CVE-2025-40274 in Linux
Summary
by MITRE • 12/07/2025
In the Linux kernel, the following vulnerability has been resolved:
KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying
When unbinding a memslot from a guest_memfd instance, remove the bindings even if the guest_memfd file is dying, i.e. even if its file refcount has gone to zero. If the memslot is freed before the file is fully released, nullifying the memslot side of the binding in kvm_gmem_release() will write to freed memory, as detected by syzbot+KASAN:
================================================================== BUG: KASAN: slab-use-after-free in kvm_gmem_release+0x176/0x440 virt/kvm/guest_memfd.c:353 Write of size 8 at addr ffff88807befa508 by task syz.0.17/6022
CPU: 0 UID: 0 PID: 6022 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025 Call Trace: <TASK> dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline]
print_report+0xca/0x240 mm/kasan/report.c:482 kasan_report+0x118/0x150 mm/kasan/report.c:595 kvm_gmem_release+0x176/0x440 virt/kvm/guest_memfd.c:353 __fput+0x44c/0xa70 fs/file_table.c:468 task_work_run+0x1d4/0x260 kernel/task_work.c:227 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7fbeeff8efc9 </TASK>
Allocated by task 6023: kasan_save_stack mm/kasan/common.c:56 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:77 poison_kmalloc_redzone mm/kasan/common.c:397 [inline]
__kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:414 kasan_kmalloc include/linux/kasan.h:262 [inline]
__kmalloc_cache_noprof+0x3e2/0x700 mm/slub.c:5758 kmalloc_noprof include/linux/slab.h:957 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
kvm_set_memory_region+0x747/0xb90 virt/kvm/kvm_main.c:2104 kvm_vm_ioctl_set_memory_region+0x6f/0xd0 virt/kvm/kvm_main.c:2154 kvm_vm_ioctl+0x957/0xc60 virt/kvm/kvm_main.c:5201 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 6023: kasan_save_stack mm/kasan/common.c:56 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:77 kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584 poison_slab_object mm/kasan/common.c:252 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:284 kasan_slab_free include/linux/kasan.h:234 [inline]
slab_free_hook mm/slub.c:2533 [inline]
slab_free mm/slub.c:6622 [inline]
kfree+0x19a/0x6d0 mm/slub.c:6829 kvm_set_memory_region+0x9c4/0xb90 virt/kvm/kvm_main.c:2130 kvm_vm_ioctl_set_memory_region+0x6f/0xd0 virt/kvm/kvm_main.c:2154 kvm_vm_ioctl+0x957/0xc60 virt/kvm/kvm_main.c:5201 vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
Deliberately don't acquire filemap invalid lock when the file is dying as the lifecycle of f_mapping is outside the purview of KVM. Dereferencing the mapping is *probably* fine, but there's no need to invalidate anything as memslot deletion is responsible for zapping SPTEs, and the only code that can access the dying file is kvm_gmem_release(), whose core code is mutual ---truncated---
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Analysis
by VulDB Data Team • 02/23/2026
The vulnerability described in CVE-2025-40274 resides within the Linux kernel's KVM subsystem, specifically in the handling of guest memory file descriptors. This flaw manifests as a use-after-free condition that occurs during the cleanup of memory slots associated with guest_memfd instances. The issue arises when a memslot is removed from a guest_memfd file that is already in the process of being destroyed, leading to memory corruption and potential system instability. The vulnerability was detected by KASAN, a kernel memory error detector, which identified that a write operation occurred to freed memory at address ffff88807befa508, indicating that the memory region had already been deallocated but was still being accessed.
The technical root cause of this vulnerability stems from improper synchronization during the cleanup process of memory mappings within KVM's guest memory management system. When a memslot is deleted, the code attempts to nullify the memslot side of the binding in the kvm_gmem_release() function. However, if the guest_memfd file is already in the dying state with a reference count of zero, this operation can proceed against freed memory structures. This scenario typically occurs during concurrent operations where memory region modifications and file cleanup happen simultaneously, creating a race condition between the memory management subsystem and the KVM subsystem. The vulnerability is classified under CWE-415 as a double free or a use-after-free condition, which represents a critical class of memory safety issues that can lead to arbitrary code execution or system crashes.
The operational impact of this vulnerability is significant for virtualized environments that rely on KVM for hypervisor functionality. Attackers could potentially exploit this condition to cause denial of service or achieve privilege escalation within the virtualized environment. The vulnerability affects systems running Linux kernels that implement KVM virtualization, particularly those managing guest memory through memfd mechanisms. The use-after-free condition can result in unpredictable behavior including system crashes, data corruption, or in more severe cases, potential code execution if the attacker can control the memory layout. The vulnerability is particularly concerning in cloud computing and containerized environments where KVM is extensively used for virtual machine management, as it could allow an unprivileged user to compromise the host system.
Mitigation strategies for this vulnerability involve ensuring that the kernel is patched with the appropriate fix that prevents the nullification of freed memory during memslot deletion. The fix implemented in the kernel ensures that when a memslot is being deleted and the associated guest_memfd file is already in the dying state, the binding removal operation does not attempt to access freed memory structures. This is achieved by not acquiring the filemap invalid lock when the file is dying, as the lifecycle of f_mapping is outside KVM's control. The patch maintains the existing functionality while preventing the memory corruption that occurs during cleanup operations. System administrators should immediately apply the kernel patches released by the Linux kernel security team to protect against this vulnerability, particularly in production environments where KVM virtualization is utilized. Additionally, monitoring for unusual system behavior or crashes related to memory management operations can help detect exploitation attempts, and implementing proper access controls and sandboxing measures can reduce the potential impact of such vulnerabilities in compromised environments.