On 28/04/20 19:32, Sean Christopherson wrote:
> Use an enum for passing around the failure code for a failed VM-Enter
> that results in VM-Exit to provide a level of indirection from the final
> resting place of the failure code, vmcs.EXIT_QUALIFICATION. The exit
> qualification field is an unsigned long, e.g. passing around
> 'u32 exit_qual' throws up red flags as it suggests KVM may be dropping
> bits when reporting errors to L1. This is a red herring because the
> only defined failure codes are 0, 2, 3, and 4, i.e. don't come remotely
> close to overflowing a u32.
>
> Setting vmcs.EXIT_QUALIFICATION on entry failure is further complicated
> by the MSR load list, which returns the (1-based) entry that failed, and
> the number of MSRs to load is a 32-bit VMCS field. At first blush, it
> would appear that overflowing a u32 is possible, but the number of MSRs
> that can be loaded is hardcapped at 4096 (limited by MSR_IA32_VMX_MISC).
>
> In other words, there are two completely disparate types of data that
> eventually get stuffed into vmcs.EXIT_QUALIFICATION, neither of which is
> an 'unsigned long' in nature. This was presumably the reasoning for
> switching to 'u32' when the related code was refactored in commit
> ca0bde28f2ed6 ("kvm: nVMX: Split VMCS checks from nested_vmx_run()").
>
> Using an enum for the failure code addresses the technically-possible-
> but-will-never-happen scenario where Intel defines a failure code that
> doesn't fit in a 32-bit integer. The enum variables and values will
> either be automatically sized (gcc 5.4 behavior) or be subjected to some
> combination of truncation. The former case will simply work, while the
> latter will trigger a compile-time warning unless the compiler is being
> particularly unhelpful.
>
> Separating the failure code from the failed MSR entry allows for
> disassociating both from vmcs.EXIT_QUALIFICATION, which avoids the
> conundrum where KVM has to choose between 'u32 exit_qual' and tracking
> values as 'unsigned long' that have no business being tracked as such.
> To cement the split, set vmcs12->exit_qualification directly from the
> entry error code or failed MSR index instead of bouncing through a local
> variable.
>
> Opportunistically rename the variables in load_vmcs12_host_state() and
> vmx_set_nested_state() to call out that they're ignored, set exit_reason
> on demand on nested VM-Enter failure, and add a comment in
> nested_vmx_load_msr() to call out that returning 'i + 1' can't wrap.
>
> No functional change intended.
>
> Reported-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>
> v2:
> - Set vmcs12->exit_qualification directly to avoid writing the failed
> MSR index (a u32) to the entry_failure_code enum. [Jim]
> - Set exit_reason on demand since the "goto vm_exit" paths need to set
> vmcs12->exit_qualification anyways, i.e. already have curly braces.
>
> arch/x86/include/asm/vmx.h | 10 +++++----
> arch/x86/kvm/vmx/nested.c | 44 ++++++++++++++++++++++----------------
> 2 files changed, 31 insertions(+), 23 deletions(-)
>
> diff --git a/arch/x86/include/asm/vmx.h b/arch/x86/include/asm/vmx.h
> index 5e090d1f03f8..cd7de4b401fe 100644
> --- a/arch/x86/include/asm/vmx.h
> +++ b/arch/x86/include/asm/vmx.h
> @@ -527,10 +527,12 @@ struct vmx_msr_entry {
> /*
> * Exit Qualifications for entry failure during or after loading guest state
> */
> -#define ENTRY_FAIL_DEFAULT 0
> -#define ENTRY_FAIL_PDPTE 2
> -#define ENTRY_FAIL_NMI 3
> -#define ENTRY_FAIL_VMCS_LINK_PTR 4
> +enum vm_entry_failure_code {
> + ENTRY_FAIL_DEFAULT = 0,
> + ENTRY_FAIL_PDPTE = 2,
> + ENTRY_FAIL_NMI = 3,
> + ENTRY_FAIL_VMCS_LINK_PTR = 4,
> +};
>
> /*
> * Exit Qualifications for EPT Violations
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 2c36f3f53108..dc00d1742480 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -922,6 +922,7 @@ static u32 nested_vmx_load_msr(struct kvm_vcpu *vcpu, u64 gpa, u32 count)
> }
> return 0;
> fail:
> + /* Note, max_msr_list_size is at most 4096, i.e. this can't wrap. */
> return i + 1;
> }
>
> @@ -1117,7 +1118,7 @@ static bool nested_vmx_transition_mmu_sync(struct kvm_vcpu *vcpu)
> * @entry_failure_code.
> */
> static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool nested_ept,
> - u32 *entry_failure_code)
> + enum vm_entry_failure_code *entry_failure_code)
> {
> if (cr3 != kvm_read_cr3(vcpu) || (!nested_ept && pdptrs_changed(vcpu))) {
> if (CC(!nested_cr3_valid(vcpu, cr3))) {
> @@ -2470,7 +2471,7 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
> * is assigned to entry_failure_code on failure.
> */
> static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> - u32 *entry_failure_code)
> + enum vm_entry_failure_code *entry_failure_code)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> struct hv_enlightened_vmcs *hv_evmcs = vmx->nested.hv_evmcs;
> @@ -2930,11 +2931,11 @@ static int nested_check_guest_non_reg_state(struct vmcs12 *vmcs12)
>
> static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
> struct vmcs12 *vmcs12,
> - u32 *exit_qual)
> + enum vm_entry_failure_code *entry_failure_code)
> {
> bool ia32e;
>
> - *exit_qual = ENTRY_FAIL_DEFAULT;
> + *entry_failure_code = ENTRY_FAIL_DEFAULT;
>
> if (CC(!nested_guest_cr0_valid(vcpu, vmcs12->guest_cr0)) ||
> CC(!nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4)))
> @@ -2949,7 +2950,7 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
> return -EINVAL;
>
> if (nested_vmx_check_vmcs_link_ptr(vcpu, vmcs12)) {
> - *exit_qual = ENTRY_FAIL_VMCS_LINK_PTR;
> + *entry_failure_code = ENTRY_FAIL_VMCS_LINK_PTR;
> return -EINVAL;
> }
>
> @@ -3241,9 +3242,9 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> + enum vm_entry_failure_code entry_failure_code;
> bool evaluate_pending_interrupts;
> - u32 exit_reason = EXIT_REASON_INVALID_STATE;
> - u32 exit_qual;
> + u32 exit_reason, failed_index;
>
> if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
> kvm_vcpu_flush_tlb_current(vcpu);
> @@ -3291,24 +3292,30 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> return NVMX_VMENTRY_VMFAIL;
> }
>
> - if (nested_vmx_check_guest_state(vcpu, vmcs12, &exit_qual))
> + if (nested_vmx_check_guest_state(vcpu, vmcs12,
> + &entry_failure_code)) {
> + exit_reason = EXIT_REASON_INVALID_STATE;
> + vmcs12->exit_qualification = entry_failure_code;
> goto vmentry_fail_vmexit;
> + }
> }
>
> enter_guest_mode(vcpu);
> if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETTING)
> vcpu->arch.tsc_offset += vmcs12->tsc_offset;
>
> - if (prepare_vmcs02(vcpu, vmcs12, &exit_qual))
> + if (prepare_vmcs02(vcpu, vmcs12, &entry_failure_code))
> goto vmentry_fail_vmexit_guest_mode;
>
> if (from_vmentry) {
> - exit_reason = EXIT_REASON_MSR_LOAD_FAIL;
> - exit_qual = nested_vmx_load_msr(vcpu,
> - vmcs12->vm_entry_msr_load_addr,
> - vmcs12->vm_entry_msr_load_count);
> - if (exit_qual)
> + failed_index = nested_vmx_load_msr(vcpu,
> + vmcs12->vm_entry_msr_load_addr,
> + vmcs12->vm_entry_msr_load_count);
> + if (failed_index) {
> + exit_reason = EXIT_REASON_MSR_LOAD_FAIL;
> + vmcs12->exit_qualification = failed_index;
> goto vmentry_fail_vmexit_guest_mode;
> + }
> } else {
> /*
> * The MMU is not initialized to point at the right entities yet and
> @@ -3372,7 +3379,6 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
>
> load_vmcs12_host_state(vcpu, vmcs12);
> vmcs12->vm_exit_reason = exit_reason | VMX_EXIT_REASONS_FAILED_VMENTRY;
> - vmcs12->exit_qualification = exit_qual;
> if (enable_shadow_vmcs || vmx->nested.hv_evmcs)
> vmx->nested.need_vmcs12_to_shadow_sync = true;
> return NVMX_VMENTRY_VMEXIT;
> @@ -4066,8 +4072,8 @@ static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
> struct vmcs12 *vmcs12)
> {
> + enum vm_entry_failure_code ignored;
> struct kvm_segment seg;
> - u32 entry_failure_code;
>
> if (vmcs12->vm_exit_controls & VM_EXIT_LOAD_IA32_EFER)
> vcpu->arch.efer = vmcs12->host_ia32_efer;
> @@ -4102,7 +4108,7 @@ static void load_vmcs12_host_state(struct kvm_vcpu *vcpu,
> * Only PDPTE load can fail as the value of cr3 was checked on entry and
> * couldn't have changed.
> */
> - if (nested_vmx_load_cr3(vcpu, vmcs12->host_cr3, false, &entry_failure_code))
> + if (nested_vmx_load_cr3(vcpu, vmcs12->host_cr3, false, &ignored))
> nested_vmx_abort(vcpu, VMX_ABORT_LOAD_HOST_PDPTE_FAIL);
>
> if (!enable_ept)
> @@ -6002,7 +6008,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> struct vmcs12 *vmcs12;
> - u32 exit_qual;
> + enum vm_entry_failure_code ignored;
> struct kvm_vmx_nested_state_data __user *user_vmx_nested_state =
> &user_kvm_nested_state->data.vmx[0];
> int ret;
> @@ -6143,7 +6149,7 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>
> if (nested_vmx_check_controls(vcpu, vmcs12) ||
> nested_vmx_check_host_state(vcpu, vmcs12) ||
> - nested_vmx_check_guest_state(vcpu, vmcs12, &exit_qual))
> + nested_vmx_check_guest_state(vcpu, vmcs12, &ignored))
> goto error_guest_mode;
>
> vmx->nested.dirty_vmcs12 = true;
>
Queued this one, actually.
Paolo
