[PATCH 5.10.y] KVM: arm64: Retry fault if vma_lookup() results become invalid

From: David Matlack <[email protected]>

commit 13ec9308a85702af7c31f3638a2720863848a7f2 upstream.

Read mmu_invalidate_seq before dropping the mmap_lock so that KVM can
detect if the results of vma_lookup() (e.g. vma_shift) become stale
before it acquires kvm->mmu_lock. This fixes a theoretical bug where a
VMA could be changed by userspace after vma_lookup() and before KVM
reads the mmu_invalidate_seq, causing KVM to install page table entries
based on a (possibly) no-longer-valid vma_shift.

Re-order the MMU cache top-up to earlier in user_mem_abort() so that it
is not done after KVM has read mmu_invalidate_seq (i.e. so as to avoid
inducing spurious fault retries).

This bug has existed since KVM/ARM's inception. It's unlikely that any
sane userspace currently modifies VMAs in such a way as to trigger this
race. And even with directed testing I was unable to reproduce it. But a
sufficiently motivated host userspace might be able to exploit this
race.

Fixes: 94f8e6418d39 ("KVM: ARM: Handle guest faults in KVM")
Cc: [email protected]
Reported-by: Sean Christopherson <[email protected]>
Signed-off-by: David Matlack <[email protected]>
Reviewed-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>
[doebel: adjust to contextual and naming differences in 5.10]
Signed-off-by: Bjoern Doebel <[email protected]>
---
The fix is already present in every newer supported stable tree, so
5.10.y is the only one left without it:

  5.15.y  00f74003edf5 (backport)
  6.1.y   e1562cc202c9 (backport)
  6.6.y   and later contain the mainline commit 13ec9308a857 itself

Tested using the Amazon Linux kernel test suite.

 arch/arm64/kvm/mmu.c | 42 ++++++++++++++++++++----------------------
 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 26068456ec0f3..2cda74acada2e 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -769,6 +769,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		return -EFAULT;
 	}
 
+	/*
+	 * Permission faults just need to update the existing leaf entry,
+	 * and so normally don't require allocations from the memcache. The
+	 * only exception to this is when dirty logging is enabled at runtime
+	 * and a write fault needs to collapse a block entry into a table.
+	 */
+	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
+		ret = kvm_mmu_topup_memory_cache(memcache,
+						 kvm_mmu_cache_min_pages(kvm));
+		if (ret)
+			return ret;
+	}
+
 	/* Let's check if we will get back a huge page backed by hugetlbfs */
 	mmap_read_lock(current->mm);
 	vma = find_vma_intersection(current->mm, hva, hva + 1);
@@ -818,32 +831,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		fault_ipa &= ~(vma_pagesize - 1);
 
 	gfn = fault_ipa >> PAGE_SHIFT;
-	mmap_read_unlock(current->mm);
 
 	/*
-	 * Permission faults just need to update the existing leaf entry,
-	 * and so normally don't require allocations from the memcache. The
-	 * only exception to this is when dirty logging is enabled at runtime
-	 * and a write fault needs to collapse a block entry into a table.
+	 * Read mmu_notifier_seq so that KVM can detect if the results of
+	 * find_vma_intersection() or gfn_to_pfn_prot() become stale prior to
+	 * acquiring kvm->mmu_lock.
+	 *
+	 * Rely on mmap_read_unlock() for an implicit smp_rmb(), which pairs
+	 * with the smp_wmb() in kvm_mmu_notifier_invalidate_range_end().
 	 */
-	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
-		ret = kvm_mmu_topup_memory_cache(memcache,
-						 kvm_mmu_cache_min_pages(kvm));
-		if (ret)
-			return ret;
-	}
-
 	mmu_seq = vcpu->kvm->mmu_notifier_seq;
-	/*
-	 * Ensure the read of mmu_notifier_seq happens before we call
-	 * gfn_to_pfn_prot (which calls get_user_pages), so that we don't risk
-	 * the page we just got a reference to gets unmapped before we have a
-	 * chance to grab the mmu_lock, which ensure that if the page gets
-	 * unmapped afterwards, the call to kvm_unmap_hva will take it away
-	 * from us again properly. This smp_rmb() interacts with the smp_wmb()
-	 * in kvm_mmu_notifier_invalidate_<page|range_end>.
-	 */
-	smp_rmb();
+	mmap_read_unlock(current->mm);
 
 	pfn = gfn_to_pfn_prot(kvm, gfn, write_fault, &writable);
 	if (pfn == KVM_PFN_ERR_HWPOISON) {
-- 
2.51.0