Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

[Andy Lutomirski <luto@amacapital.net>]

> On Jan 12, 2021, at 7:17 AM, Maxim Levitsky <mlevitsk@redhat.com> wrote:
> 
> On Tue, 2021-01-12 at 07:11 -0800, Andy Lutomirski wrote:
>>>> On Jan 12, 2021, at 4:15 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>> 
>>> Wei Huang <wei.huang2@amd.com> writes:
>>> 
>>>> From: Bandan Das <bsd@redhat.com>
>>>> 
>>>> While running VM related instructions (VMRUN/VMSAVE/VMLOAD), some AMD
>>>> CPUs check EAX against reserved memory regions (e.g. SMM memory on host)
>>>> before checking VMCB's instruction intercept. If EAX falls into such
>>>> memory areas, #GP is triggered before VMEXIT. This causes problem under
>>>> nested virtualization. To solve this problem, KVM needs to trap #GP and
>>>> check the instructions triggering #GP. For VM execution instructions,
>>>> KVM emulates these instructions; otherwise it re-injects #GP back to
>>>> guest VMs.
>>>> 
>>>> Signed-off-by: Bandan Das <bsd@redhat.com>
>>>> Co-developed-by: Wei Huang <wei.huang2@amd.com>
>>>> Signed-off-by: Wei Huang <wei.huang2@amd.com>
>>>> ---
>>>> arch/x86/include/asm/kvm_host.h |   8 +-
>>>> arch/x86/kvm/mmu.h              |   1 +
>>>> arch/x86/kvm/mmu/mmu.c          |   7 ++
>>>> arch/x86/kvm/svm/svm.c          | 157 +++++++++++++++++++-------------
>>>> arch/x86/kvm/svm/svm.h          |   8 ++
>>>> arch/x86/kvm/vmx/vmx.c          |   2 +-
>>>> arch/x86/kvm/x86.c              |  37 +++++++-
>>>> 7 files changed, 146 insertions(+), 74 deletions(-)
>>>> 
>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>> index 3d6616f6f6ef..0ddc309f5a14 100644
>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>> @@ -1450,10 +1450,12 @@ extern u64 kvm_mce_cap_supported;
>>>> *                 due to an intercepted #UD (see EMULTYPE_TRAP_UD).
>>>> *                 Used to test the full emulator from userspace.
>>>> *
>>>> - * EMULTYPE_VMWARE_GP - Set when emulating an intercepted #GP for VMware
>>>> + * EMULTYPE_PARAVIRT_GP - Set when emulating an intercepted #GP for VMware
>>>> *            backdoor emulation, which is opt in via module param.
>>>> *            VMware backoor emulation handles select instructions
>>>> - *            and reinjects the #GP for all other cases.
>>>> + *            and reinjects #GP for all other cases. This also
>>>> + *            handles other cases where #GP condition needs to be
>>>> + *            handled and emulated appropriately
>>>> *
>>>> * EMULTYPE_PF - Set when emulating MMIO by way of an intercepted #PF, in which
>>>> *         case the CR2/GPA value pass on the stack is valid.
>>>> @@ -1463,7 +1465,7 @@ extern u64 kvm_mce_cap_supported;
>>>> #define EMULTYPE_SKIP            (1 << 2)
>>>> #define EMULTYPE_ALLOW_RETRY_PF        (1 << 3)
>>>> #define EMULTYPE_TRAP_UD_FORCED        (1 << 4)
>>>> -#define EMULTYPE_VMWARE_GP        (1 << 5)
>>>> +#define EMULTYPE_PARAVIRT_GP        (1 << 5)
>>>> #define EMULTYPE_PF            (1 << 6)
>>>> 
>>>> int kvm_emulate_instruction(struct kvm_vcpu *vcpu, int emulation_type);
>>>> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>>>> index 581925e476d6..1a2fff4e7140 100644
>>>> --- a/arch/x86/kvm/mmu.h
>>>> +++ b/arch/x86/kvm/mmu.h
>>>> @@ -219,5 +219,6 @@ int kvm_arch_write_log_dirty(struct kvm_vcpu *vcpu);
>>>> 
>>>> int kvm_mmu_post_init_vm(struct kvm *kvm);
>>>> void kvm_mmu_pre_destroy_vm(struct kvm *kvm);
>>>> +bool kvm_is_host_reserved_region(u64 gpa);
>>> 
>>> Just a suggestion: "kvm_gpa_in_host_reserved()" maybe? 
>>> 
>>>> #endif
>>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>>> index 6d16481aa29d..c5c4aaf01a1a 100644
>>>> --- a/arch/x86/kvm/mmu/mmu.c
>>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>>> @@ -50,6 +50,7 @@
>>>> #include <asm/io.h>
>>>> #include <asm/vmx.h>
>>>> #include <asm/kvm_page_track.h>
>>>> +#include <asm/e820/api.h>
>>>> #include "trace.h"
>>>> 
>>>> extern bool itlb_multihit_kvm_mitigation;
>>>> @@ -5675,6 +5676,12 @@ void kvm_mmu_slot_set_dirty(struct kvm *kvm,
>>>> }
>>>> EXPORT_SYMBOL_GPL(kvm_mmu_slot_set_dirty);
>>>> 
>>>> +bool kvm_is_host_reserved_region(u64 gpa)
>>>> +{
>>>> +    return e820__mbapped_raw_any(gpa-1, gpa+1, E820_TYPE_RESERVED);
>>>> +}
>>> 
>>> While _e820__mapped_any()'s doc says '..  checks if any part of the
>>> range <start,end> is mapped ..' it seems to me that the real check is
>>> [start, end) so we should use 'gpa' instead of 'gpa-1', no?
>> 
>> Why do you need to check GPA at all?
>> 
> To reduce the scope of the workaround.
> 
> The errata only happens when you use one of SVM instructions
> in the guest with EAX that happens to be inside one
> of the host reserved memory regions (for example SMM).

This code reduces the scope of the workaround at the cost of increasing the complexity of the workaround and adding a nonsensical coupling between KVM and host details and adding an export that really doesn’t deserve to be exported.

Is there an actual concrete benefit to this check?