KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot

In the Linux kernel, the following vulnerability has been resolved:

KVM: Disallow toggling KVM_MEM_GUEST_MEMFD on an existing memslot

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