Re: [PATCH v3 7/9] KVM: VMX: Add guest physical address check in EPT violation and misconfig

Sean Christopherson
2020-07-13T18:32:17Z

On Fri, Jul 10, 2020 at 05:48:09PM +0200, Mohammed Gamal wrote:
> Check guest physical address against it's maximum physical memory. If
> the guest's physical address exceeds the maximum (i.e. has reserved bits
> set), inject a guest page fault with PFERR_RSVD_MASK set.
>
> This has to be done both in the EPT violation and page fault paths, as
> there are complications in both cases with respect to the computation
> of the correct error code.
>
> For EPT violations, unfortunately the only possibility is to emulate,
> because the access type in the exit qualification might refer to an
> access to a paging structure, rather than to the access performed by
> the program.
>
> Trapping page faults instead is needed in order to correct the error code,
> but the access type can be obtained from the original error code and
> passed to gva_to_gpa. The corrections required in the error code are
> subtle. For example, imagine that a PTE for a supervisor page has a reserved
> bit set. On a supervisor-mode access, the EPT violation path would trigger.
> However, on a user-mode access, the processor will not notice the reserved
> bit and not include PFERR_RSVD_MASK in the error code.
>
> Co-developed-by: Mohammed Gamal <mgamal@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++++---
> arch/x86/kvm/vmx/vmx.h | 3 ++-
> 2 files changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 770b090969fb..de3f436b2d32 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4790,9 +4790,15 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>
> if (is_page_fault(intr_info)) {
> cr2 = vmx_get_exit_qual(vcpu);
> - /* EPT won't cause page fault directly */
> - WARN_ON_ONCE(!vcpu->arch.apf.host_apf_flags && enable_ept);
> - return kvm_handle_page_fault(vcpu, error_code, cr2, NULL, 0);
> + if (enable_ept && !vcpu->arch.apf.host_apf_flags) {
> + /*
> + * EPT will cause page fault only if we need to
> + * detect illegal GPAs.
> + */

It'd be nice to retain a WARN_ON_ONCE() here, e.g.

WARN_ON_ONCE(!vmx_need_pf_intercept(vcpu));

This WARN has fired for me when I've botched the nested VM-Exit routing,
debugging a spurious L2 #PF without would be less than fun.

> + kvm_fixup_and_inject_pf_error(vcpu, cr2, error_code);
> + return 1;
> + } else
> + return kvm_handle_page_fault(vcpu, error_code, cr2, NULL, 0);
> }
>
> ex_no = intr_info & INTR_INFO_VECTOR_MASK;
> @@ -5308,6 +5314,18 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
> PFERR_GUEST_FINAL_MASK : PFERR_GUEST_PAGE_MASK;
>
> vcpu->arch.exit_qualification = exit_qualification;
> +
> + /*
> + * 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(kvm_mmu_is_illegal_gpa(vcpu, gpa)))
> + return kvm_emulate_instruction(vcpu, 0);
> +
> return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> }
>
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index b0e5e210f1c1..0d06951e607c 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -11,6 +11,7 @@
> #include "kvm_cache_regs.h"
> #include "ops.h"
> #include "vmcs.h"
> +#include "cpuid.h"
>
> extern const u32 vmx_msr_index[];
>
> @@ -552,7 +553,7 @@ static inline bool vmx_has_waitpkg(struct vcpu_vmx *vmx)
>
> static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
> {
> - return !enable_ept;
> + return !enable_ept || cpuid_maxphyaddr(vcpu) < boot_cpu_data.x86_phys_bits;
> }
>
> void dump_vmcs(void);
> --
> 2.26.2
>