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

[Paolo Bonzini]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  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