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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 18:19, Wincy Van wrote:
> On Tue, Aug 9, 2016 at 5:32 PM, Yang Zhang <yang.zhang.wz@gmail.com> wrote:
>> 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.
>>
>>
>
> Nice to see you, Yang :-)

Niec to see you too.

>
> I think vmx->nested.msr_bitmap is filled by L0 and L1's settings, and
> used by hardware.
> L1's msr_bitmap is managed by itself, and L1 will write MSR_BITMAP field to
> vmcs12->msr_bitmap, then L0 can get L1's settings to emulate MSR_BITMAP.
>
> Am I missed something?

You are correct!

>
> Thanks,
> Wincy
>
>>> 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


--
Yang
Alibaba Cloud Computing

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