KVM: selftests: Add a test to force emulation with a pending exception

Add a VMX specific test to verify that KVM doesn't explode if userspace
attempts KVM_RUN when emulation is required with a pending exception.
KVM VMX's emulation support for !unrestricted_guest punts exceptions to
userspace instead of attempting to synthesize the exception with all the
correct state (and stack switching, etc...).

Punting is acceptable as there's never been a request to support
injecting exceptions when emulating due to invalid state, but KVM has
historically assumed that userspace will do the right thing and either
clear the exception or kill the guest.  Deliberately do the opposite and
attempt to re-enter the guest with a pending exception and emulation
required to verify KVM continues to punt the combination to userspace,
e.g. doesn't explode, WARN, etc...
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211228232437.1875318-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/.gitignore

@@ -37,6 +37,7 @@
 /x86_64/vmx_apic_access_test
 /x86_64/vmx_close_while_nested_test
 /x86_64/vmx_dirty_log_test
+/x86_64/vmx_exception_with_invalid_guest_state
 /x86_64/vmx_invalid_nested_guest_state
 /x86_64/vmx_preemption_timer_test
 /x86_64/vmx_set_nested_state_test

tools/testing/selftests/kvm/Makefile

@@ -70,6 +70,7 @@ 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_exception_with_invalid_guest_state
 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

tools/testing/selftests/kvm/x86_64/vmx_exception_with_invalid_guest_state.c

+// SPDX-License-Identifier: GPL-2.0-only
+#include "test_util.h"
+#include "kvm_util.h"
+#include "processor.h"
+
+#include <signal.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/time.h>
+
+#include "kselftest.h"
+
+#define VCPU_ID	0
+
+static struct kvm_vm *vm;
+
+static void guest_ud_handler(struct ex_regs *regs)
+{
+	/* Loop on the ud2 until guest state is made invalid. */
+}
+
+static void guest_code(void)
+{
+	asm volatile("ud2");
+}
+
+static void __run_vcpu_with_invalid_state(void)
+{
+	struct kvm_run *run = vcpu_state(vm, VCPU_ID);
+
+	vcpu_run(vm, VCPU_ID);
+
+	TEST_ASSERT(run->exit_reason == KVM_EXIT_INTERNAL_ERROR,
+		    "Expected KVM_EXIT_INTERNAL_ERROR, got %d (%s)\n",
+		    run->exit_reason, exit_reason_str(run->exit_reason));
+	TEST_ASSERT(run->emulation_failure.suberror == KVM_INTERNAL_ERROR_EMULATION,
+		    "Expected emulation failure, got %d\n",
+		    run->emulation_failure.suberror);
+}
+
+static void run_vcpu_with_invalid_state(void)
+{
+	/*
+	 * Always run twice to verify KVM handles the case where _KVM_ queues
+	 * an exception with invalid state and then exits to userspace, i.e.
+	 * that KVM doesn't explode if userspace ignores the initial error.
+	 */
+	__run_vcpu_with_invalid_state();
+	__run_vcpu_with_invalid_state();
+}
+
+static void set_timer(void)
+{
+	struct itimerval timer;
+
+	timer.it_value.tv_sec  = 0;
+	timer.it_value.tv_usec = 200;
+	timer.it_interval = timer.it_value;
+	ASSERT_EQ(setitimer(ITIMER_REAL, &timer, NULL), 0);
+}
+
+static void set_or_clear_invalid_guest_state(bool set)
+{
+	static struct kvm_sregs sregs;
+
+	if (!sregs.cr0)
+		vcpu_sregs_get(vm, VCPU_ID, &sregs);
+	sregs.tr.unusable = !!set;
+	vcpu_sregs_set(vm, VCPU_ID, &sregs);
+}
+
+static void set_invalid_guest_state(void)
+{
+	set_or_clear_invalid_guest_state(true);
+}
+
+static void clear_invalid_guest_state(void)
+{
+	set_or_clear_invalid_guest_state(false);
+}
+
+static void sigalrm_handler(int sig)
+{
+	struct kvm_vcpu_events events;
+
+	TEST_ASSERT(sig == SIGALRM, "Unexpected signal = %d", sig);
+
+	vcpu_events_get(vm, VCPU_ID, &events);
+
+	/*
+	 * If an exception is pending, attempt KVM_RUN with invalid guest,
+	 * otherwise rearm the timer and keep doing so until the timer fires
+	 * between KVM queueing an exception and re-entering the guest.
+	 */
+	if (events.exception.pending) {
+		set_invalid_guest_state();
+		run_vcpu_with_invalid_state();
+	} else {
+		set_timer();
+	}
+}
+
+int main(int argc, char *argv[])
+{
+	if (!is_intel_cpu() || vm_is_unrestricted_guest(NULL)) {
+		print_skip("Must be run with kvm_intel.unrestricted_guest=0");
+		exit(KSFT_SKIP);
+	}
+
+	vm = vm_create_default(VCPU_ID, 0, (void *)guest_code);
+
+	vm_init_descriptor_tables(vm);
+	vcpu_init_descriptor_tables(vm, VCPU_ID);
+
+	vm_install_exception_handler(vm, UD_VECTOR, guest_ud_handler);
+
+	/*
+	 * 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.
+	 */
+	set_invalid_guest_state();
+	run_vcpu_with_invalid_state();
+
+	/*
+	 * Verify KVM also handles the case where userspace gains control while
+	 * an exception is pending and stuffs invalid state.  Run with valid
+	 * guest state and a timer firing every 200us, and attempt to enter the
+	 * guest with invalid state when the handler interrupts KVM with an
+	 * exception pending.
+	 */
+	clear_invalid_guest_state();
+	TEST_ASSERT(signal(SIGALRM, sigalrm_handler) != SIG_ERR,
+		    "Failed to register SIGALRM handler, errno = %d (%s)",
+		    errno, strerror(errno));
+
+	set_timer();
+	run_vcpu_with_invalid_state();
+}