linux-cve-announce - CVE-2025-40274: KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying

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Message-ID: <2025120716-CVE-2025-40274-53be@gregkh>
Date: Sun,  7 Dec 2025 06:52:21 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-40274: KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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
mutually exclusive with unbinding.

Note, the mutual exclusivity is also what makes it safe to access the
bindings on a dying gmem instance.  Unbinding either runs with slots_lock
held, or after the last reference to the owning "struct kvm" is put, and
kvm_gmem_release() nullifies the slot pointer under slots_lock, and puts
its reference to the VM after that is done.

The Linux kernel CVE team has assigned CVE-2025-40274 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.8 with commit a7800aa80ea4d5356b8474c2302812e9d4926fa6 and fixed in 6.12.59 with commit a8ac2bd0f98e1a230f1eb3260fa552bf2ef1753b
	Issue introduced in 6.8 with commit a7800aa80ea4d5356b8474c2302812e9d4926fa6 and fixed in 6.17.9 with commit 393893693a523e053f84d69320d090b93503f79f
	Issue introduced in 6.8 with commit a7800aa80ea4d5356b8474c2302812e9d4926fa6 and fixed in 6.18 with commit ae431059e75d36170a5ae6b44cc4d06d43613215

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-40274
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	virt/kvm/guest_memfd.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a8ac2bd0f98e1a230f1eb3260fa552bf2ef1753b
	https://git.kernel.org/stable/c/393893693a523e053f84d69320d090b93503f79f
	https://git.kernel.org/stable/c/ae431059e75d36170a5ae6b44cc4d06d43613215