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SubjectRe: [PATCH 1/2] KVM: nVMX: fix msr bitmaps to prevent L2 from accessing L0 x2APIC
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On 2016/8/9 2:16, Radim Krčmář wrote:
> msr bitmap can be used to avoid a VM exit (interception) on guest MSR
> accesses. In some configurations of VMX controls, the guest can even
> directly access host's x2APIC MSRs. See SDM 29.5 VIRTUALIZING MSR-BASED
> APIC ACCESSES.
>
> L2 could read all L0's x2APIC MSRs and write TPR, EOI, and SELF_IPI.
> To do so, L1 would first trick KVM to disable all possible interceptions
> by enabling APICv features and then would turn those features off;
> nested_vmx_merge_msr_bitmap() only disabled interceptions, so VMX would
> not intercept previously enabled MSRs even though they were not safe
> with the new configuration.
>
> Correctly re-enabling interceptions is not enough as a second bug would
> still allow L1+L2 to access host's MSRs: msr bitmap was shared for all
> VMCSs, so L1 could trigger a race to get the desired combination of msr
> bitmap and VMX controls.
>
> This fix allocates a msr bitmap for every L1 VCPU, allows only safe
> x2APIC MSRs from L1's msr bitmap, and disables msr bitmaps if they would
> have to intercept everything anyway.
>
> Fixes: 3af18d9c5fe9 ("KVM: nVMX: Prepare for using hardware MSR bitmap")
> Reported-by: Jim Mattson <jmattson@google.com>
> Suggested-by: Wincy Van <fanwenyi0529@gmail.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> arch/x86/kvm/vmx.c | 107 ++++++++++++++++++++++-------------------------------
> 1 file changed, 44 insertions(+), 63 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index a45d8580f91e..c66ac2c70d22 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -435,6 +435,8 @@ struct nested_vmx {
> bool pi_pending;
> u16 posted_intr_nv;
>
> + unsigned long *msr_bitmap;
> +
> struct hrtimer preemption_timer;
> bool preemption_timer_expired;
>
> @@ -924,7 +926,6 @@ static unsigned long *vmx_msr_bitmap_legacy;
> static unsigned long *vmx_msr_bitmap_longmode;
> static unsigned long *vmx_msr_bitmap_legacy_x2apic;
> static unsigned long *vmx_msr_bitmap_longmode_x2apic;
> -static unsigned long *vmx_msr_bitmap_nested;
> static unsigned long *vmx_vmread_bitmap;
> static unsigned long *vmx_vmwrite_bitmap;
>
> @@ -2508,7 +2509,7 @@ static void vmx_set_msr_bitmap(struct kvm_vcpu *vcpu)
> unsigned long *msr_bitmap;
>
> if (is_guest_mode(vcpu))
> - msr_bitmap = vmx_msr_bitmap_nested;
> + msr_bitmap = to_vmx(vcpu)->nested.msr_bitmap;
> else if (cpu_has_secondary_exec_ctrls() &&
> (vmcs_read32(SECONDARY_VM_EXEC_CONTROL) &
> SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE)) {
> @@ -6363,13 +6364,6 @@ static __init int hardware_setup(void)
> if (!vmx_msr_bitmap_longmode_x2apic)
> goto out4;
>
> - if (nested) {
> - vmx_msr_bitmap_nested =
> - (unsigned long *)__get_free_page(GFP_KERNEL);
> - if (!vmx_msr_bitmap_nested)
> - goto out5;
> - }
> -
> vmx_vmread_bitmap = (unsigned long *)__get_free_page(GFP_KERNEL);
> if (!vmx_vmread_bitmap)
> goto out6;
> @@ -6392,8 +6386,6 @@ static __init int hardware_setup(void)
>
> memset(vmx_msr_bitmap_legacy, 0xff, PAGE_SIZE);
> memset(vmx_msr_bitmap_longmode, 0xff, PAGE_SIZE);
> - if (nested)
> - memset(vmx_msr_bitmap_nested, 0xff, PAGE_SIZE);
>
> if (setup_vmcs_config(&vmcs_config) < 0) {
> r = -EIO;
> @@ -6529,9 +6521,6 @@ out8:
> out7:
> free_page((unsigned long)vmx_vmread_bitmap);
> out6:
> - if (nested)
> - free_page((unsigned long)vmx_msr_bitmap_nested);
> -out5:
> free_page((unsigned long)vmx_msr_bitmap_longmode_x2apic);
> out4:
> free_page((unsigned long)vmx_msr_bitmap_longmode);
> @@ -6557,8 +6546,6 @@ static __exit void hardware_unsetup(void)
> free_page((unsigned long)vmx_io_bitmap_a);
> free_page((unsigned long)vmx_vmwrite_bitmap);
> free_page((unsigned long)vmx_vmread_bitmap);
> - if (nested)
> - free_page((unsigned long)vmx_msr_bitmap_nested);
>
> free_kvm_area();
> }
> @@ -6995,16 +6982,21 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
> return 1;
> }
>
> + if (cpu_has_vmx_msr_bitmap()) {
> + vmx->nested.msr_bitmap =
> + (unsigned long *)__get_free_page(GFP_KERNEL);
> + if (!vmx->nested.msr_bitmap)
> + goto out_msr_bitmap;
> + }
> +

We export msr_bitmap to guest even it is not supported by hardware. So
we need to allocate msr_bitmap for L1 unconditionally.

> vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
> if (!vmx->nested.cached_vmcs12)
> - return -ENOMEM;
> + goto out_cached_vmcs12;
>
> if (enable_shadow_vmcs) {
> shadow_vmcs = alloc_vmcs();
> - if (!shadow_vmcs) {
> - kfree(vmx->nested.cached_vmcs12);
> - return -ENOMEM;
> - }
> + if (!shadow_vmcs)
> + goto out_shadow_vmcs;
> /* mark vmcs as shadow */
> shadow_vmcs->revision_id |= (1u << 31);
> /* init shadow vmcs */
> @@ -7024,6 +7016,15 @@ static int handle_vmon(struct kvm_vcpu *vcpu)
> skip_emulated_instruction(vcpu);
> nested_vmx_succeed(vcpu);
> return 1;
> +
> +out_shadow_vmcs:
> + kfree(vmx->nested.cached_vmcs12);
> +
> +out_cached_vmcs12:
> + free_page((unsigned long)vmx->nested.msr_bitmap);
> +
> +out_msr_bitmap:
> + return -ENOMEM;
> }
>
> /*
> @@ -7098,6 +7099,10 @@ static void free_nested(struct vcpu_vmx *vmx)
> vmx->nested.vmxon = false;
> free_vpid(vmx->nested.vpid02);
> nested_release_vmcs12(vmx);
> + if (vmx->nested.msr_bitmap) {
> + free_page((unsigned long)vmx->nested.msr_bitmap);
> + vmx->nested.msr_bitmap = NULL;
> + }
> if (enable_shadow_vmcs)
> free_vmcs(vmx->nested.current_shadow_vmcs);
> kfree(vmx->nested.cached_vmcs12);
> @@ -9472,8 +9477,10 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
> {
> int msr;
> struct page *page;
> - unsigned long *msr_bitmap;
> + unsigned long *msr_bitmap_l1;
> + unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.msr_bitmap;
>
> + /* This shortcut is ok because we support only x2APIC MSRs so far. */
> if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
> return false;
>
> @@ -9482,63 +9489,37 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
> WARN_ON(1);
> return false;
> }
> - msr_bitmap = (unsigned long *)kmap(page);
> - if (!msr_bitmap) {
> + msr_bitmap_l1 = (unsigned long *)kmap(page);
> + if (!msr_bitmap_l1) {
> nested_release_page_clean(page);
> WARN_ON(1);
> return false;
> }
>
> + memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
> +
> if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {
> if (nested_cpu_has_apic_reg_virt(vmcs12))
> for (msr = 0x800; msr <= 0x8ff; msr++)
> nested_vmx_disable_intercept_for_msr(
> - msr_bitmap,
> - vmx_msr_bitmap_nested,
> + msr_bitmap_l1, msr_bitmap_l0,
> msr, MSR_TYPE_R);
> - /* TPR is allowed */
> - nested_vmx_disable_intercept_for_msr(msr_bitmap,
> - vmx_msr_bitmap_nested,
> +
> + nested_vmx_disable_intercept_for_msr(
> + msr_bitmap_l1, msr_bitmap_l0,
> APIC_BASE_MSR + (APIC_TASKPRI >> 4),
> MSR_TYPE_R | MSR_TYPE_W);
> +
> if (nested_cpu_has_vid(vmcs12)) {
> - /* EOI and self-IPI are allowed */
> nested_vmx_disable_intercept_for_msr(
> - msr_bitmap,
> - vmx_msr_bitmap_nested,
> + msr_bitmap_l1, msr_bitmap_l0,
> APIC_BASE_MSR + (APIC_EOI >> 4),
> MSR_TYPE_W);
> nested_vmx_disable_intercept_for_msr(
> - msr_bitmap,
> - vmx_msr_bitmap_nested,
> + msr_bitmap_l1, msr_bitmap_l0,
> APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
> MSR_TYPE_W);
> }
> - } else {
> - /*
> - * Enable reading intercept of all the x2apic
> - * MSRs. We should not rely on vmcs12 to do any
> - * optimizations here, it may have been modified
> - * by L1.
> - */
> - for (msr = 0x800; msr <= 0x8ff; msr++)
> - __vmx_enable_intercept_for_msr(
> - vmx_msr_bitmap_nested,
> - msr,
> - MSR_TYPE_R);
> -
> - __vmx_enable_intercept_for_msr(
> - vmx_msr_bitmap_nested,
> - APIC_BASE_MSR + (APIC_TASKPRI >> 4),
> - MSR_TYPE_W);
> - __vmx_enable_intercept_for_msr(
> - vmx_msr_bitmap_nested,
> - APIC_BASE_MSR + (APIC_EOI >> 4),
> - MSR_TYPE_W);
> - __vmx_enable_intercept_for_msr(
> - vmx_msr_bitmap_nested,
> - APIC_BASE_MSR + (APIC_SELF_IPI >> 4),
> - MSR_TYPE_W);
> }
> kunmap(page);
> nested_release_page_clean(page);
> @@ -9957,10 +9938,10 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
> }
>
> if (cpu_has_vmx_msr_bitmap() &&
> - exec_control & CPU_BASED_USE_MSR_BITMAPS) {
> - nested_vmx_merge_msr_bitmap(vcpu, vmcs12);
> - /* MSR_BITMAP will be set by following vmx_set_efer. */
> - } else
> + exec_control & CPU_BASED_USE_MSR_BITMAPS &&
> + nested_vmx_merge_msr_bitmap(vcpu, vmcs12))
> + ; /* MSR_BITMAP will be set by following vmx_set_efer. */
> + else
> exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;
>
> /*
>


--
Yang
Alibaba Cloud Computing

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