption can destabilize the kernel, leading to service interruptions for all processes running on the host. Given that binder is widely used in Android-based systems and increasingly in other Linux environments for inter-process communication, the potential attack surface is significant. The vulnerability highlights the complexity of maintaining consistency between filesystem layer abstractions like dentries and internal driver state machines when dealing with concurrent access patterns.
Mitigation strategies primarily involve applying kernel patches that correct the ordering of operations during device creation. Developers must ensure that the binder_add_device call occurs while holding appropriate locks or before releasing references that could trigger premature inode eviction. Implementing stricter reference counting for the dentry until after global registration is complete would prevent the race condition from being exploitable. Additionally, using lockless data structures with atomic operations where possible can reduce window of vulnerability in high-concurrency scenarios. System administrators should ensure their systems are updated to versions containing this fix and monitor for any signs of kernel instability or unexpected crashes that might indicate attempted exploitation attempts. Regular auditing of ioctl handlers against race conditions is recommended as a best practice for maintaining robustness in device drivers interacting with virtual file systems.