Re: [PATCH 59/89] KVM: arm64: Do not support MTE for protected VMs

From: Peter Collingbourne <pcc@google.com>
To: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
	kernel-team@android.com, kvm@vger.kernel.org,
	Andy Lutomirski <luto@amacapital.net>,
	Michael Roth <michael.roth@amd.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Chao Peng <chao.p.peng@linux.intel.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 59/89] KVM: arm64: Do not support MTE for protected VMs
Date: Thu, 2 Jun 2022 20:00:29 -0700	[thread overview]
Message-ID: <Ypl5TdMN3J/tttNe@google.com> (raw)
In-Reply-To: <CA+EHjTx328na4FDfKU-cdLX+SV4MmKfMKKrTHo5H0=iB2GTQ+A@mail.gmail.com>

Hi Fuad,

On Fri, May 27, 2022 at 08:55:42AM +0100, Fuad Tabba wrote:
> Hi Peter,
> 
> On Thu, May 26, 2022 at 9:08 PM Peter Collingbourne <pcc@google.com> wrote:
> >
> > On Thu, May 19, 2022 at 7:40 AM Will Deacon <will@kernel.org> wrote:
> > >
> > > From: Fuad Tabba <tabba@google.com>
> > >
> > > Return an error (-EINVAL) if trying to enable MTE on a protected
> > > vm.
> >
> > I think this commit message needs more explanation as to why MTE is
> > not currently supported in protected VMs.
> 
> Yes, we need to explain this more. Basically this is an extension of
> restricting features for protected VMs done earlier [*].
> 
> Various VM feature configurations are allowed in KVM/arm64, each requiring
> specific handling logic to deal with traps, context-switching and potentially
> emulation. Achieving feature parity in pKVM therefore requires either elevating
> this logic to EL2 (and substantially increasing the TCB) or continuing to trust
> the host handlers at EL1. Since neither of these options are especially
> appealing, pKVM instead limits the CPU features exposed to a guest to a fixed
> configuration based on the underlying hardware and which can mostly be provided
> straightforwardly by EL2.
> 
> This of course can change in the future and we can support more
> features for protected VMs as needed. We'll expand on this commit
> message when we respin.
> 
> Also note that this only applies to protected VMs. Non-protected VMs
> in protected mode support MTE.

I see. In this case unless I'm missing something the EL2 side seems
quite trivial though (flipping some bits in HCR_EL2). The patch below
(in place of this one) seems to make MTE work in my test environment
(patched [1] crosvm on Android in MTE-enabled QEMU).

[1] https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3689015

From c87965cd14515586d487872486e7670874209113 Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <pcc@google.com>
Date: Thu, 2 Jun 2022 19:16:02 -0700
Subject: [PATCH] arm64: support MTE in protected VMs

Enable HCR_EL2.ATA while running a vCPU with MTE enabled.

To avoid exposing MTE tags from the host to protected VMs, sanitize
tags before donating pages.

Signed-off-by: Peter Collingbourne <pcc@google.com>
---
 arch/arm64/include/asm/kvm_pkvm.h | 4 +++-
 arch/arm64/kvm/hyp/nvhe/pkvm.c    | 6 +++---
 arch/arm64/kvm/mmu.c              | 4 +++-
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pkvm.h b/arch/arm64/include/asm/kvm_pkvm.h
index 952e3c3fa32d..9ca9296f2a25 100644
--- a/arch/arm64/include/asm/kvm_pkvm.h
+++ b/arch/arm64/include/asm/kvm_pkvm.h
@@ -73,10 +73,12 @@ void kvm_shadow_destroy(struct kvm *kvm);
  * Allow for protected VMs:
  * - Branch Target Identification
  * - Speculative Store Bypassing
+ * - Memory Tagging Extension
  */
 #define PVM_ID_AA64PFR1_ALLOW (\
 	ARM64_FEATURE_MASK(ID_AA64PFR1_BT) | \
-	ARM64_FEATURE_MASK(ID_AA64PFR1_SSBS) \
+	ARM64_FEATURE_MASK(ID_AA64PFR1_SSBS) | \
+	ARM64_FEATURE_MASK(ID_AA64PFR1_MTE) \
 	)
 
 /*
diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index e33ba9067d7b..46ddd9093ac7 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -88,7 +88,7 @@ static void pvm_init_traps_aa64pfr1(struct kvm_vcpu *vcpu)
 	/* Memory Tagging: Trap and Treat as Untagged if not supported. */
 	if (!FIELD_GET(ARM64_FEATURE_MASK(ID_AA64PFR1_MTE), feature_ids)) {
 		hcr_set |= HCR_TID5;
-		hcr_clear |= HCR_DCT | HCR_ATA;
+		hcr_clear |= HCR_ATA;
 	}
 
 	vcpu->arch.hcr_el2 |= hcr_set;
@@ -179,8 +179,8 @@ static void pvm_init_trap_regs(struct kvm_vcpu *vcpu)
 	 * - Feature id registers: to control features exposed to guests
 	 * - Implementation-defined features
 	 */
-	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS |
-			     HCR_TID3 | HCR_TACR | HCR_TIDCP | HCR_TID1;
+	vcpu->arch.hcr_el2 = HCR_GUEST_FLAGS | HCR_TID3 | HCR_TACR | HCR_TIDCP |
+			     HCR_TID1 | HCR_ATA;
 
 	if (cpus_have_const_cap(ARM64_HAS_RAS_EXTN)) {
 		/* route synchronous external abort exceptions to EL2 */
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 392ff7b2362d..f513852357f7 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1206,8 +1206,10 @@ static int pkvm_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		goto dec_account;
 	}
 
-	write_lock(&kvm->mmu_lock);
 	pfn = page_to_pfn(page);
+	sanitise_mte_tags(kvm, pfn, PAGE_SIZE);
+
+	write_lock(&kvm->mmu_lock);
 	ret = pkvm_host_map_guest(pfn, fault_ipa >> PAGE_SHIFT);
 	if (ret) {
 		if (ret == -EAGAIN)
-- 
2.36.1.255.ge46751e96f-goog
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