Re: [PATCH v18] arm64: mm: Handle Granule Protection Faults (GPFs)
From: Suzuki K Poulose
Date: Tue Sep 29 2026 - 06:56:47 EST
On 29/09/2026 11:40, Catalin Marinas wrote:
On Mon, Sep 28, 2026 at 12:01:18PM +0100, Suzuki K Poulose wrote:
On 22/09/2026 15:49, Catalin Marinas wrote:
On Tue, Sep 22, 2026 at 02:21:13PM +0100, Suzuki K Poulose wrote:
I had another look and we could handle this via kvm_fault_is_gmem_abort()
see in arch/arm64/kvm/mmu.c:
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 87e49251e0447..af5a4bf961aae 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1731,6 +1731,9 @@ static int gmem_abort(const struct kvm_s2_fault_desc
*s2fd)
gfn_t gfn;
int ret;
+ if (!kvm_slot_has_gmem(s2fd->memslot))
+ return -EINVAL;
I wonder whether we should add a KVM_BUG_ON() here. With the rest of the
changes, we should never get in this situation. Well, to be revisited
for private devices.
Also maybe move it to the caller, kvm_vm_mem_abort(), and not change
kvm_fault_is_gmem_abort(). Something like:
if (private_ipa_fault(kvm, s2fd->fault_ipa) &&
KVM_BUG_ON(!kvm_slot_has_gmem(s2fd->memslot), kvm))
return -EIO;
To me it makes more sense for gmem_abort() to be called only *if* it's a
gmem slot. So any inconsistency, avoiding user_mem_abort() for private
memory, should be done in the caller. I assume the caller will also have
to route the private device path as well rather than rely on
gmem_abort().
+
if (!perm_fault) {
memcache = get_mmu_memcache(vcpu);
ret = topup_mmu_memcache(vcpu, memcache);
@@ -2277,10 +2280,12 @@ static bool private_ipa_fault(struct kvm *kvm,
phys_addr_t fault_ipa);
static bool kvm_fault_is_gmem_abort(struct kvm *kvm,
const struct kvm_s2_fault_desc *s2fd)
{
- if (!kvm_slot_has_gmem(s2fd->memslot))
- return false;
if (kvm_memslot_is_gmem_only(s2fd->memslot))
return true;
+ /*
+ * For Realms, all private faults must be backed by GMEM.
+ * TODO: Handle Trusted device private memory mappings.
+ */
if (private_ipa_fault(kvm, s2fd->fault_ipa))
return true;
return false;
Also, I have the following hunk for preventing memslot modifications.
I will add this to v20 integration branch, which is almost ready ;-)
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 582b48e34486b..87e49251e0447 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2783,6 +2783,18 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
}
}
+static bool kvm_prevents_memslot_change(struct kvm *kvm, enum kvm_mr_change change)
+{
+ /* Cannot modify memslots once a pVM has run or Realm created */
+ if (change != KVM_MR_DELETE && change != KVM_MR_MOVE)
+ return false;
+
+ if ((kvm_vm_is_protected_pkvm(kvm) && pkvm_hyp_vm_is_created(kvm)) ||
+ kvm_realm_is_created(kvm))
+ return true;
+ return false;
+}
+
This needs to be tweaked for Realm to support non-secure device assignment.
Aneesh reports that the Device assignment fails now,
because the Device BAR reset deletes the memory slot and re-registers
it, which the above change prevents.
I will modify that to
1. Prevent "Guest-memfd" backed memory slot deletion. Makes sure that
nothing can replace a private memory slot.
2. Allow non-Guest-memfd backed memory slots to be created
after the Realm is created. We anyways prevent "private" memory
to be mapped from a non-Guest-memfd memslot.
Yes, I think this should be fine. But at least with the last version I
looked at, we did not prevent private memory from being mapped from
non-guest_memfd slots (there was a way to delete the gmem slot, add a
normal one while the guest does a private IPA access). If we prevent
gmem slot deletion, we no longer have this issue, although we should
warn somewhere on the user_mem_abort() both.
Anyway, to be discussed on the KVM patches, not here. I raised it
initially here as I was looking whether a non-gmem slot ever ends up
private and trigger the GPF.
Agree. For the record here is what the change looks like, to be included
in v21 kvm-integration. We can discuss it with KVM changes.
KVM: arm64: CCA: Prevent memslot modifications after Realm creation
Once the Realm is created, prevent any changes to guest_memfd backed
memslots, as the private memory is provided by slots backed by gmem.
This is also validated when we handle a fault for Realm Stage2.
Additionally, similar to pKVM, we do not support dirty logging or
read-only memslots.
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 2e584a187602f..ea14f66ed9f29 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2726,17 +2726,28 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
hva_t hva, reg_end;
int ret = 0;
+ if (kvm_vm_is_protected(kvm)) {
+ if (new &&
+ new->flags & (KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY)) {
+ return -EPERM;
+ }
+ }
+
if (kvm_vm_is_protected_pkvm(kvm)) {
/* Cannot modify memslots once a pVM has run. */
if (pkvm_hyp_vm_is_created(kvm) &&
(change == KVM_MR_DELETE || change == KVM_MR_MOVE)) {
return -EPERM;
}
-
- if (new &&
- new->flags & (KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY)) {
+ } else if (kvm_realm_is_created(kvm)) {
+ /*
+ * Once the Realm is created, we cannot modify any slots that
+ * could be providing private memory. i.e., guest_memfd backed
+ * slots.
+ * TODO: Handle trusted device private memory slots
+ */
+ if (kvm_slot_has_gmem(old) || kvm_slot_has_gmem(new))
return -EPERM;
- }
}
if (change != KVM_MR_CREATE && change != KVM_MR_MOVE &&
diff --git a/arch/arm64/kvm/rmi.c b/arch/arm64/kvm/rmi.c
index 8756ea4c76619..5755422ddcb49 100644
--- a/arch/arm64/kvm/rmi.c
+++ b/arch/arm64/kvm/rmi.c
@@ -1196,6 +1196,7 @@ int kvm_activate_realm(struct kvm *kvm)
if (kvm_realm_state(kvm) >= REALM_STATE_ACTIVE)
return 0;
+ guard(mutex)(&kvm->slots_lock);
guard(mutex)(&kvm->arch.config_lock);
/* Check again with the lock held */
if (kvm_realm_state(kvm) >= REALM_STATE_ACTIVE)