[PATCH 2/2] KVM: Do not speculatively mark pfn cache valid to "fix" race

[Sean Christopherson <seanjc@google.com>]

When refreshing a stale pfn cache, mark it valid only after the new pfn
(and maybe kernel mapping) has been acquired.  Speculatively marking the
cache as valid with garbage pfn/khva values can lead to other tasks using
the garbage values.  Presumably, the cache was previously marked valid
before dropping gpc->lock to address a race where an mmu_notifier event
could invalidate the cache between retrieving the pfn via hva_to_pfn()
and re-acquiring gpc->lock.  That race has been plugged by waiting for
mn_active_invalidate_count to reach zero before returning from
hva_to_pfn_retry(), i.e. waiting for any notifiers to fully complete
before returning.

Note, this could also be "fixed" by having kvm_gfn_to_pfn_cache_check()
also verify gpc->hkva, but that's unnecessary now that the aforementioned
race between refresh and mmu_notifier invalidation has been eliminated.

  CPU0                                    CPU1
  ----                                    ----

                                          gpc->valid == false

                                          kvm_gfn_to_pfn_cache_refresh()
                                          |
                                          |-> gpc->valid = true;

                                          hva_to_pfn_retry()
                                          |
                                          -> drop gpc->lock

  acquire gpc->lock for read
  kvm_gfn_to_pfn_cache_check()
  |
  |-> gpc->valid == true
      gpc->khva  == NULL

  caller dereferences NULL khva

Opportunistically add a lockdep to the check() helper to make it slightly
more obvious that there is no memory ordering issue when setting "valid"
versus "pfn" and/or "khva".

Fixes: 982ed0de4753 ("KVM: Reinstate gfn_to_pfn_cache with invalidation support")
Cc: stable@vger.kernel.org
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Mingwei Zhang <mizhang@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/pfncache.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 71c84a43024c..72eee096a7cd 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -81,6 +81,8 @@ bool kvm_gfn_to_pfn_cache_check(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 {
 	struct kvm_memslots *slots = kvm_memslots(kvm);
 
+	lockdep_assert_held_read(&gpc->lock);
+
 	if ((gpa & ~PAGE_MASK) + len > PAGE_SIZE)
 		return false;
 
@@ -226,11 +228,6 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 	if (!old_valid || old_uhva != gpc->uhva) {
 		void *new_khva = NULL;
 
-		/* Placeholders for "hva is valid but not yet mapped" */
-		gpc->pfn = KVM_PFN_ERR_FAULT;
-		gpc->khva = NULL;
-		gpc->valid = true;
-
 		new_pfn = hva_to_pfn_retry(kvm, gpc);
 		if (is_error_noslot_pfn(new_pfn)) {
 			ret = -EFAULT;
@@ -259,7 +256,7 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 			gpc->pfn = KVM_PFN_ERR_FAULT;
 			gpc->khva = NULL;
 		} else {
-			/* At this point, gpc->valid may already have been cleared */
+			gpc->valid = true;
 			gpc->pfn = new_pfn;
 			gpc->khva = new_khva;
 		}
-- 
2.36.0.rc0.470.gd361397f0d-goog