by Tian, Kevin

[permalink] [raw]

Subject: RE: [PATCH v6 04/21] kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID

> From: Like Xu <[email protected]>
> Sent: Sunday, January 23, 2022 2:22 PM
>
> On 8/1/2022 2:54 am, Paolo Bonzini wrote:
> > From: Jing Liu <[email protected]>
> >
> > KVM_GET_SUPPORTED_CPUID should not include any dynamic xstates in
> > CPUID[0xD] if they have not been requested with prctl. Otherwise
> > a process which directly passes KVM_GET_SUPPORTED_CPUID to
> > KVM_SET_CPUID2 would now fail even if it doesn't intend to use a
> > dynamically enabled feature. Userspace must know that prctl is
> > required and allocate >4K xstate buffer before setting any dynamic
> > bit.
> >
> > Suggested-by: Paolo Bonzini <[email protected]>
> > Signed-off-by: Jing Liu <[email protected]>
> > Signed-off-by: Yang Zhong <[email protected]>
> > Message-Id: <[email protected]>
> > Signed-off-by: Paolo Bonzini <[email protected]>
> > ---
> > Documentation/virt/kvm/api.rst | 4 ++++
> > arch/x86/kvm/cpuid.c | 9 ++++++---
> > 2 files changed, 10 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst
> b/Documentation/virt/kvm/api.rst
> > index 6b683dfea8f2..f4ea5e41a4d0 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -1687,6 +1687,10 @@ userspace capabilities, and with user
> requirements (for example, the
> > user may wish to constrain cpuid to emulate older hardware, or for
> > feature consistency across a cluster).
> >
> > +Dynamically-enabled feature bits need to be requested with
> > +``arch_prctl()`` before calling this ioctl. Feature bits that have not
> > +been requested are excluded from the result.
> > +
> > Note that certain capabilities, such as KVM_CAP_X86_DISABLE_EXITS, may
> > expose cpuid features (e.g. MONITOR) which are not supported by kvm in
> > its default configuration. If userspace enables such capabilities, it
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index f3e6fda6b858..eb52dde5deec 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -815,11 +815,13 @@ static inline int __do_cpuid_func(struct
> kvm_cpuid_array *array, u32 function)
> > goto out;
> > }
> > break;
> > - case 0xd:
> > - entry->eax &= supported_xcr0;
> > + case 0xd: {
> > + u64 guest_perm = xstate_get_guest_group_perm();
> > +
> > + entry->eax &= supported_xcr0 & guest_perm;
> > entry->ebx = xstate_required_size(supported_xcr0, false);
>
> If we choose to exclude unpermitted xfeatures in the entry->eax, why do
> we choose to expose the size of unpermitted xfeatures in ebx and ecx?
>
> This seems to be an inconsistency, how about:
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 1bd4d560cbdd..193cbf56a5fa 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -888,12 +888,12 @@ static inline int __do_cpuid_func(struct
> kvm_cpuid_array
> *array, u32 function)
> }
> break;
> case 0xd: {
> - u64 guest_perm = xstate_get_guest_group_perm();
> + u64 supported_xcr0 = supported_xcr0 &
> xstate_get_guest_group_perm();
>
> - entry->eax &= supported_xcr0 & guest_perm;
> + entry->eax &= supported_xcr0;
> entry->ebx = xstate_required_size(supported_xcr0, false);
> entry->ecx = entry->ebx;
> - entry->edx &= (supported_xcr0 & guest_perm) >> 32;
> + entry->edx &= supported_xcr0 >> 32;
> if (!supported_xcr0)
> break;
>
> It also helps to fix the CPUID_D_1_EBX and later for (i = 2; i < 64; ++i);
>
> Is there anything I've missed ?

No, you are correct. Would you please send out a formal fix?

>
> > entry->ecx = entry->ebx;
> > - entry->edx &= supported_xcr0 >> 32;
> > + entry->edx &= (supported_xcr0 & guest_perm) >> 32;
> > if (!supported_xcr0)
> > break;
> >
> > @@ -866,6 +868,7 @@ static inline int __do_cpuid_func(struct
> kvm_cpuid_array *array, u32 function)
> > entry->edx = 0;
> > }
> > break;
> > + }
> > case 0x12:
> > /* Intel SGX */
> > if (!kvm_cpu_cap_has(X86_FEATURE_SGX)) {