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

Oliver Upton
2022-09-19T21:24:09Z

On Thu, Sep 08, 2022 at 11:31:33PM +0000, Sean Christopherson wrote:
> 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));

Should we just keep the assertions consistent for both cases (patched
and unpatched)?

--
Thanks,
Oliver

> + } else {
> + GUEST_ASSERT(!ret);
> + GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
> + HYPERCALL_INSN_SIZE));
> + }
> +
> GUEST_DONE();
> }
>
> --
> 2.37.2.789.g6183377224-goog
>