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Message-ID: <3b563b01-5090-4c9d-a47c-a0aaa13c474b@arm.com>
Date: Mon, 7 Apr 2025 17:34:28 +0100
From: Steven Price <steven.price@....com>
To: Gavin Shan <gshan@...hat.com>, kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
Oliver Upton <oliver.upton@...ux.dev>,
Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
<yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
Alexandru Elisei <alexandru.elisei@....com>,
Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
linux-coco@...ts.linux.dev,
Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
Shanker Donthineni <sdonthineni@...dia.com>, Alper Gun
<alpergun@...gle.com>, "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v7 18/45] arm64: RME: Handle RMI_EXIT_RIPAS_CHANGE
On 04/03/2025 04:35, Gavin Shan wrote:
> On 2/14/25 2:13 AM, Steven Price wrote:
>> The guest can request that a region of it's protected address space is
>> switched between RIPAS_RAM and RIPAS_EMPTY (and back) using
>> RSI_IPA_STATE_SET. This causes a guest exit with the
>> RMI_EXIT_RIPAS_CHANGE code. We treat this as a request to convert a
>> protected region to unprotected (or back), exiting to the VMM to make
>> the necessary changes to the guest_memfd and memslot mappings. On the
>> next entry the RIPAS changes are committed by making RMI_RTT_SET_RIPAS
>> calls.
>>
>> The VMM may wish to reject the RIPAS change requested by the guest. For
>> now it can only do with by no longer scheduling the VCPU as we don't
>> currently have a usecase for returning that rejection to the guest, but
>> by postponing the RMI_RTT_SET_RIPAS changes to entry we leave the door
>> open for adding a new ioctl in the future for this purpose.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>> New patch for v7: The code was previously split awkwardly between two
>> other patches.
>> ---
>> arch/arm64/kvm/rme.c | 87 ++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 87 insertions(+)
>>
>
> With the following comments addressed:
>
> Reviewed-by: Gavin Shan <gshan@...hat.com>
>
>> diff --git a/arch/arm64/kvm/rme.c b/arch/arm64/kvm/rme.c
>> index 507eb4b71bb7..f965869e9ef7 100644
>> --- a/arch/arm64/kvm/rme.c
>> +++ b/arch/arm64/kvm/rme.c
>> @@ -624,6 +624,64 @@ void kvm_realm_unmap_range(struct kvm *kvm,
>> unsigned long start, u64 size,
>> realm_unmap_private_range(kvm, start, end);
>> }
>> +static int realm_set_ipa_state(struct kvm_vcpu *vcpu,
>> + unsigned long start,
>> + unsigned long end,
>> + unsigned long ripas,
>> + unsigned long *top_ipa)
>> +{
>> + struct kvm *kvm = vcpu->kvm;
>> + struct realm *realm = &kvm->arch.realm;
>> + struct realm_rec *rec = &vcpu->arch.rec;
>> + phys_addr_t rd_phys = virt_to_phys(realm->rd);
>> + phys_addr_t rec_phys = virt_to_phys(rec->rec_page);
>> + struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
>> + unsigned long ipa = start;
>> + int ret = 0;
>> +
>> + while (ipa < end) {
>> + unsigned long next;
>> +
>> + ret = rmi_rtt_set_ripas(rd_phys, rec_phys, ipa, end, &next);
>> +
>
> This doesn't look correct to me. Looking at RMM::smc_rtt_set_ripas(),
> it's possible
> the SMC call is returned without updating 'next' to a valid address. In
> this case,
> the garbage content resident in 'next' can be used to updated to 'ipa'
> in next
> iternation. So we need to initialize it in advance, like below.
>
> unsigned long ipa = start;
> unsigned long next = start;
>
> while (ipa < end) {
> ret = rmi_rtt_set_ripas(rd_phys, rec_phys, ipa, end, &next);
I agree this might not be the clearest code, but 'next' should be set if
the return state is RMI_SUCCESS, and we don't actually get to the "ipa =
next" line unless that is the case. But I'll rejig things because it's
not clear.
>> + if (RMI_RETURN_STATUS(ret) == RMI_ERROR_RTT) {
>> + int walk_level = RMI_RETURN_INDEX(ret);
>> + int level = find_map_level(realm, ipa, end);
>> +
>> + /*
>> + * If the RMM walk ended early then more tables are
>> + * needed to reach the required depth to set the RIPAS.
>> + */
>> + if (walk_level < level) {
>> + ret = realm_create_rtt_levels(realm, ipa,
>> + walk_level,
>> + level,
>> + memcache);
>> + /* Retry with RTTs created */
>> + if (!ret)
>> + continue;
>> + } else {
>> + ret = -EINVAL;
>> + }
>> +
>> + break;
>> + } else if (RMI_RETURN_STATUS(ret) != RMI_SUCCESS) {
>> + WARN(1, "Unexpected error in %s: %#x\n", __func__,
>> + ret);
>> + ret = -EINVAL;
>
> ret = -ENXIO;
Ack
>> + break;
>> + }
>> + ipa = next;
>> + }
>> +
>> + *top_ipa = ipa;
>> +
>> + if (ripas == RMI_EMPTY && ipa != start)
>> + realm_unmap_private_range(kvm, start, ipa);
>> +
>> + return ret;
>> +}
>> +
>> static int realm_init_ipa_state(struct realm *realm,
>> unsigned long ipa,
>> unsigned long end)
>> @@ -863,6 +921,32 @@ void kvm_destroy_realm(struct kvm *kvm)
>> kvm_free_stage2_pgd(&kvm->arch.mmu);
>> }
>> +static void kvm_complete_ripas_change(struct kvm_vcpu *vcpu)
>> +{
>> + struct kvm *kvm = vcpu->kvm;
>> + struct realm_rec *rec = &vcpu->arch.rec;
>> + unsigned long base = rec->run->exit.ripas_base;
>> + unsigned long top = rec->run->exit.ripas_top;
>> + unsigned long ripas = rec->run->exit.ripas_value;
>> + unsigned long top_ipa;
>> + int ret;
>> +
>
> Some checks are needed here to ensure the addresses (@base and @top)
> falls inside
> the protected (private) space for two facts: (1) Those parameters
> originates from
> the guest, which can be misbehaving. (2) RMM::smc_rtt_set_ripas() isn't
> limited to
> the private space, meaning it also can change RIPAS for the ranges in
> the shared
> space.
I might be missing something, but AFAICT this is safe:
1. The RMM doesn't permit RIPAS changes within the shared space:
RSI_IPA_STATE_SET has a precondition [rgn_bound]:
AddrRangeIsProtected(base, top, realm)
So a malicious guest shouldn't get passed the RMM.
2. The RMM validates that the range passed here is (a subset of) the
one provided to the NS-world [base_bound / top_bound].
And even if somehow a malicious guest managed to bypass these checks I
don't see how it would cause harm to the host operating on the wrong region.
I'm happy to be corrected though! What am I missing?
Thank,
Steve
>> + do {
>> + kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_page_cache,
>> + kvm_mmu_cache_min_pages(vcpu->arch.hw_mmu));
>> + write_lock(&kvm->mmu_lock);
>> + ret = realm_set_ipa_state(vcpu, base, top, ripas, &top_ipa);
>> + write_unlock(&kvm->mmu_lock);
>> +
>> + if (WARN_RATELIMIT(ret && ret != -ENOMEM,
>> + "Unable to satisfy RIPAS_CHANGE for %#lx - %#lx,
>> ripas: %#lx\n",
>> + base, top, ripas))
>> + break;
>> +
>> + base = top_ipa;
>> + } while (top_ipa < top);
>> +}
>> +
>> int kvm_rec_enter(struct kvm_vcpu *vcpu)
>> {
>> struct realm_rec *rec = &vcpu->arch.rec;
>> @@ -873,6 +957,9 @@ int kvm_rec_enter(struct kvm_vcpu *vcpu)
>> for (int i = 0; i < REC_RUN_GPRS; i++)
>> rec->run->enter.gprs[i] = vcpu_get_reg(vcpu, i);
>> break;
>> + case RMI_EXIT_RIPAS_CHANGE:
>> + kvm_complete_ripas_change(vcpu);
>> + break;
>> }
>> if (kvm_realm_state(vcpu->kvm) != REALM_STATE_ACTIVE)
>
> Thanks,
> Gavin
>
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