Re: [PATCH v3 08/11] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall

["Singh, Brijesh" <brijesh.singh@amd.com>]

On 7/22/19 12:12 PM, Cfir Cohen wrote:
> In addition, it seems that svm_page_enc_status_hc() accepts 'gpa',
> 'npages', 'enc' directly from the guest, and so these can take
> arbitrary values. A very large 'npages' could lead to an int overflow
> in 'gfn_end = gfn_start + npages', making gfn_end < gfn_start. This
> could an OOB access in the bitmap. Concrete example: gfn_start = 2,
> npages = -1, gfn_end = 2+(-1) = 1, sev_resize_page_enc_bitmap
> allocates a bitmap for a single page (new_size=1), __bitmap_set access
> offset gfn_end - gfn_start = -1.
> 

Good point. I will add a check for it, something like

if (gfn_end <= gfn_start)
	return -EINVAL;


> 
> On Sun, Jul 21, 2019 at 1:57 PM David Rientjes <rientjes@google.com> wrote:
>>
>> On Wed, 10 Jul 2019, Singh, Brijesh wrote:
>>
>>> diff --git a/Documentation/virtual/kvm/hypercalls.txt b/Documentation/virtual/kvm/hypercalls.txt
>>> index da24c138c8d1..94f0611f4d88 100644
>>> --- a/Documentation/virtual/kvm/hypercalls.txt
>>> +++ b/Documentation/virtual/kvm/hypercalls.txt
>>> @@ -141,3 +141,17 @@ a0 corresponds to the APIC ID in the third argument (a2), bit 1
>>>   corresponds to the APIC ID a2+1, and so on.
>>>
>>>   Returns the number of CPUs to which the IPIs were delivered successfully.
>>> +
>>> +7. KVM_HC_PAGE_ENC_STATUS
>>> +-------------------------
>>> +Architecture: x86
>>> +Status: active
>>> +Purpose: Notify the encryption status changes in guest page table (SEV guest)
>>> +
>>> +a0: the guest physical address of the start page
>>> +a1: the number of pages
>>> +a2: encryption attribute
>>> +
>>> +   Where:
>>> +     * 1: Encryption attribute is set
>>> +     * 0: Encryption attribute is cleared
>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>> index 26d1eb83f72a..b463a81dc176 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -1199,6 +1199,8 @@ struct kvm_x86_ops {
>>>        uint16_t (*nested_get_evmcs_version)(struct kvm_vcpu *vcpu);
>>>
>>>        bool (*need_emulation_on_page_fault)(struct kvm_vcpu *vcpu);
>>> +     int (*page_enc_status_hc)(struct kvm *kvm, unsigned long gpa,
>>> +                               unsigned long sz, unsigned long mode);
>>>   };
>>>
>>>   struct kvm_arch_async_pf {
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index 3089942f6630..431718309359 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -135,6 +135,8 @@ struct kvm_sev_info {
>>>        int fd;                 /* SEV device fd */
>>>        unsigned long pages_locked; /* Number of pages locked */
>>>        struct list_head regions_list;  /* List of registered regions */
>>> +     unsigned long *page_enc_bmap;
>>> +     unsigned long page_enc_bmap_size;
>>>   };
>>>
>>>   struct kvm_svm {
>>> @@ -1910,6 +1912,8 @@ static void sev_vm_destroy(struct kvm *kvm)
>>>
>>>        sev_unbind_asid(kvm, sev->handle);
>>>        sev_asid_free(kvm);
>>> +
>>> +     kvfree(sev->page_enc_bmap);
>>>   }
>>>
>>>   static void avic_vm_destroy(struct kvm *kvm)
>>
>> Adding Cfir who flagged this kvfree().
>>
>> Other freeing of sev->page_enc_bmap in this patch also set
>> sev->page_enc_bmap_size to 0 and neither set sev->page_enc_bmap to NULL
>> after freeing it.
>>
>> For extra safety, is it possible to sev->page_enc_bmap = NULL anytime the
>> bitmap is kvfreed?
>>
>>> @@ -2084,6 +2088,7 @@ static void avic_set_running(struct kvm_vcpu *vcpu, bool is_run)
>>>
>>>   static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>>   {
>>> +     struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info;
>>>        struct vcpu_svm *svm = to_svm(vcpu);
>>>        u32 dummy;
>>>        u32 eax = 1;
>>> @@ -2105,6 +2110,12 @@ static void svm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>>
>>>        if (kvm_vcpu_apicv_active(vcpu) && !init_event)
>>>                avic_update_vapic_bar(svm, APIC_DEFAULT_PHYS_BASE);
>>> +
>>> +     /* reset the page encryption bitmap */
>>> +     if (sev_guest(vcpu->kvm)) {
>>> +             kvfree(sev->page_enc_bmap);
>>> +             sev->page_enc_bmap_size = 0;
>>> +     }
>>>   }
>>>
>>>   static int avic_init_vcpu(struct vcpu_svm *svm)
>>
>> What is protecting sev->page_enc_bmap and sev->page_enc_bmap_size in calls
>> to svm_vcpu_reset()?