[PATCH 6.12 211/567] KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot

Greg Kroah-Hartman
2026-01-06T17:27:40Z

From: Sean Christopherson <seanjc@google.com>

commit 9935df5333aa503a18de5071f53762b65c783c4c upstream.

Reject attempts to disable KVM_MEM_GUEST_MEMFD on a memslot that was
initially created with a guest_memfd binding, as KVM doesn't support
toggling KVM_MEM_GUEST_MEMFD on existing memslots. KVM prevents enabling
KVM_MEM_GUEST_MEMFD, but doesn't prevent clearing the flag.

Failure to reject the new memslot results in a use-after-free due to KVM
not unbinding from the guest_memfd instance. Unbinding on a FLAGS_ONLY
change is easy enough, and can/will be done as a hardening measure (in
anticipation of KVM supporting dirty logging on guest_memfd at some point),
but fixing the use-after-free would only address the immediate symptom.

==================================================================
BUG: KASAN: slab-use-after-free in kvm_gmem_release+0x362/0x400 [kvm]
Write of size 8 at addr ffff8881111ae908 by task repro/745

CPU: 7 UID: 1000 PID: 745 Comm: repro Not tainted 6.18.0-rc6-115d5de2eef3-next-kasan #3 NONE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
Call Trace:
<TASK>
dump_stack_lvl+0x51/0x60
print_report+0xcb/0x5c0
kasan_report+0xb4/0xe0
kvm_gmem_release+0x362/0x400 [kvm]
__fput+0x2fa/0x9d0
task_work_run+0x12c/0x200
do_exit+0x6ae/0x2100
do_group_exit+0xa8/0x230
__x64_sys_exit_group+0x3a/0x50
x64_sys_call+0x737/0x740
do_syscall_64+0x5b/0x900
entry_SYSCALL_64_after_hwframe+0x4b/0x53
RIP: 0033:0x7f581f2eac31
</TASK>

Allocated by task 745 on cpu 6 at 9.746971s:
kasan_save_stack+0x20/0x40
kasan_save_track+0x13/0x50
__kasan_kmalloc+0x77/0x90
kvm_set_memory_region.part.0+0x652/0x1110 [kvm]
kvm_vm_ioctl+0x14b0/0x3290 [kvm]
__x64_sys_ioctl+0x129/0x1a0
do_syscall_64+0x5b/0x900
entry_SYSCALL_64_after_hwframe+0x4b/0x53

Freed by task 745 on cpu 6 at 9.747467s:
kasan_save_stack+0x20/0x40
kasan_save_track+0x13/0x50
__kasan_save_free_info+0x37/0x50
__kasan_slab_free+0x3b/0x60
kfree+0xf5/0x440
kvm_set_memslot+0x3c2/0x1160 [kvm]
kvm_set_memory_region.part.0+0x86a/0x1110 [kvm]
kvm_vm_ioctl+0x14b0/0x3290 [kvm]
__x64_sys_ioctl+0x129/0x1a0
do_syscall_64+0x5b/0x900
entry_SYSCALL_64_after_hwframe+0x4b/0x53

Reported-by: Alexander Potapenko <glider@google.com>
Fixes: a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20251202020334.1171351-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
virt/kvm/kvm_main.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2062,7 +2062,7 @@ int __kvm_set_memory_region(struct kvm *
return -EINVAL;
if ((mem->userspace_addr != old->userspace_addr) ||
(npages != old->npages) ||
- ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
+ ((mem->flags ^ old->flags) & (KVM_MEM_READONLY | KVM_MEM_GUEST_MEMFD)))
return -EINVAL;

if (base_gfn != old->base_gfn)