Re: [PATCH] KVM: nVMX: Inject #GP when nested_vmx_get_vmptr() fails to read guest memory

Paolo Bonzini
2020-06-04T14:41:00Z

On 04/06/20 16:31, Vitaly Kuznetsov wrote:
> Syzbot reports the following issue:
>
> WARNING: CPU: 0 PID: 6819 at arch/x86/kvm/x86.c:618 kvm_inject_emulated_page_fault+0x210/0x290 arch/x86/kvm/x86.c:618
> ...
> Call Trace:
> ...
> RIP: 0010:kvm_inject_emulated_page_fault+0x210/0x290 arch/x86/kvm/x86.c:618
> ...
> nested_vmx_get_vmptr+0x1f9/0x2a0 arch/x86/kvm/vmx/nested.c:4638
> handle_vmon arch/x86/kvm/vmx/nested.c:4767 [inline]
> handle_vmon+0x168/0x3a0 arch/x86/kvm/vmx/nested.c:4728
> vmx_handle_exit+0x29c/0x1260 arch/x86/kvm/vmx/vmx.c:6067
>
> 'exception' we're trying to inject with kvm_inject_emulated_page_fault() comes from
> nested_vmx_get_vmptr()
> kvm_read_guest_virt()
> kvm_read_guest_virt_helper()
> vcpu->arch.walk_mmu->gva_to_gpa()
>
> but it is only set when GVA to GPA conversion fails. In case it doesn't but
> we still fail kvm_vcpu_read_guest_page(), X86EMUL_IO_NEEDED is returned and
> nested_vmx_get_vmptr() calls kvm_inject_emulated_page_fault() with zeroed
> 'exception'. This happen when e.g. VMXON/VMPTRLD/VMCLEAR argument is MMIO.
>
> KVM could've handled the request correctly by going to userspace and
> performing I/O but there doesn't seem to be a good need for such requests
> in the first place. Sane guests should not call VMXON/VMPTRLD/VMCLEAR with
> anything but normal memory. Just inject #GP to find insane ones.
>
> Reported-by: syzbot+2a7156e11dc199bdbd8a@syzkaller.appspotmail.com
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx/nested.c | 19 +++++++++++++++++--
> 1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 9c74a732b08d..05d57c3cb1ce 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4628,14 +4628,29 @@ static int nested_vmx_get_vmptr(struct kvm_vcpu *vcpu, gpa_t *vmpointer)
> {
> gva_t gva;
> struct x86_exception e;
> + int r;
>
> if (get_vmx_mem_address(vcpu, vmx_get_exit_qual(vcpu),
> vmcs_read32(VMX_INSTRUCTION_INFO), false,
> sizeof(*vmpointer), &gva))
> return 1;
>
> - if (kvm_read_guest_virt(vcpu, gva, vmpointer, sizeof(*vmpointer), &e)) {
> - kvm_inject_emulated_page_fault(vcpu, &e);
> + r = kvm_read_guest_virt(vcpu, gva, vmpointer, sizeof(*vmpointer), &e);
> + if (r != X86EMUL_CONTINUE) {
> + if (r == X86EMUL_PROPAGATE_FAULT) {
> + kvm_inject_emulated_page_fault(vcpu, &e);
> + } else {
> + /*
> + * X86EMUL_IO_NEEDED is returned when kvm_vcpu_read_guest_page()
> + * fails to read guest's memory (e.g. when 'gva' points to MMIO
> + * space). While KVM could've handled the request correctly by
> + * exiting to userspace and performing I/O, there doesn't seem
> + * to be a real use-case behind such requests, just inject #GP
> + * for now.
> + */
> + kvm_inject_gp(vcpu, 0);
> + }
> +
> return 1;
> }
>
>

Hi Vitaly,

looks good but we need to do the same in handle_vmread, handle_vmwrite,
handle_invept and handle_invvpid. Which probably means adding something
like nested_inject_emulation_fault to commonize the inner "if".

Thanks,

Paolo