KVM: VMX: Properly handle kvm_read/write_guest_virt*() result

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 the argument is MMIO.

Paolo also noticed that nested_vmx_get_vmptr() is not the only place in
KVM code where kvm_read/write_guest_virt*() return result is mishandled.
VMX instructions along with INVPCID have the same issue. This was already
noticed before, e.g. see commit 541ab2aeb282 ("KVM: x86: work around
leak of uninitialized stack contents") but was never fully fixed.

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.

Introduce vmx_handle_memory_failure() as an interim solution.

Note, nested_vmx_get_vmptr() now has three possible outcomes: OK, PF,
KVM_EXIT_INTERNAL_ERROR and callers need to know if userspace exit is
needed (for KVM_EXIT_INTERNAL_ERROR) in case of failure. We don't seem
to have a good enum describing this tristate, just add "int *ret" to
nested_vmx_get_vmptr() interface to pass the information.

Reported-by: syzbot+2a7156e11dc199bdbd8a@syzkaller.appspotmail.com
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200605115906.532682-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 30 insertions, 4 deletions

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0e4f7ff54e6a..08e26a9518c2 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1600,6 +1600,32 @@ static int skip_emulated_instruction(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
+/*
+ * Handles kvm_read/write_guest_virt*() result and either injects #PF or returns
+ * KVM_EXIT_INTERNAL_ERROR for cases not currently handled by KVM. Return value
+ * indicates whether exit to userspace is needed.
+ */
+int vmx_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
+			      struct x86_exception *e)
+{
+	if (r == X86EMUL_PROPAGATE_FAULT) {
+		kvm_inject_emulated_page_fault(vcpu, e);
+		return 1;
+	}
+
+	/*
+	 * In case kvm_read/write_guest_virt*() failed with X86EMUL_IO_NEEDED
+	 * while handling a VMX instruction KVM could've handled the request
+	 * correctly by exiting to userspace and performing I/O but there
+	 * doesn't seem to be a real use-case behind such requests, just return
+	 * KVM_EXIT_INTERNAL_ERROR for now.
+	 */
+	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
+	vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_EMULATION;
+	vcpu->run->internal.ndata = 0;
+
+	return 0;
+}
 
 /*
  * Recognizes a pending MTF VM-exit and records the nested state for later
@@ -5486,6 +5512,7 @@ static int handle_invpcid(struct kvm_vcpu *vcpu)
 		u64 pcid;
 		u64 gla;
 	} operand;
+	int r;
 
 	if (!guest_cpuid_has(vcpu, X86_FEATURE_INVPCID)) {
 		kvm_queue_exception(vcpu, UD_VECTOR);
@@ -5508,10 +5535,9 @@ static int handle_invpcid(struct kvm_vcpu *vcpu)
 				sizeof(operand), &gva))
 		return 1;
 
-	if (kvm_read_guest_virt(vcpu, gva, &operand, sizeof(operand), &e)) {
-		kvm_inject_emulated_page_fault(vcpu, &e);
-		return 1;
-	}
+	r = kvm_read_guest_virt(vcpu, gva, &operand, sizeof(operand), &e);
+	if (r != X86EMUL_CONTINUE)
+		return vmx_handle_memory_failure(vcpu, r, &e);
 
 	if (operand.pcid >> 12 != 0) {
 		kvm_inject_gp(vcpu, 0);