[PATCH v6 1/5] KVM: arm64: Block cacheable PFNMAP mapping

ankita
2025-05-24T01:40:00Z

From: Ankit Agrawal <ankita@nvidia.com>

Fixes a security bug due to mismatched attributes between S1 and
S2 mapping.

Currently, it is possible for a region to be cacheable in S1, but mapped
non cached in S2. This creates a potential issue where the VMM may
sanitize cacheable memory across VMs using cacheable stores, ensuring
it is zeroed. However, if KVM subsequently assigns this memory to a VM
as uncached, the VM could end up accessing stale, non-zeroed data from
a previous VM, leading to unintended data exposure. This is a security
risk.

Block such mismatch attributes case by returning EINVAL when userspace
try to map PFNMAP cacheable. Only allow NORMAL_NC and DEVICE_*.

CC: Oliver Upton <oliver.upton@linux.dev>
CC: Sean Christopherson <seanjc@google.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
---
arch/arm64/kvm/mmu.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 2feb6c6b63af..305a0e054f81 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1466,6 +1466,18 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
return vma->vm_flags & VM_MTE_ALLOWED;
}

+/*
+ * Determine the memory region cacheability from VMA's pgprot. This
+ * is used to set the stage 2 PTEs.
+ */
+static unsigned long mapping_type_noncacheable(pgprot_t page_prot)
+{
+ unsigned long mt = FIELD_GET(PTE_ATTRINDX_MASK, pgprot_val(page_prot));
+
+ return (mt == MT_NORMAL_NC || mt == MT_DEVICE_nGnRnE ||
+ mt == MT_DEVICE_nGnRE);
+}
+
static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_s2_trans *nested,
struct kvm_memory_slot *memslot, unsigned long hva,
@@ -1612,6 +1624,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,

vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;

+ if ((vma->vm_flags & VM_PFNMAP) &&
+ !mapping_type_noncacheable(vma->vm_page_prot))
+ return -EINVAL;
+
/* Don't use the VMA after the unlock -- it may have vanished */
vma = NULL;

@@ -2207,6 +2223,12 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
ret = -EINVAL;
break;
}
+
+ /* Cacheable PFNMAP is not allowed */
+ if (!mapping_type_noncacheable(vma->vm_page_prot)) {
+ ret = -EINVAL;
+ break;
+ }
}
hva = min(reg_end, vma->vm_end);
} while (hva < reg_end);
--
2.34.1