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

Vitaly Kuznetsov
2020-06-04T14:32:27Z

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;
}

--
2.25.4