On Wed, Aug 14, 2024 at 11:11:29AM +1200,
"Huang, Kai" <kai.huang@xxxxxxxxx> wrote:
>
>
> On 14/08/2024 5:50 am, Isaku Yamahata wrote:
> > On Tue, Aug 13, 2024 at 01:12:55PM +0800,
> > Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:
> >
> > > Check whether a KVM hypercall needs to exit to userspace or not based on
> > > hypercall_exit_enabled field of struct kvm_arch.
> > >
> > > Userspace can request a hypercall to exit to userspace for handling by
> > > enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in
> > > hypercall_exit_enabled. Make the check code generic based on it.
> > >
> > > Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/x86.c | 4 ++--
> > > arch/x86/kvm/x86.h | 7 +++++++
> > > 2 files changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index af6c8cf6a37a..6e16c9751af7 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > > cpl = kvm_x86_call(get_cpl)(vcpu);
> > > ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> > > - if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> > > - /* MAP_GPA tosses the request to the user space. */
> > > + if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr))
> > > + /* The hypercall is requested to exit to userspace. */
> > > return 0;
> > > if (!op_64_bit)
> > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> > > index 50596f6f8320..0cbec76b42e6 100644
> > > --- a/arch/x86/kvm/x86.h
> > > +++ b/arch/x86/kvm/x86.h
> > > @@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
> > > unsigned int port, void *data, unsigned int count,
> > > int in);
> > > +static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr)
> > > +{
> > > + if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8))
> > > + return false;
> >
> > Is this to detect potential bug? Maybe
> > BUILD_BUG_ON(__builtin_constant_p(hc_nr) &&
> > !(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK));
> > Overkill?
>
> I don't think this is the correct way to use __builtin_constant_p(), i.e. it
> doesn't make sense to use __builtin_constant_p() in BUILD_BUG_ON().
>
> IIUC you need some build time guarantee here, but __builtin_constant_p() can
> return false, in which case the above BUILD_BUG_ON() does nothing, which
> defeats the purpose.
It depends on what we'd like to detect. BUILT_BUG_ON(__builtin_constant_p())
can detect the usage in the patch 2/2,
is_kvm_hc_exit_enabled(vcpu->kvm, KVM_HC_MAP_GPA_RANGE). The potential
future use of is_kvm_hc_exit_enabled(, KVM_HC_MAP_future_hypercall).
Although this version doesn't help for the one in kvm_emulate_hypercall(),
!ret check is done first to avoid WARN_ON_ONCE() to hit here.
Maybe we can just drop this WARN_ON_ONCE().
--
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
