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

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Message-ID: <ZrwI-927_7cBxYT1@google.com>
Date: Tue, 13 Aug 2024 18:31:39 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: Kai Huang <kai.huang@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, pbonzini@...hat.com, rick.p.edgecombe@...el.com, 
	michael.roth@....com
Subject: Re: [PATCH v2 1/2] KVM: x86: Check hypercall's exit to userspace generically

On Tue, Aug 13, 2024, Isaku Yamahata wrote:
> On Wed, Aug 14, 2024 at 11:11:29AM +1200,
> Kai Huang <kai.huang@...el.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@...ux.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@...ux.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)

I would rather have "hypercall" in the name, "hc" never jumps out to me as being
"hypercall". Maybe is_hypercall_exit_enabled(), user_exit_on_hypercall(), or just
exit_on_hypercall()?

I'd probably vote for user_exit_on_hypercall(), as that clarifies it's all about
exiting to userspace, not from the guest.

> > > > +{
> > > > +	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().

KVM does use __builtin_constant_p() to effectively disable some assertions when
it's allowed (by KVM's arbitrary rules) to pass in a non-constant value.  E.g.
see all the vmcs_checkNN() helpers.  If we didn't waive the assertion for values
that aren't constant at compile-time, all of the segmentation code would need to
be unwound into switch statements.

But for things like guest_cpuid_has(), the rule is that the input must be a
compile-time constant.

> > 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().

Yeah, I think it makes sense to drop the WARN, otherwise I suspect we'll end up
dancing around the helper just to avoid the warning.

I'm 50/50 on the BUILD_BUG_ON().  One one hand, it's kinda overkill.  On the other
hand, it's zero generated code.