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KVM: arm64: Isolate mmap_read_lock inside new kvm_s2_fault_get_vma_info() helper

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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>
Signed-off-by: Marc Zyngier <maz@kernel.org>
This commit is contained in:
Fuad Tabba 2026-03-06 14:02:23 +00:00 committed by Marc Zyngier
parent bae99813c6
commit f5a5bb8de1
1 changed files with 12 additions and 4 deletions
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16
arch/arm64/kvm/mmu.c
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@ -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);