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

[Oliver Upton]

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@xxxxxxxxxx>
> ---
>  .../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
>