Re: [PATCH v2] i386/kvm: fix a use-after-free when vcpu plug/unplug

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 13 May 2020 09:26:30 -0400
Pan Nengyuan <pannengyuan@huawei.com> wrote:

> When we hotplug vcpus, cpu_update_state is added to vm_change_state_head
> in kvm_arch_init_vcpu(). But it forgot to delete in kvm_arch_destroy_vcpu() after
> unplug. Then it will cause a use-after-free access. This patch delete it in
> kvm_arch_destroy_vcpu() to fix that.
> 
> Reproducer:
>     virsh setvcpus vm1 4 --live
>     virsh setvcpus vm1 2 --live
>     virsh suspend vm1
>     virsh resume vm1
> 
> The UAF stack:
> ==qemu-system-x86_64==28233==ERROR: AddressSanitizer: heap-use-after-free on address 0x62e00002e798 at pc 0x5573c6917d9e bp 0x7fff07139e50 sp 0x7fff07139e40
> WRITE of size 1 at 0x62e00002e798 thread T0
>     #0 0x5573c6917d9d in cpu_update_state /mnt/sdb/qemu/target/i386/kvm.c:742
>     #1 0x5573c699121a in vm_state_notify /mnt/sdb/qemu/vl.c:1290
>     #2 0x5573c636287e in vm_prepare_start /mnt/sdb/qemu/cpus.c:2144
>     #3 0x5573c6362927 in vm_start /mnt/sdb/qemu/cpus.c:2150
>     #4 0x5573c71e8304 in qmp_cont /mnt/sdb/qemu/monitor/qmp-cmds.c:173
>     #5 0x5573c727cb1e in qmp_marshal_cont qapi/qapi-commands-misc.c:835
>     #6 0x5573c7694c7a in do_qmp_dispatch /mnt/sdb/qemu/qapi/qmp-dispatch.c:132
>     #7 0x5573c7694c7a in qmp_dispatch /mnt/sdb/qemu/qapi/qmp-dispatch.c:175
>     #8 0x5573c71d9110 in monitor_qmp_dispatch /mnt/sdb/qemu/monitor/qmp.c:145
>     #9 0x5573c71dad4f in monitor_qmp_bh_dispatcher /mnt/sdb/qemu/monitor/qmp.c:234
> 
> Reported-by: Euler Robot <euler.robot@huawei.com>
> Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Reviewed-by: Igor Mammedov <imammedo@redhat.com>

> ---
> - v2: remove unnecessary set vmsentry to null(there is no non-null check).
> ---
>  target/i386/cpu.h | 1 +
>  target/i386/kvm.c | 4 +++-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index e818fc712a..afbd11b7a3 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1631,6 +1631,7 @@ struct X86CPU {
>  
>      CPUNegativeOffsetState neg;
>      CPUX86State env;
> +    VMChangeStateEntry *vmsentry;
>  
>      uint64_t ucode_rev;
>  
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index 4901c6dd74..0a4eca5a85 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -1770,7 +1770,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>          }
>      }
>  
> -    qemu_add_vm_change_state_handler(cpu_update_state, env);
> +    cpu->vmsentry = qemu_add_vm_change_state_handler(cpu_update_state, env);
>  
>      c = cpuid_find_entry(&cpuid_data.cpuid, 1, 0);
>      if (c) {
> @@ -1883,6 +1883,8 @@ int kvm_arch_destroy_vcpu(CPUState *cs)
>          env->nested_state = NULL;
>      }
>  
> +    qemu_del_vm_change_state_handler(cpu->vmsentry);
> +
>      return 0;
>  }
>