x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

Web:        https://git.kernel.org/torvalds/c/d391f1207067268261add0485f0f34503539c5b0
Commit:     d391f1207067268261add0485f0f34503539c5b0
Parent:     e46b469278a59781f9b25ff608af84892963821b
Refname:    refs/heads/master
Author:     Vitaly Kuznetsov <[email protected]>
AuthorDate: Thu Jan 25 16:37:07 2018 +0100
Committer:  Radim Krčmář <[email protected]>
CommitDate: Wed Jan 31 18:25:34 2018 +0100

    x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested
    
    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 <[email protected]>
    Suggested-by: Paolo Bonzini <[email protected]>
    Signed-off-by: Vitaly Kuznetsov <[email protected]>
    Acked-by: Michael S. Tsirkin <[email protected]>
    Signed-off-by: Radim Krčmář <[email protected]>
---
 arch/x86/kvm/vmx.c | 16 +++++++++++++++-
 arch/x86/kvm/x86.c |  3 ++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 1e2ca9e8662f..438802d0b01d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6577,7 +6577,21 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
 	if (!is_guest_mode(vcpu) &&
 	    !kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
 		trace_kvm_fast_mmio(gpa);
-		return kvm_skip_emulated_instruction(vcpu);
+		/*
+		 * 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))
+			return kvm_skip_emulated_instruction(vcpu);
+		else
+			return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
+						       NULL, 0) == EMULATE_DONE;
 	}
 
 	ret = kvm_mmu_page_fault(vcpu, gpa, PFERR_RSVD_MASK, NULL, 0);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0204b2b8a293..01571102b50c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5773,7 +5773,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		 * 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;
--
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