Re: [PATCH 4/4] KVM: selftests: Add test to verify TRIPLE_FAULT on invalid L2 guest state

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2021-12-07 at 19:30 +0000, Sean Christopherson wrote:
> Add a selftest to attempt to enter L2 with invalid guests state by
> exiting to userspace via I/O from L2, and then using KVM_SET_SREGS to set
> invalid guest state (marking TR unusable is arbitrary chosen for its
> relative simplicity).
> 
> This is a regression test for a bug introduced by commit c8607e4a086f
> ("KVM: x86: nVMX: don't fail nested VM entry on invalid guest state if
> !from_vmentry"), which incorrectly set vmx->fail=true when L2 had invalid
> guest state and ultimately triggered a WARN due to nested_vmx_vmexit()
> seeing vmx->fail==true while attempting to synthesize a nested VM-Exit.
> 
> The is also a functional test to verify that KVM sythesizes TRIPLE_FAULT
> for L2, which is somewhat arbitrary behavior, instead of emulating L2.
> KVM should never emulate L2 due to invalid guest state, as it's
> architecturally impossible for L1 to run an L2 guest with invalid state
> as nested VM-Enter should always fail, i.e. L1 needs to do the emulation.
> Stuffing state via KVM ioctl() is a non-architctural, out-of-band case,
> hence the TRIPLE_FAULT being rather arbitrary.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/.gitignore        |   1 +
>  tools/testing/selftests/kvm/Makefile          |   1 +
>  .../x86_64/vmx_invalid_nested_guest_state.c   | 105 ++++++++++++++++++
>  3 files changed, 107 insertions(+)
>  create mode 100644 tools/testing/selftests/kvm/x86_64/vmx_invalid_nested_guest_state.c
> 
> diff --git a/tools/testing/selftests/kvm/.gitignore b/tools/testing/selftests/kvm/.gitignore
> index 3763105029fb..de41afb01a77 100644
> --- a/tools/testing/selftests/kvm/.gitignore
> +++ b/tools/testing/selftests/kvm/.gitignore
> @@ -34,6 +34,7 @@
>  /x86_64/vmx_apic_access_test
>  /x86_64/vmx_close_while_nested_test
>  /x86_64/vmx_dirty_log_test
> +/x86_64/vmx_invalid_nested_guest_state
>  /x86_64/vmx_preemption_timer_test
>  /x86_64/vmx_set_nested_state_test
>  /x86_64/vmx_tsc_adjust_test
> diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
> index c4e34717826a..6be4144c8d25 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -63,6 +63,7 @@ TEST_GEN_PROGS_x86_64 += x86_64/userspace_msr_exit_test
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_apic_access_test
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_close_while_nested_test
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_dirty_log_test
> +TEST_GEN_PROGS_x86_64 += x86_64/vmx_invalid_nested_guest_state
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_set_nested_state_test
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_tsc_adjust_test
>  TEST_GEN_PROGS_x86_64 += x86_64/vmx_nested_tsc_scaling_test
> diff --git a/tools/testing/selftests/kvm/x86_64/vmx_invalid_nested_guest_state.c b/tools/testing/selftests/kvm/x86_64/vmx_invalid_nested_guest_state.c
> new file mode 100644
> index 000000000000..489fbed4ca6f
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/x86_64/vmx_invalid_nested_guest_state.c
> @@ -0,0 +1,105 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#include "test_util.h"
> +#include "kvm_util.h"
> +#include "processor.h"
> +#include "vmx.h"
> +
> +#include <string.h>
> +#include <sys/ioctl.h>
> +
> +#include "kselftest.h"
> +
> +#define VCPU_ID	0
> +#define ARBITRARY_IO_PORT 0x2000
> +
> +static struct kvm_vm *vm;
> +
> +static void l2_guest_code(void)
> +{
> +	/*
> +	 * Generate an exit to L0 userspace, i.e. main(), via I/O to an
> +	 * arbitrary port.
> +	 */
> +	asm volatile("inb %%dx, %%al"
> +		     : : [port] "d" (ARBITRARY_IO_PORT) : "rax");
> +}
> +
> +static void l1_guest_code(struct vmx_pages *vmx_pages)
> +{
> +#define L2_GUEST_STACK_SIZE 64
> +	unsigned long l2_guest_stack[L2_GUEST_STACK_SIZE];
> +
> +	GUEST_ASSERT(prepare_for_vmx_operation(vmx_pages));
> +	GUEST_ASSERT(load_vmcs(vmx_pages));
> +
> +	/* Prepare the VMCS for L2 execution. */
> +	prepare_vmcs(vmx_pages, l2_guest_code,
> +		     &l2_guest_stack[L2_GUEST_STACK_SIZE]);
> +
> +	/*
> +	 * L2 must be run without unrestricted guest, verify that the selftests
> +	 * library hasn't enabled it.  Because KVM selftests jump directly to
> +	 * 64-bit mode, unrestricted guest support isn't required.
> +	 */
> +	GUEST_ASSERT(!(vmreadz(CPU_BASED_VM_EXEC_CONTROL) & CPU_BASED_ACTIVATE_SECONDARY_CONTROLS) ||
> +		     !(vmreadz(SECONDARY_VM_EXEC_CONTROL) & SECONDARY_EXEC_UNRESTRICTED_GUEST));
> +
> +	GUEST_ASSERT(!vmlaunch());
> +
> +	/* L2 should triple fault after main() stuffs invalid guest state. */
> +	GUEST_ASSERT(vmreadz(VM_EXIT_REASON) == EXIT_REASON_TRIPLE_FAULT);
> +	GUEST_DONE();
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +	vm_vaddr_t vmx_pages_gva;
> +	struct kvm_sregs sregs;
> +	struct kvm_run *run;
> +	struct ucall uc;
> +
> +	nested_vmx_check_supported();
> +
> +	vm = vm_create_default(VCPU_ID, 0, (void *) l1_guest_code);
> +
> +	/* Allocate VMX pages and shared descriptors (vmx_pages). */
> +	vcpu_alloc_vmx(vm, &vmx_pages_gva);
> +	vcpu_args_set(vm, VCPU_ID, 1, vmx_pages_gva);
> +
> +	vcpu_run(vm, VCPU_ID);
> +
> +	run = vcpu_state(vm, VCPU_ID);
> +
> +	/*
> +	 * The first exit to L0 userspace should be an I/O access from L2.
> +	 * Running L1 should launch L2 without triggering an exit to userspace.
> +	 */
> +	TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
> +		    "Expected KVM_EXIT_IO, got: %u (%s)\n",
> +		    run->exit_reason, exit_reason_str(run->exit_reason));
> +
> +	TEST_ASSERT(run->io.port == ARBITRARY_IO_PORT,
> +		    "Expected IN from port %d from L2, got port %d",
> +		    ARBITRARY_IO_PORT, run->io.port);
> +
> +	/*
> +	 * Stuff invalid guest state for L2 by making TR unusuable.  The next
> +	 * KVM_RUN should induce a TRIPLE_FAULT in L2 as KVM doesn't support
> +	 * emulating invalid guest state for L2.
> +	 */
> +	memset(&sregs, 0, sizeof(sregs));
> +	vcpu_sregs_get(vm, VCPU_ID, &sregs);
> +	sregs.tr.unusable = 1;
> +	vcpu_sregs_set(vm, VCPU_ID, &sregs);
> +
> +	vcpu_run(vm, VCPU_ID);
> +
> +	switch (get_ucall(vm, VCPU_ID, &uc)) {
> +	case UCALL_DONE:
> +		break;
> +	case UCALL_ABORT:
> +		TEST_FAIL("%s", (const char *)uc.args[0]);
> +	default:
> +		TEST_FAIL("Unexpected ucall: %lu", uc.cmd);
> +	}
> +}

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Best regards,
	Maxim Levitsky