[PATCH 4/5] KVM: selftests: Explicitly verify KVM doesn't patch hypercall if quirk==off

Sean Christopherson
2022-09-08T23:34:32Z

Explicitly verify that KVM doesn't patch in the native hypercall if the
FIX_HYPERCALL_INSN quirk is disabled. The test currently verifies that
a #UD occurred, but doesn't actually verify that no patching occurred.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
.../selftests/kvm/x86_64/fix_hypercall_test.c | 35 ++++++++++++++-----
1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
index dde97be3e719..5925da3b3648 100644
--- a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
+++ b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
@@ -21,8 +21,8 @@ static bool ud_expected;

static void guest_ud_handler(struct ex_regs *regs)
{
- GUEST_ASSERT(ud_expected);
- GUEST_DONE();
+ regs->rax = -EFAULT;
+ regs->rip += HYPERCALL_INSN_SIZE;
}

extern unsigned char svm_hypercall_insn[HYPERCALL_INSN_SIZE];
@@ -57,17 +57,18 @@ static void guest_main(void)
{
unsigned char *native_hypercall_insn, *hypercall_insn;
uint8_t apic_id;
+ uint64_t ret;

apic_id = GET_APIC_ID_FIELD(xapic_read_reg(APIC_ID));

if (is_intel_cpu()) {
native_hypercall_insn = vmx_hypercall_insn;
hypercall_insn = svm_hypercall_insn;
- svm_do_sched_yield(apic_id);
+ ret = svm_do_sched_yield(apic_id);
} else if (is_amd_cpu()) {
native_hypercall_insn = svm_hypercall_insn;
hypercall_insn = vmx_hypercall_insn;
- vmx_do_sched_yield(apic_id);
+ ret = vmx_do_sched_yield(apic_id);
} else {
GUEST_ASSERT(0);
/* unreachable */
@@ -75,12 +76,28 @@ static void guest_main(void)
}

/*
- * The hypercall didn't #UD (guest_ud_handler() signals "done" if a #UD
- * occurs). Verify that a #UD is NOT expected and that KVM patched in
- * the native hypercall.
+ * If the quirk is disabled, verify that guest_ud_handler() "returned"
+ * -EFAULT and that KVM did NOT patch the hypercall. If the quirk is
+ * enabled, verify that the hypercall succeeded and that KVM patched in
+ * the "right" hypercall.
*/
- GUEST_ASSERT(!ud_expected);
- GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn, HYPERCALL_INSN_SIZE));
+ if (ud_expected) {
+ GUEST_ASSERT(ret == (uint64_t)-EFAULT);
+
+ /*
+ * Divergence should occur only on the last byte, as the VMCALL
+ * (0F 01 C1) and VMMCALL (0F 01 D9) share the first two bytes.
+ */
+ GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
+ HYPERCALL_INSN_SIZE - 1));
+ GUEST_ASSERT(memcmp(native_hypercall_insn, hypercall_insn,
+ HYPERCALL_INSN_SIZE));
+ } else {
+ GUEST_ASSERT(!ret);
+ GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
+ HYPERCALL_INSN_SIZE));
+ }
+
GUEST_DONE();
}

--
2.37.2.789.g6183377224-goog