[PATCH 7/7] KVM: arm64: Separate write faults on read-only memslots from MMIO

[Will Deacon]

Although taking a write-fault on a read-only memslot triggers an MMIO
exit back to userspace, lumping the handling together in
kvm_handle_guest_abort() causes some of the early triage to have weird
effects on userspace. For example, if a guest generates a stage-2 fault
on a stage-1 translation fault when trying to fetch an instruction from
a read-only memslot, it will be mistakenly treated as an attempt to
execute from MMIO and a prefetch abort will be re-injected into the
guest.

Separate the MMIO handling from the read-only memslot handling, so that
the latter is left entirely up to userspace. Note that this _will_
result in more exits than before for read-only memslots, since userspace
will now see some cache maintenance and instruction-side aborts.

Cc: Marc Zyngier <maz at kernel.org>
Cc: Quentin Perret <qperret at google.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/kvm/mmu.c | 45 ++++++++++++++++++++++++--------------------
 1 file changed, 25 insertions(+), 20 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 9e72e7f4a2c2..2edc6f2412dc 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2117,9 +2117,30 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	memslot = gfn_to_memslot(vcpu->kvm, gfn);
 	hva = gfn_to_hva_memslot_prot(memslot, gfn, &writable);
 	write_fault = kvm_is_write_fault(vcpu);
-	if (kvm_is_error_hva(hva) || (write_fault && !writable)) {
+
+	/*
+	 * The IPA is reported as [MAX:12], so we need to complement it with
+	 * the bottom 12 bits from the faulting VA. This is always 12 bits,
+	 * irrespective of the page size.
+	 */
+	fault_ipa |= kvm_vcpu_get_hfar(vcpu) & ((1 << 12) - 1);
+
+	/*
+	 * We can perform some early fault triage based purely on the memslot
+	 * information:
+	 *
+	 * Faults on IPAs falling outside of any memslot are re-injected
+	 * into the guest as external aborts if they were either signalled as
+	 * instruction aborts or as a stage-2 fault on a translation table walk.
+	 * If the instruction was a cache maintenance instruction then it is
+	 * quietly skipped, otherwise we exit to userspace for MMIO emulation.
+	 *
+	 * Write faults on IPAs falling within a read-only memslot are reported
+	 * to userspace as MMIO exits. This includes cache maintenance and
+	 * stage-2 faults on translation table walks,
+	 */
+	if (kvm_is_error_hva(hva)) {
 		if (kvm_vcpu_trap_is_iabt(vcpu)) {
-			/* Prefetch Abort on I/O address */
 			ret = -ENOEXEC;
 			goto out;
 		}
@@ -2129,30 +2150,12 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
 			goto out;
 		}
 
-		/*
-		 * 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.
-		 *
-		 * So let's assume that the guest is just being
-		 * cautious, and skip the instruction.
-		 */
 		if (kvm_vcpu_dabt_is_cm(vcpu)) {
 			kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
 			ret = 1;
 			goto out_unlock;
 		}
 
-		/*
-		 * The IPA is reported as [MAX:12], so we need to
-		 * complement it with the bottom 12 bits from the
-		 * faulting VA. This is always 12 bits, irrespective
-		 * of the page size.
-		 */
-		fault_ipa |= kvm_vcpu_get_hfar(vcpu) & ((1 << 12) - 1);
-
 		/*
 		 * No valid syndrome? Ask userspace for help if it has
 		 * volunteered to do so, and bail out otherwise.
@@ -2161,7 +2164,9 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu, struct kvm_run *run)
 			ret = handle_error_invalid_dabt(vcpu, run, fault_ipa);
 			goto out_unlock;
 		}
+	}
 
+	if (kvm_is_error_hva(hva) || (write_fault && !writable)) {
 		ret = io_mem_abort(vcpu, run, fault_ipa);
 		goto out_unlock;
 	}
-- 
2.28.0.rc0.142.g3c755180ce-goog