[PATCH v1 04/13] KVM: arm64: Isolate mmap_read_lock inside new kvm_s2_fault_get_vma_info() helper

[Fuad Tabba <tabba@google.com>]

Extract the VMA lookup and metadata snapshotting logic from
kvm_s2_fault_pin_pfn() into a tightly-scoped sub-helper.

This refactoring structurally fixes a TOCTOU (Time-Of-Check to
Time-Of-Use) vulnerability and Use-After-Free risk involving the vma
pointer. In the previous layout, the mmap_read_lock is taken, the vma is
looked up, and then the lock is dropped before the function continues to
map the PFN. While an explicit vma = NULL safeguard was present, the vma
variable was still lexically in scope for the remainder of the function.

By isolating the locked region into kvm_s2_fault_get_vma_info(), the vma
pointer becomes a local variable strictly confined to that sub-helper.
Because the pointer's scope literally ends when the sub-helper returns,
it is not possible for the subsequent page fault logic in
kvm_s2_fault_pin_pfn() to accidentally access the vanished VMA,
eliminating this bug class by design.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
 arch/arm64/kvm/mmu.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index d56c6422ca5f..344a477e1bff 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1740,7 +1740,7 @@ struct kvm_s2_fault {
 	vm_flags_t vm_flags;
 };
 
-static int kvm_s2_fault_pin_pfn(struct kvm_s2_fault *fault)
+static int kvm_s2_fault_get_vma_info(struct kvm_s2_fault *fault)
 {
 	struct vm_area_struct *vma;
 	struct kvm *kvm = fault->vcpu->kvm;
@@ -1774,9 +1774,6 @@ static int kvm_s2_fault_pin_pfn(struct kvm_s2_fault *fault)
 
 	fault->is_vma_cacheable = kvm_vma_is_cacheable(vma);
 
-	/* Don't use the VMA after the unlock -- it may have vanished */
-	vma = NULL;
-
 	/*
 	 * Read mmu_invalidate_seq so that KVM can detect if the results of
 	 * vma_lookup() or __kvm_faultin_pfn() become stale prior to
@@ -1788,6 +1785,17 @@ static int kvm_s2_fault_pin_pfn(struct kvm_s2_fault *fault)
 	fault->mmu_seq = kvm->mmu_invalidate_seq;
 	mmap_read_unlock(current->mm);
 
+	return 0;
+}
+
+static int kvm_s2_fault_pin_pfn(struct kvm_s2_fault *fault)
+{
+	int ret;
+
+	ret = kvm_s2_fault_get_vma_info(fault);
+	if (ret)
+		return ret;
+
 	fault->pfn = __kvm_faultin_pfn(fault->memslot, fault->gfn,
 				       fault->write_fault ? FOLL_WRITE : 0,
 				       &fault->writable, &fault->page);
-- 
2.53.0.473.g4a7958ca14-goog