Re: [PATCH 1/4] KVM: X86: Fix tlb flush for tdp in kvm_invalidate_pcid()

[Sean Christopherson <seanjc@google.com>]

On Wed, Oct 20, 2021, Lai Jiangshan wrote:
> On 2021/10/19 23:25, Sean Christopherson wrote:
> I just read some interception policy in vmx.c, if EPT=1 but vmx_need_pf_intercept()
> return true for some reasons/configs, #PF is intercepted.  But CR3 write is not
> intercepted, which means there will be an EPT fault _after_ (IIUC) the CR3 write if
> the GPA of the new CR3 exceeds the guest maxphyaddr limit.  And kvm queues a fault to
> the guest which is also _after_ the CR3 write, but the guest expects the fault before
> the write.
> 
> IIUC, it can be fixed by intercepting CR3 write or reversing the CR3 write in EPT
> violation handler.

KVM implicitly does the latter by emulating the faulting instruction.

  static int handle_ept_violation(struct kvm_vcpu *vcpu)
  {
	...

	/*
	 * Check that the GPA doesn't exceed physical memory limits, as that is
	 * a guest page fault.  We have to emulate the instruction here, because
	 * if the illegal address is that of a paging structure, then
	 * EPT_VIOLATION_ACC_WRITE bit is set.  Alternatively, if supported we
	 * would also use advanced VM-exit information for EPT violations to
	 * reconstruct the page fault error code.
	 */
	if (unlikely(allow_smaller_maxphyaddr && kvm_vcpu_is_illegal_gpa(vcpu, gpa)))
		return kvm_emulate_instruction(vcpu, 0);

	return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
  }

and injecting a #GP when kvm_set_cr3() fails.

  static int em_cr_write(struct x86_emulate_ctxt *ctxt)
  {
	if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
		return emulate_gp(ctxt, 0);

	/* Disable writeback. */
	ctxt->dst.type = OP_NONE;
	return X86EMUL_CONTINUE;
  }