Re: [PATCH] KVM: arm64: Handle CMOs on Read Only memslots

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

Played with this for a bit to try to understand the problem better, wrote a simple
MMIO device in kvmtool which maps the memory as a read-only memslot [1] and poked
it with kvm-unit-tests [2].

[1] https://gitlab.arm.com/linux-arm/kvmtool-ae/-/tree/mmiodev-wip1

[2] https://gitlab.arm.com/linux-arm/kvm-unit-tests-ae/-/tree/mmiodev-wip1

On 2/11/21 2:27 PM, Marc Zyngier wrote:
> It appears that when a guest traps into KVM because it is
> performing a CMO on a Read Only memslot, our handling of
> this operation is "slightly suboptimal", as we treat it as
> an MMIO access without a valid syndrome.
>
> The chances that userspace is adequately equiped to deal
> with such an exception being slim, it would be better to
> handle it in the kernel.
>
> What we need to provide is roughly as follows:
>
> (a) if a CMO hits writeable memory, handle it as a normal memory acess
> (b) if a CMO hits non-memory, skip it
> (c) if a CMO hits R/O memory, that's where things become fun:
>   (1) if the CMO is DC IVAC, the architecture says this should result
>       in a permission fault
>   (2) if the CMO is DC CIVAC, it should work similarly to (a)
>
> We already perform (a) and (b) correctly, but (c) is a total mess.
> Hence we need to distinguish between IVAC (c.1) and CIVAC (c.2).
>
> One way to do it is to treat CMOs generating a translation fault as
> a *read*, even when they are on a RW memslot. This allows us to
> further triage things:
>
> If they come back with a permission fault, that is because this is
> a DC IVAC instruction:
> - inside a RW memslot: no problem, treat it as a write (a)(c.2)
> - inside a RO memslot: inject a data abort in the guest (c.1)
>
> The only drawback is that DC IVAC on a yet unmapped page faults
> twice: one for the initial translation fault that result in a RO
> mapping, and once for the permission fault. I think we can live with
> that.
>
> Reported-by: Jianyong Wu <jianyong.wu@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>
> Notes:
>     I have taken the option to inject an abort in the guest when
>     it issues a DC IVAC on a R/O memslot, but another option would
>     be to just perform the invalidation ourselves as a DC CIAVAC.
>     
>     This would have the advantage of being consistent with what we
>     do for emulated MMIO.
>
>  arch/arm64/kvm/mmu.c | 53 ++++++++++++++++++++++++++++++++++----------
>  1 file changed, 41 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 7d2257cc5438..c7f4388bea45 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -760,7 +760,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	struct kvm_pgtable *pgt;
>  
>  	fault_granule = 1UL << ARM64_HW_PGTABLE_LEVEL_SHIFT(fault_level);
> -	write_fault = kvm_is_write_fault(vcpu);
> +	/*
> +	 * Treat translation faults on CMOs as read faults. Should
> +	 * this further generate a permission fault on a R/O memslot,
> +	 * it will be caught in kvm_handle_guest_abort(), with
> +	 * prejudice. Permission faults on non-R/O memslot will be
> +	 * gracefully handled as writes.
> +	 */
> +	if (fault_status == FSC_FAULT && kvm_vcpu_dabt_is_cm(vcpu))
> +		write_fault = false;

This means that every DC CIVAC will map the IPA with read permissions in the stage
2 tables, regardless of the IPA being already mapped. It's harmless, but a bit
unexpected.

> +	else
> +		write_fault = kvm_is_write_fault(vcpu);
>  	exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
>  	VM_BUG_ON(write_fault && exec_fault);
>  
> @@ -1013,19 +1023,37 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>  		}
>  
>  		/*
> -		 * Check for a cache maintenance operation. Since we
> -		 * ended-up here, we know it is outside of any memory
> -		 * slot. But we can't find out if that is for a device,
> -		 * or if the guest is just being stupid. The only thing
> -		 * we know for sure is that this range cannot be cached.
> +		 * Check for a cache maintenance operation. Three cases:
> +		 *
> +		 * - It is outside of any memory slot. But we can't find out
> +		 *   if that is for a device, or if the guest is just being
> +		 *   stupid. The only thing we know for sure is that this
> +		 *   range cannot be cached.  So let's assume that the guest
> +		 *   is just being cautious, and skip the instruction.
> +		 *
> +		 * - Otherwise, check whether this is a permission fault.
> +		 *   If so, that's a DC IVAC on a R/O memslot, which is a
> +		 *   pretty bad idea, and we tell the guest so.
>  		 *
> -		 * So let's assume that the guest is just being
> -		 * cautious, and skip the instruction.
> +		 * - If this wasn't a permission fault, pass it along for
> +		 *   further handling (including faulting the page in if it
> +		 *   was a translation fault).
>  		 */
> -		if (kvm_is_error_hva(hva) && kvm_vcpu_dabt_is_cm(vcpu)) {
> -			kvm_incr_pc(vcpu);
> -			ret = 1;
> -			goto out_unlock;
> +		if (kvm_vcpu_dabt_is_cm(vcpu)) {
> +			if (kvm_is_error_hva(hva)) {
> +				kvm_incr_pc(vcpu);
> +				ret = 1;
> +				goto out_unlock;
> +			}
> +
> +			if (fault_status == FSC_PERM) {
> +				/* DC IVAC on a R/O memslot */
> +				kvm_inject_dabt(vcpu, kvm_vcpu_get_hfar(vcpu));
> +				ret = 1;
> +				goto out_unlock;
> +			}

I don't like the inconsistency. We go from exiting to userspace for both DC
IVAC/DC CIVAC to mapping the IPA with read permissions for DC CIVAC, but injecting
a DABT for a DC IVAC. DC IVAC acts just like a DC CIVAC and requires the same
permissions when executed by a guest, so I'm not sure we should be handling them
differently.

Thanks,

Alex

> +
> +			goto handle_access;
>  		}
>  
>  		/*
> @@ -1039,6 +1067,7 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>  		goto out_unlock;
>  	}
>  
> +handle_access:
>  	/* Userspace should not be able to register out-of-bounds IPAs */
>  	VM_BUG_ON(fault_ipa >= kvm_phys_size(vcpu->kvm));
>  

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