Re: [RFC PATCH 2/3] KVM: arm64: Allow userspace to trap SMCCC sub-ranges

[Oliver Upton <oliver.upton@linux.dev>]

On Thu, Nov 10, 2022 at 12:22:12PM +0000, Marc Zyngier wrote:
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index e33ed7c09a28..cc3872f1900c 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -52,6 +52,9 @@
> >  
> >  #define KVM_HAVE_MMU_RWLOCK
> >  
> > +#define KVM_ARM_USER_HYPERCALL_FLAGS	\
> > +		GENMASK_ULL(KVM_ARM_USER_HYPERCALL_FLAGS_COUNT - 1, 0)
> > +
> >  /*
> >   * Mode of operation configurable with kvm-arm.mode early param.
> >   * See Documentation/admin-guide/kernel-parameters.txt for more information.
> > @@ -104,11 +107,13 @@ struct kvm_arch_memory_slot {
> >  /**
> >   * struct kvm_smccc_features: Descriptor of the hypercall services exposed to the guests
> >   *
> > + * @user_trap_bmap: Bitmap of SMCCC function ranges trapped to userspace
> >   * @std_bmap: Bitmap of standard secure service calls
> >   * @std_hyp_bmap: Bitmap of standard hypervisor service calls
> >   * @vendor_hyp_bmap: Bitmap of vendor specific hypervisor service calls
> >   */
> >  struct kvm_smccc_features {
> > +	unsigned long user_trap_bmap;
> 
> nit: I strongly object to the word 'trap'. By definition, this is a
> trap. The difference here is that you *forward* something to userspace
> instead of implementing it in the kernel.

I think you're being polite calling this a 'nit' :-)

Naming came about lazily to shorten some names, but completely breaks
the notion of what a trap is. Oops.

> > diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> > index 62ce45d0d957..22a23b12201d 100644
> > --- a/arch/arm64/kvm/hypercalls.c
> > +++ b/arch/arm64/kvm/hypercalls.c
> > @@ -92,6 +92,49 @@ static bool kvm_hvc_call_default_allowed(u32 func_id)
> >  	}
> >  }
> >  
> > +static bool kvm_hvc_call_user_trapped(struct kvm_vcpu *vcpu, u32 func_id)
> > +{
> > +	struct kvm *kvm = vcpu->kvm;
> > +	unsigned long *bmap = &kvm->arch.smccc_feat.user_trap_bmap;
> > +
> > +	switch (ARM_SMCCC_OWNER_NUM(func_id)) {
> > +	case ARM_SMCCC_OWNER_ARCH:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_ARCH, bmap);
> > +	case ARM_SMCCC_OWNER_CPU:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_CPU, bmap);
> > +	case ARM_SMCCC_OWNER_SIP:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_SIP, bmap);
> > +	case ARM_SMCCC_OWNER_OEM:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_OEM, bmap);
> > +	case ARM_SMCCC_OWNER_STANDARD:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_STANDARD, bmap);
> > +	case ARM_SMCCC_OWNER_STANDARD_HYP:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_STANDARD_HYP, bmap);
> > +	case ARM_SMCCC_OWNER_VENDOR_HYP:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_VENDOR_HYP, bmap);
> > +	case ARM_SMCCC_OWNER_TRUSTED_APP ... ARM_SMCCC_OWNER_TRUSTED_APP_END:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_TRUSTED_APP, bmap);
> > +	case ARM_SMCCC_OWNER_TRUSTED_OS ... ARM_SMCCC_OWNER_TRUSTED_OS_END:
> > +		return test_bit(KVM_ARM_USER_HYPERCALL_OWNER_TRUSTED_OS, bmap);
> > +	default:
> > +		return false;
> > +	}
> 
> You have multiple problems here:
> 
> - the granularity is way too coarse. You want to express arbitrary
>   ranges, and not necessarily grab a whole owner range.
> 
> - you have now an overlap between ranges that are handled in the
>   kernel (PSCI, spectre mitigations) and ranges that userspace wants
>   to observe. Not good.

We need to come to agreement on what degree of mix-and-match should be
supported.

Spectre really ought to be in the kernel, and I don't think anyone is
particularly excited about reimplementing PSCI. Right now my interest
in this starts and ends with forwarding the vendor-specific hypercall
range to userspace, allowing something like Hyper-V PV on KVM.

> If we are going down this road, this can only be done at the
> *function* level. And userspace must know that the kernel will refuse
> to forward some ranges.

The goal of what I was trying to get at is that either the kernel or
userspace takes ownership of a range that has an ABI, but not both. i.e.
you really wouldn't want some VMM or cloud provider trapping portions of
KVM's vendor-specific range while still reporting a 'vanilla' ABI at the
time of discovery. Same goes for PSCI, TRNG, etc.

> So obviously, this cannot be a simple bitmap. Making it a radix tree
> (or an xarray, which is basically the same thing) could work. And the
> filtering request from userspace can be similar to what we have for
> the PMU filters.

Right, we'll need a more robust data structure for all this.

My only concern is that communicating the hypercall filter between
user/kernel with a set of ranges or function numbers is that we could be
mutating what KVM *doesn't* already implement into an ABI of sorts.

i.e. suppose that userspace wants to filter function(s) in an
unallocated/unused range of function numbers. Later down the line KVM
adds support for a new shiny thing and the filter becomes a subset of a
now allocated range of calls. We then reject the filter due to the
incongruence.

> > +}
> > +
> > +static void kvm_hvc_prepare_user_trap(struct kvm_vcpu *vcpu)
> > +{
> > +	struct kvm_run *run = vcpu->run;
> > +
> > +	run->exit_reason	= KVM_EXIT_HYPERCALL;
> > +	run->hypercall.nr	= smccc_get_function(vcpu);
> > +	run->hypercall.args[0]	= smccc_get_arg(vcpu, 1);
> > +	run->hypercall.args[1]	= smccc_get_arg(vcpu, 2);
> > +	run->hypercall.args[2]	= smccc_get_arg(vcpu, 3);
> > +	run->hypercall.args[3]	= smccc_get_arg(vcpu, 4);
> > +	run->hypercall.args[4]	= smccc_get_arg(vcpu, 5);
> > +	run->hypercall.args[5]	= smccc_get_arg(vcpu, 6);
> 
> All of which is readily available through the ONE_REG interface. I'm
> mildly reluctant to expose another interface that disclose the same
> information (yes, I understand the performance impact).

I can drop this bit for now, always easy to add it back in and advertize
with a flag if the overhead is too great.

--
Thanks,
Oliver