linux-kernel - [PATCH 4.9 68/71] x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

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Message-Id: <20190107105336.633052729@linuxfoundation.org>
Date:   Mon,  7 Jan 2019 13:33:37 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Mike Haboustak <haboustak@...il.com>
Subject: [PATCH 4.9 68/71] x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vitaly Kuznetsov <vkuznets@...hat.com>

commit d391f1207067268261add0485f0f34503539c5b0 upstream.

I was investigating an issue with seabios >= 1.10 which stopped working
for nested KVM on Hyper-V. The problem appears to be in
handle_ept_violation() function: when we do fast mmio we need to skip
the instruction so we do kvm_skip_emulated_instruction(). This, however,
depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS.
However, this is not the case.

Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when
EPT MISCONFIG occurs. While on real hardware it was observed to be set,
some hypervisors follow the spec and don't set it; we end up advancing
IP with some random value.

I checked with Microsoft and they confirmed they don't fill
VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG.

Fix the issue by doing instruction skip through emulator when running
nested.

Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
Suggested-by: Radim Krčmář <rkrcmar@...hat.com>
Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
[mhaboustak: backport to 4.9.y]
Signed-off-by: Mike Haboustak <haboustak@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/kvm/vmx.c |   19 +++++++++++++++++--
 arch/x86/kvm/x86.c |    3 ++-
 2 files changed, 19 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6548,9 +6548,24 @@ static int handle_ept_misconfig(struct k
 
 	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
 	if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
-		skip_emulated_instruction(vcpu);
 		trace_kvm_fast_mmio(gpa);
-		return 1;
+		/*
+		* Doing kvm_skip_emulated_instruction() depends on undefined
+		* behavior: Intel's manual doesn't mandate
+		* VM_EXIT_INSTRUCTION_LEN to be set in VMCS when EPT MISCONFIG
+		* occurs and while on real hardware it was observed to be set,
+		* other hypervisors (namely Hyper-V) don't set it, we end up
+		* advancing IP with some random value. Disable fast mmio when
+		* running nested and keep it for real hardware in hope that
+		* VM_EXIT_INSTRUCTION_LEN will always be set correctly.
+		*/
+		if (!static_cpu_has(X86_FEATURE_HYPERVISOR)) {
+			skip_emulated_instruction(vcpu);
+			return 1;
+		}
+		else
+			return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
+						       NULL, 0) == EMULATE_DONE;
 	}
 
 	ret = handle_mmio_page_fault(vcpu, gpa, true);
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5707,7 +5707,8 @@ int x86_emulate_instruction(struct kvm_v
 		 * handle watchpoints yet, those would be handled in
 		 * the emulate_ops.
 		 */
-		if (kvm_vcpu_check_breakpoint(vcpu, &r))
+		if (!(emulation_type & EMULTYPE_SKIP) &&
+		    kvm_vcpu_check_breakpoint(vcpu, &r))
 			return r;
 
 		ctxt->interruptibility = 0;