Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically

[Isaku Yamahata <isaku.yamahata@intel.com>]

On Wed, Aug 14, 2024 at 11:11:29AM +1200,
"Huang, Kai" <kai.huang@intel.com> 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@linux.intel.com> 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@linux.intel.com>
> > > ---
> > >   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@intel.com>